Cyclohexene 3-Bromocyclohexene 1,3-Cyclohexadiene (76\%) -

Simple conjugated dienes used in polymer synthesis include 1,3-butadiene, chloroprene (2-chloro-1,3-butadiene), and isoprene (2-methyl-1,3-butadiene). Isoprene has been prepared industrially by several methods, including the acid-catalyzed double dehydration of 3-methyl-1,3-butanediol.
<img class="imgSvg" id = "m6sn3nrg1pl903qul93" src="data:image/svg+xml;base64,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"/>

3-Methyl-1,3-butanediol
<img class="imgSvg" id = "m6sn3nrgnqs4brbkrz" src="data:image/svg+xml;base64,<svg id="smiles-m6sn3nrgnqs4brbkrz" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 220 89.2500550946915" style="width: 220.39899049303781px; height: 89.2500550946915px; overflow: visible;"><defs><linearGradient id="line-m6sn3nrgnqs4brbkrz-1" gradientUnits="userSpaceOnUse" x1="151.11920087681455" y1="52.500036729794346" x2="178.39899049303781" y2="68.2500550946915"><stop stop-color="currentColor" offset="20%"></stop><stop stop-color="currentColor" offset="100%"></stop></linearGradient><linearGradient id="line-m6sn3nrgnqs4brbkrz-3" gradientUnits="userSpaceOnUse" x1="129.40236783116615" y1="71.58538614937356" x2="151.22621648891337" y2="58.985400841297036"><stop stop-color="currentColor" offset="20%"></stop><stop stop-color="currentColor" offset="100%"></stop></linearGradient><linearGradient id="line-m6sn3nrgnqs4brbkrz-5" gradientUnits="userSpaceOnUse" x1="123.83939005463051" y1="68.25001836489002" x2="151.11920087681455" y2="52.500036729794346"><stop stop-color="currentColor" offset="20%"></stop><stop stop-color="currentColor" offset="100%"></stop></linearGradient><linearGradient id="line-m6sn3nrgnqs4brbkrz-7" gradientUnits="userSpaceOnUse" x1="96.55960043840727" y1="52.499999999992866" x2="123.83939005463051" y2="68.25001836489002"><stop stop-color="currentColor" offset="20%"></stop><stop stop-color="currentColor" offset="100%"></stop></linearGradient><linearGradient id="line-m6sn3nrgnqs4brbkrz-9" gradientUnits="userSpaceOnUse" x1="96.55960043840727" y1="52.499999999992866" x2="96.55962164436804" y2="21"><stop stop-color="currentColor" offset="20%"></stop><stop stop-color="currentColor" offset="100%"></stop></linearGradient><linearGradient id="line-m6sn3nrgnqs4brbkrz-11" gradientUnits="userSpaceOnUse" x1="74.84276739275887" y1="71.58534941957208" x2="96.6666160505061" y2="58.98536411149556"><stop stop-color="currentColor" offset="20%"></stop><stop stop-color="currentColor" offset="100%"></stop></linearGradient><linearGradient id="line-m6sn3nrgnqs4brbkrz-13" gradientUnits="userSpaceOnUse" x1="69.27978961622325" y1="68.24998163508855" x2="96.55960043840727" y2="52.499999999992866"><stop stop-color="currentColor" offset="20%"></stop><stop stop-color="currentColor" offset="100%"></stop></linearGradient><linearGradient id="line-m6sn3nrgnqs4brbkrz-15" gradientUnits="userSpaceOnUse" x1="42" y1="52.49996327019139" x2="69.27978961622325" y2="68.24998163508855"><stop stop-color="currentColor" offset="20%"></stop><stop stop-color="currentColor" offset="100%"></stop></linearGradient></defs><mask id="text-mask-m6sn3nrgnqs4brbkrz"><rect x="0" y="0" width="100%" height="100%" fill="white"></rect><circle cx="178.39899049303781" cy="68.2500550946915" r="7.875" fill="black"></circle></mask><style>
                .element-m6sn3nrgnqs4brbkrz {
                    font: 14px Helvetica, Arial, sans-serif;
                    alignment-baseline: 'middle';
                }
                .sub-m6sn3nrgnqs4brbkrz {
                    font: 8.4px Helvetica, Arial, sans-serif;
                }
            </style><g mask="url(#text-mask-m6sn3nrgnqs4brbkrz)"><line x1="151.11920087681455" y1="52.500036729794346" x2="178.39899049303781" y2="68.2500550946915" style="stroke-linecap:round;stroke-dasharray:none;stroke-width:1.26" stroke="url('#line-m6sn3nrgnqs4brbkrz-1')"></line><line x1="129.40236783116615" y1="71.58538614937356" x2="151.22621648891337" y2="58.985400841297036" style="stroke-linecap:round;stroke-dasharray:none;stroke-width:1.26" stroke="url('#line-m6sn3nrgnqs4brbkrz-3')"></line><line x1="123.83939005463051" y1="68.25001836489002" x2="151.11920087681455" y2="52.500036729794346" style="stroke-linecap:round;stroke-dasharray:none;stroke-width:1.26" stroke="url('#line-m6sn3nrgnqs4brbkrz-5')"></line><line x1="96.55960043840727" y1="52.499999999992866" x2="123.83939005463051" y2="68.25001836489002" style="stroke-linecap:round;stroke-dasharray:none;stroke-width:1.26" stroke="url('#line-m6sn3nrgnqs4brbkrz-7')"></line><line x1="96.55960043840727" y1="52.499999999992866" x2="96.55962164436804" y2="21" style="stroke-linecap:round;stroke-dasharray:none;stroke-width:1.26" stroke="url('#line-m6sn3nrgnqs4brbkrz-9')"></line><line x1="74.84276739275887" y1="71.58534941957208" x2="96.6666160505061" y2="58.98536411149556" style="stroke-linecap:round;stroke-dasharray:none;stroke-width:1.26" stroke="url('#line-m6sn3nrgnqs4brbkrz-11')"></line><line x1="69.27978961622325" y1="68.24998163508855" x2="96.55960043840727" y2="52.499999999992866" style="stroke-linecap:round;stroke-dasharray:none;stroke-width:1.26" stroke="url('#line-m6sn3nrgnqs4brbkrz-13')"></line><line x1="42" y1="52.49996327019139" x2="69.27978961622325" y2="68.24998163508855" style="stroke-linecap:round;stroke-dasharray:none;stroke-width:1.26" stroke="url('#line-m6sn3nrgnqs4brbkrz-15')"></line></g><g><text x="174.46149049303781" y="73.5000550946915" class="element-m6sn3nrgnqs4brbkrz" fill="currentColor" style="
                text-anchor: start;
                writing-mode: horizontal-tb;
                text-orientation: mixed;
                letter-spacing: normal;
                direction: ltr;
            "><tspan>H<tspan baseline-shift="super" class="sub-m6sn3nrgnqs4brbkrz">1</tspan></tspan></text><text x="178.39899049303781" y="68.2500550946915" class="debug" fill="#ff0000" style="
                font: 5px Droid Sans, sans-serif;
            "></text><text x="151.11920087681455" y="52.500036729794346" class="debug" fill="#ff0000" style="
                font: 5px Droid Sans, sans-serif;
            "></text><text x="123.83939005463051" y="68.25001836489002" class="debug" fill="#ff0000" style="
                font: 5px Droid Sans, sans-serif;
            "></text><text x="96.55960043840727" y="52.499999999992866" class="debug" fill="#ff0000" style="
                font: 5px Droid Sans, sans-serif;
            "></text><text x="96.55962164436804" y="21" class="debug" fill="#ff0000" style="
                font: 5px Droid Sans, sans-serif;
            "></text><text x="69.27978961622325" y="68.24998163508855" class="debug" fill="#ff0000" style="
                font: 5px Droid Sans, sans-serif;
            "></text><text x="42" y="52.49996327019139" class="debug" fill="#ff0000" style="
                font: 5px Droid Sans, sans-serif;
            "></text></g></svg>"/>


Isoprene
2-Methyl-1,3-butadiene


One of the properties that distinguishes conjugated from nonconjugated dienes is that the central single bond is shorter than might be expected. The C2-C3 single bond in 1,3-butadiene, for instance, has a length of 147 pm , some 6 pm shorter than the C2-C3 bond in butane ( 153 pm ).


Butane


Another distinctive property of conjugated dienes is their unusual stability, as evidenced by their heats of hydrogenation (TABLE 14.1). We saw in Section 7.6 that monosubstituted alkenes, such as 1-butene, have $\Delta H^{\circ}{ }_{\text {hydrog }}$ near $-126 \mathrm{~kJ} / \mathrm{mol}(-30.1 \mathrm{kcal} / \mathrm{mol})$, whereas disubstituted alkenes, such as 2-methylpropene, have $\Delta H^{\circ}{ }_{\text {hydrog }}$ near $-119 \mathrm{~kJ} / \mathrm{mol}(-28.4 \mathrm{kcal} / \mathrm{mol})$, which is approximately $7 \mathrm{~kJ} / \mathrm{mol}$ less negative. We concluded from these data that more highly substituted alkenes are more stable than less substituted ones. That is, more highly substituted alkenes release less heat on hydrogenation because they contain less energy to start with. A similar conclusion can be drawn for conjugated dienes.


TABLE 14.1 Heats of Hydrogenation for Some Alkenes and Dienes


| Alkene or diene | Product | $\Delta H^{\circ} \mathrm{hydrog}$ |  |
| :---: | :---: | :---: | :---: |
|  |  | (kJ/mol) | ( $\mathrm{kcal} / \mathrm{mol}$ ) |
| $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CH}_{2}$ | $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}$ | -126 | -30.1 |
| <smiles>C=C(C)C</smiles> | <smiles>CC(C)C</smiles> | -119 | -28.4 |
| $\mathrm{H}_{2} \mathrm{C}=\mathrm{CHCH}_{2} \mathrm{CH}=\mathrm{CH}_{2}$ | $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}$ | -253 | -60.5 |
| $\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}_{2}$ | $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}$ | -236 | -56.4 |
| <smiles>C=CC(=C)C</smiles> | <smiles>CCC(C)C</smiles> | -229 | -54.7 |


Because a monosubstituted alkene has a $\Delta H^{\circ}{ }_{\text {hydrog }}$ of approximately $-126 \mathrm{~kJ} / \mathrm{mol}$, we might expect that a compound with two monosubstituted double bonds would have a $\Delta H^{\circ}{ }_{\text {hydrog }}$ approximately twice that value, or $-252 \mathrm{~kJ} / \mathrm{mol}$. Nonconjugated dienes, such as 1,4 -pentadiene ( $\Delta H^{\circ}{ }_{\text {hydrog }}=-253 \mathrm{~kJ} / \mathrm{mol}$ ), meet this expectation, but the conjugated diene 1,3-butadiene ( $\Delta H^{\circ}{ }_{\text {hydrog }}=-236 \mathrm{~kJ} / \mathrm{mol}$ ) does not. 1,3-Butadiene is approximately $16 \mathrm{~kJ} / \mathrm{mol}(3.8 \mathrm{kcal} / \mathrm{mol})$ more stable than expected.


|  | $\Delta \boldsymbol{H}^{\circ}$ hydrog $(\mathbf{k J} / \mathbf{m o l})$ |  |
| :---: | :--- | :--- |
| $\mathrm{H}_{2} \mathrm{C}=\mathrm{CHCH}_{2} \mathrm{CH}=\mathrm{CH}_{2}$ | $-126+(-126)=-252$ | Expected
 1,4-Pentadiene |
|  | -253 | Observed
 Difference |
| $\mathrm{H}_{2} \mathrm{C}=\mathrm{CHCH}=\mathrm{CH}_{2}$ | $-126+(-126)=-252$ | Expected |
| 1,3-Butadiene | -236 | Observed |
|  | -16 | Difference |


What accounts for the stability of conjugated dienes? According to valence bond theory (Section 1.5 and Section 1.8), their stability is due to orbital hybridization. Typical $\mathrm{C}-\mathrm{C}$ single bonds, like those in alkanes, result from $\sigma$ overlap of $s p^{3}$ orbitals on both carbons, but in a conjugated diene, the central $\mathrm{C}-\mathrm{C}$ single bond results from $\sigma$ overlap of $s p^{2}$ orbitals on both carbons. Because $s p^{2}$ orbitals have more $s$ character ( $33 \% s$ ) than $s p^{3}$ orbitals $(25 \% s)$, the electrons in $s p^{2}$ orbitals are closer to the nucleus and the bonds they form are somewhat shorter
and stronger. Thus, the "extra" stability of a conjugated diene results in part from the greater amount of $s$ character in the orbitals forming the $\mathrm{C}-\mathrm{C}$ single bond.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0449.jpg?height=177&width=345&top_left_y=302&top_left_x=706)
<img class="imgSvg" id = "m6sn3ns0e9khgagkjsb" src="data:image/svg+xml;base64,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"/>


Bond formed by overlap of $\boldsymbol{s p} \boldsymbol{p}^{2}$ orbitals
According to molecular orbital theory (Section 1.11), the stability of conjugated dienes arises because of an interaction between the $\pi$ orbitals of the two double bonds. To review briefly, when two $p$ atomic orbitals combine to form a $\pi$ bond, two $\pi$ molecular orbitals (MOs) result. One is lower in energy than the starting $p$ orbitals and is therefore bonding; the other is higher in energy, has a node between nuclei, and is antibonding. The two $\pi$ electrons occupy the low-energy, bonding orbital, resulting in formation of a stable bond between atoms (FIGURE 14.2).
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0449.jpg?height=684&width=1226&top_left_y=758&top_left_x=452)


FIGURE 14.2 Two $\boldsymbol{p}$ orbitals combine to form two $\boldsymbol{\pi}$ molecular orbitals. Both electrons occupy the low-energy, bonding orbital, leading to a net lowering of energy and formation of a stable bond. The asterisk on $\psi_{2} *$ indicates an antibonding orbital.

Now let's combine four adjacent $p$ atomic orbitals, as occurs in a conjugated diene. In so doing, we generate a set of four $\pi$ molecular orbitals, two of which are bonding and two of which are antibonding (FIGURE 14.3). The four $\pi$ electrons occupy the two bonding orbitals, leaving the antibonding orbitals vacant.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0449.jpg?height=722&width=1232&top_left_y=1704&top_left_x=452)


FIGURE 14.3 Four $\boldsymbol{\pi}$ molecular orbitals in 1,3-butadiene. Note that the number of nodes between nuclei increases as the energy level of the orbital increases.


The lowest-energy $\pi$ molecular orbital (denoted $\psi_{1}$, Greek psi) has no nodes between the nuclei and is therefore bonding. The $\pi$ MO of next-lowest energy, $\psi_{2}$, has one node between nuclei and is also bonding. Above $\psi_{1}$ and $\psi_{2}$ in energy are the two antibonding $\pi \operatorname{MOs}, \psi_{3} *$ and $\psi_{4}{ }^{*}$. (The asterisks indicate antibonding orbitals.) Note that the number of nodes between nuclei increases as the energy level of the orbital increases. The $\psi_{3} *$ orbital has two nodes between nuclei, and $\psi_{4}{ }^{*}$, the highest-energy MO, has three nodes between nuclei.


Comparing the $\pi$ molecular orbitals of 1,3-butadiene (two conjugated double bonds) with those of 1,4-pentadiene (two isolated double bonds) shows why the conjugated diene is more stable. In a conjugated diene, the lowest-energy $\pi \mathrm{MO}\left(\psi_{1}\right)$ has a favorable bonding interaction between C 2 and C 3 that is absent in a nonconjugated diene. As a result, there is a certain amount of double-bond character to the $\mathrm{C} 2-\mathrm{C} 3$ single bond, making that bond both stronger and shorter than a typical single bond. Electrostatic potential maps show clearly the additional electron density in the central single bond (FIGURE 14.4).
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0450.jpg?height=367&width=405&top_left_y=716&top_left_x=616)
<img class="imgSvg" id = "m6sn3ns1647gdhprng7" src="data:image/svg+xml;base64,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"/>

1,3-Butadiene (conjugated)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0450.jpg?height=269&width=467&top_left_y=814&top_left_x=1046)
<img class="imgSvg" id = "m6sn3ns2shhzazngny" src="data:image/svg+xml;base64,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"/>


1,4-Pentadiene
(nonconjugated)


FIGURE 14.4 Electrostatic potential maps of 1,3-butadiene (conjugated) and 1,4-pentadiene (nonconjugated). Additional electron density is present in the central $\mathrm{C}-\mathrm{C}$ bond of 1,3-butadiene, corresponding to partial double-bond character.


In describing 1,3-butadiene, we say that the $\pi$ electrons are spread out, or delocalized, over the entire $\pi$ framework, rather than localized between two specific nuclei. Delocalization allows the bonding electrons to be closer to more nuclei, thus leading to lower energy and greater stability.


PROBLEM Allene, $\mathrm{H}_{2} \mathrm{C}=\mathrm{C}=\mathrm{CH}_{2}$, has a heat of hydrogenation of $-298 \mathrm{~kJ} / \mathrm{mol}(-71.3 \mathrm{kcal} / \mathrm{mol})$. Rank a 14-1 conjugated diene, a nonconjugated diene, and an allene in order of stability.


Cyclohexene 3-Bromocyclohexene 1,3-Cyclohexadiene (76\%) - 14.2 Electrophilic Additions to Conjugated Dienes: Allylic Carbocations

One of the most striking differences between conjugated dienes and typical alkenes is their behavior in electrophilic addition reactions. To review briefly, the addition of an electrophile to a carbon-carbon double bond is a general reaction of alkenes (Section 7.7). Markovnikov regiochemistry is observed because the more stable carbocation is formed as an intermediate. Thus, addition of HCl to 2-methylpropene yields 2-chloro-2-methylpropane rather than 1-chloro-2-methylpropane, and addition of 2 equivalents of HCl to the nonconjugated diene 1,4-pentadiene yields 2,4-dichloropentane.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0451.jpg?height=274&width=944&top_left_y=191&top_left_x=588)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0451.jpg?height=216&width=746&top_left_y=472&top_left_x=684)

Conjugated dienes also undergo electrophilic addition reactions readily, but mixtures of products are invariably obtained. Addition of HBr to 1,3 -butadiene, for instance, yields a mixture of two products (not counting cis-trans isomers). 3-Bromo-1-butene is the typical Markovnikov product of $\mathbf{1 , 2}$-addition to a double bond, but 1-bromo-2-butene seems unusual. The double bond in this product has moved to a position between carbons 2 and 3 , and HBr has added to carbons 1 and 4, a result described as $\mathbf{1 , 4}$-addition.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0451.jpg?height=465&width=1091&top_left_y=922&top_left_x=517)

Many other electrophiles besides HBr add to conjugated dienes, and mixtures of products are usually formed. For example, $\mathrm{Br}_{2}$ adds to 1,3-butadiene to give a mixture of 3,4-dibromo-1-butene and 1,4-dibromo-2-butene.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0451.jpg?height=302&width=1060&top_left_y=1497&top_left_x=527)


How can we account for the formation of 1,4-addition products? The answer is that allylic carbocations are involved as intermediates (recall that the word allylic means "next to a double bond"). When 1,3-butadiene reacts with an electrophile such as $\mathrm{H}^{+}$, two carbocation intermediates are possible-a primary nonallylic carbocation and a secondary allylic cation. Because an allylic cation is stabilized by resonance between two forms (Section 11.5), it is more stable and forms faster than a nonallylic carbocation.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0452.jpg?height=584&width=1277&top_left_y=185&top_left_x=424)


When the allylic cation reacts with $\mathrm{Br}^{-}$to complete the electrophilic addition, the reaction can occur either at C 1 or at C3 because both carbons share the positive charge (FIGURE 14.5). Thus, a mixture of 1,2-and 1,4-addition products results. You might recall that a similar product mixture was seen for NBS bromination of alkenes in Section 10.3, a reaction that proceeds through an allylic radical.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0452.jpg?height=611&width=673&top_left_y=968&top_left_x=460)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0452.jpg?height=375&width=497&top_left_y=967&top_left_x=1166)


FIGURE 14.5 An electrostatic potential map of the allylic carbocation produced by protonation of 1,3-butadiene shows that the positive charge is shared by carbons 1 and 3. Reaction of $\mathrm{Br}^{-}$with the more positive carbon (C3) predominantly yields the 1,2-addition product.


WORKED EXAMPLE 14.1 -


Predicting the Product of an Electrophilic Addition Reaction of a Conjugated Diene -

Give the structures of the likely products from reaction of 1 equivalent of HCl with 2 -methyl-1,3-cyclohexadiene. Show both 1,2 and 1,4 adducts.


Strategy -

Electrophilic addition of HCl to a conjugated diene involves the formation of allylic carbocation intermediate. Thus, the first step is to protonate the two ends of the diene and draw the resonance forms of the two allylic carbocations that result. Then, allow each resonance form to react with $\mathrm{Cl}^{-}$, generating a maximum of four possible products.


Strategy -

In the present instance, protonation of the $\mathrm{C} 1-\mathrm{C} 2$ double bond gives a carbocation that can react further to give the 1,2 adduct 3 -chloro-3-methylcyclohexene and the 1,4 adduct 3-chloro-1-methylcyclohexene. Protonation of the C3-C4 double bond gives a symmetrical carbocation, whose two resonance forms are equivalent. Thus, the 1,2 adduct and the 1,4 adduct have the same structure: 6 -chloro-1-methylcyclohexene. Of the two possible modes of protonation, the first is more likely because it yields a more stable, tertiary allylic cation rather than a less-stable, secondary allylic cation.


Solution -


Solution -

PROBLEM Give the structures of both 1,2 and 1,4 adducts resulting from reaction of 1 equivalent of HCl with 14-2 1,3-pentadiene.

PROBLEM Look at the possible carbocation intermediates produced during addition of HCl to 1,3-pentadiene 14-3 (Problem 14-2), and predict which 1,2 adduct predominates. Which 1,4 adduct predominates?


Solution -

PROBLEM Give the structures of both 1,2 and 1,4 adducts resulting from reaction of 1 equivalent of HBr with
14-4 the following compound:
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0453.jpg?height=366&width=377&top_left_y=1259&top_left_x=424)


Solution - 14.3 Kinetic versus Thermodynamic Control of Reactions

Electrophilic addition to a conjugated diene at or below room temperature normally leads to a mixture of products in which the 1,2 adduct predominates over the 1,4 adduct. When the same reaction is carried out at higher temperatures, however, the product ratio often changes and the 1,4 adduct predominates. For example, addition of HBr to 1,3 -butadiene at $0^{\circ} \mathrm{C}$ yields a $71: 29$ mixture of 1,2 and 1,4 adducts, but the same reaction carried out at $40^{\circ} \mathrm{C}$ yields a $15: 85$ mixture. Furthermore, when the product mixture formed at $0{ }^{\circ} \mathrm{C}$ is heated to $40^{\circ} \mathrm{C}$ in the presence of HBr , the ratio of adducts slowly changes from $71: 29$ to $15: 85$. Why?
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0453.jpg?height=375&width=1036&top_left_y=1992&top_left_x=539)


To understand the effect of temperature on product distribution, let's briefly review what we said in Section 6.7 about rates and equilibria. Imagine a reaction that can give either or both of two products, B and C.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0454.jpg?height=117&width=296&top_left_y=188&top_left_x=912)


Let's assume that $\mathbf{B}$ forms faster than $\mathbf{C}$ (in other words, $\Delta G^{\ddagger} \mathrm{B}^{<\Delta G^{\ddagger}} \mathrm{C}$ ) but that $\mathbf{C}$ is more stable than $\mathbf{B}$ (in other words, $\Delta G^{\circ}{ }_{\mathrm{C}}>\Delta G^{\circ}{ }_{\mathrm{B}}$ ). An energy diagram for the two processes might look like that shown in FIGURE 14.6.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0454.jpg?height=617&width=1261&top_left_y=472&top_left_x=429)

FIGURE 14.6 An energy diagram for two competing reactions in which the less stable product $B$ forms faster than the more stable product C .


Let's first carry out the reaction at a lower temperature so that both processes are irreversible and no equilibrium is reached. Because $\mathbf{B}$ forms faster than $\mathbf{C}, \mathbf{B}$ is the major product. It doesn't matter that $\mathbf{C}$ is more stable than B, because the two are not in equilibrium. The product of an irreversible reaction depends only on relative rates, not on stability. Such reactions are said to be under kinetic control.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0454.jpg?height=113&width=655&top_left_y=1380&top_left_x=733)

Now let's carry out the same reaction at some higher temperature so that both processes are readily reversible and an equilibrium is reached. Because $\mathbf{C}$ is more stable than $\mathbf{B}, \mathbf{C}$ is the major product obtained. It doesn't matter that $\mathbf{C}$ forms more slowly than $\mathbf{B}$, because the two are in equilibrium. The product of a readily reversible reaction depends only on stability, not on relative rates. Such reactions are said to be under equilibrium control, or thermodynamic control.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0454.jpg?height=123&width=687&top_left_y=1738&top_left_x=719)


We can now explain the effect of temperature on the electrophilic addition reactions of conjugated dienes. At low temperature $\left(0^{\circ} \mathrm{C}\right)$, HBr adds to 1,3-butadiene under kinetic control to give a $71: 29$ mixture of products, with the more rapidly formed 1,2 adduct predominating. Because these low-temperature conditions don't allow the reaction to reach equilibrium, the product that forms faster predominates. At higher temperature $\left(40^{\circ} \mathrm{C}\right)$, however, the reaction occurs under thermodynamic control to give a $15: 85$ mixture of products, with the more stable 1,4 adduct predominating. The higher temperature allows the addition process to become reversible, so an equilibrium mixture of products results. FIGURE 14.7 shows this situation in an energy diagram.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0455.jpg?height=638&width=1171&top_left_y=191&top_left_x=477)

FIGURE 14.7 Energy diagram for the electrophilic addition of $\mathbf{H B r}$ to 1,3-butadiene. The 1,2 adduct is the kinetic product because it forms faster, but the 1,4 adduct is the thermodynamic product because it is more stable.

The electrophilic addition of HBr to 1,3-butadiene is a good example of how a change in experimental conditions can change the product of a reaction. The concept of thermodynamic control versus kinetic control is a useful one that we can sometimes take advantage of in the laboratory.


PROBLEM The 1,2 adduct and the 1,4 adduct formed by reaction of HBr with 1,3-butadiene are in equilibrium 14-5 at $40^{\circ} \mathrm{C}$. Propose a mechanism by which the interconversion of products takes place.


PROBLEM Why do you suppose 1,4 adducts of 1,3-butadiene are generally more stable than 1,2 adducts?
14-6


Solution - 14.4 The Diels-Alder Cycloaddition Reaction

Perhaps the most striking difference between conjugated and nonconjugated dienes is that conjugated dienes undergo an addition reaction with alkenes to yield substituted cyclohexene products. For example, 1,3-butadiene and 3-buten-2-one give 3-cyclohexenyl methyl ketone.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0455.jpg?height=372&width=953&top_left_y=1511&top_left_x=586)


This process, named the Diels-Alder cycloaddition reaction after its discoverers, is extremely useful in the laboratory because it forms two carbon-carbon bonds in a single step and is one of the few general methods available for making cyclic molecules. (As the name implies, a cycloaddition reaction is one in which two reactants add together to give a cyclic product.) The 1950 Nobel Prize in Chemistry was awarded to Otto Diels and Kurt Alder in recognition of the importance of their discovery.

The mechanism of Diels-Alder cycloaddition is different from that of other reactions we've studied because it is neither polar nor radical. Rather, the Diels-Alder reaction is a pericyclic process. Pericyclic reactions, which we'll discuss in more detail in Chapter 30, take place in a single step by a cyclic redistribution of bonding electrons. The two reactants simply join together through a cyclic transition state in which the two new $\mathrm{C}-\mathrm{C}$ bonds form at the same time.


We can picture a Diels-Alder addition as occurring by head-on ( $\sigma$ ) overlap of the two alkene $p$ orbitals with the two $p$ orbitals on carbons 1 and 4 of the diene (FIGURE 14.8). This is, of course, a cyclic orientation of the reactants.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0456.jpg?height=527&width=459&top_left_y=189&top_left_x=234)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0456.jpg?height=272&width=1162&top_left_y=346&top_left_x=758)


FIGURE 14.8 Mechanism of the Diels-Alder cycloaddition reaction. The reaction occurs in a single step through a cyclic transition state in which the two new $\mathrm{C}-\mathrm{C}$ bonds form simultaneously.


In the Diels-Alder transition state, the two alkene carbons and carbons 1 and 4 of the diene rehybridize from $s p^{2}$ to $s p^{3}$ to form two new single bonds, while carbons 2 and 3 of the diene remain $s p^{2}$-hybridized to form the new double bond in the cyclohexene product. We'll study this mechanism in more detail in Section $\mathbf{3 0 . 5}$ but will concentrate for the present on learning about the characteristics and uses of the Diels-Alder reaction.


Solution - 14.5 Characteristics of the Diels-Alder Reaction


The Dienophile -

The Diels-Alder cycloaddition reaction occurs most rapidly if the alkene component, called the dienophile ("diene lover"), has an electron-withdrawing substituent group. Thus, ethylene itself reacts sluggishly, but propenal, ethyl propenoate, maleic anhydride, benzoquinone, propenenitrile, and similar compounds are highly reactive. Note also that alkynes, such as methyl propynoate, can act as Diels-Alder dienophiles.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0456.jpg?height=738&width=1304&top_left_y=1336&top_left_x=405)


The Dienophile -

In all cases, the double or triple bond of the dienophile is adjacent to the positively polarized carbon of an electron-withdrawing substituent. As a result, the double-bond carbons in these substances are substantially less electron-rich than the carbons in ethylene, as indicated by the electrostatic potential maps in FIGURE 14.9.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0457.jpg?height=478&width=1237&top_left_y=192&top_left_x=439)


The Dienophile -

FIGURE 14.9 Electrostatic potential maps of ethylene, propenal, and propenenitrile show that electron-withdrawing groups make the double-bond carbons less electron-rich.


The Dienophile -

One of the most useful features of the Diels-Alder reaction is that it is stereospecific, meaning that a single product stereoisomer is formed. Furthermore, the stereochemistry of the dienophile is retained. If we carry out cycloaddition with methyl cis-2-butenoate, only the cis-substituted cyclohexene product is formed. With methyl trans-2-butenoate, only the trans-substituted cyclohexene product is formed.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0457.jpg?height=234&width=900&top_left_y=943&top_left_x=599)


The Dienophile -

1,3-Butadiene Methyl(Z)-2-butenoate
Cis product
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0457.jpg?height=291&width=909&top_left_y=1259&top_left_x=608)


The Dienophile -

Another stereochemical feature of the Diels-Alder reaction is that the diene and dienophile partners orient so that the endo product, rather than the alternative exo product, is formed. The words endo and exo are used to indicate relative stereochemistry when referring to bicyclic structures like substituted norbornanes (Section 4.9). A substituent on one bridge is said to be endo if it is syn (cis) to the larger of the other two bridges and is said to be exo if it is anti (trans) to the larger of the other two.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0457.jpg?height=421&width=1361&top_left_y=1795&top_left_x=363)

Endo products result from Diels-Alder reactions because the amount of orbital overlap between diene and dienophile is greater when the reactants lie directly on top of one another, so that the electron-withdrawing substituent on the dienophile is underneath the diene double bonds. In the reaction of 1,3-cyclopentadiene with maleic anhydride, for instance, the following result is obtained:
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0458.jpg?height=400&width=1313&top_left_y=185&top_left_x=409)


The Dienophile -

Maleic anhydride


WORKED EXAMPLE 14.2 -


Predicting the Product of a Diels-Alder Reaction -

Predict the product of the following Diels-Alder reaction:
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0458.jpg?height=250&width=535&top_left_y=875&top_left_x=795)


Strategy -

Draw the diene so that the ends of its two double bonds are near the dienophile double bond. Then form two single bonds between the partners, convert the three double bonds into single bonds, and convert the former single bond of the diene into a double bond. Because the dienophile double bond is cis to begin with, the two attached hydrogens must remain cis in the product.


Solution -


Solution -

PROBLEM Predict the product of the following Diels-Alder reaction:
14-7
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0458.jpg?height=188&width=693&top_left_y=1901&top_left_x=407)


The Diene -

Just as the dienophile component has certain constraints that affect its reactivity, so too does the conjugated diene component. The diene must adopt what is called an $\boldsymbol{s}$-cis conformation, meaning "cis-like" about the single bond, to undergo a Diels-Alder reaction. Only in the $s$-cis conformation are carbons 1 and 4 of the diene close enough to react through a cyclic transition state.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0459.jpg?height=196&width=613&top_left_y=192&top_left_x=756)


$s$-Cis conformation -


$s$-Cis conformation -

No reaction (ends too far apart)


$s$-Cis conformation -

Two examples of dienes that can't adopt an $s$-cis conformation, and thus don't undergo Diels-Alder reactions, are shown in FIGURE 14.10. In the bicyclic diene, the double bonds are rigidly fixed in an $s$-trans arrangement by geometric constraints of the rings. In $(2 Z, 4 Z)-2,4$-hexadiene, steric strain between the two methyl groups prevents the molecule from adopting $s$-cis geometry.
<img class="imgSvg" id = "m6sn3ns7v480h4r7hp" src="data:image/svg+xml;base64,<svg id="smiles-m6sn3ns7v480h4r7hp" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 194 123.83943600088905" style="width: 194.24998775672177px; height: 123.83943600088905px; overflow: visible;"><defs><linearGradient id="line-m6sn3ns7v480h4r7hp-1" gradientUnits="userSpaceOnUse" x1="133.1646334638487" y1="53.84281408747262" x2="145.76462366923374" y2="75.66665991776115"><stop stop-color="currentColor" offset="20%"></stop><stop stop-color="currentColor" offset="100%"></stop></linearGradient><linearGradient id="line-m6sn3ns7v480h4r7hp-3" gradientUnits="userSpaceOnUse" x1="136.4999999999905" y1="48.27983556247493" x2="152.24998775672177" y2="75.55964285033559"><stop stop-color="currentColor" offset="20%"></stop><stop stop-color="currentColor" offset="100%"></stop></linearGradient><linearGradient id="line-m6sn3ns7v480h4r7hp-5" gradientUnits="userSpaceOnUse" x1="136.4999755134562" y1="102.83943600088905" x2="152.24998775672177" y2="75.55964285033559"><stop stop-color="currentColor" offset="20%"></stop><stop stop-color="currentColor" offset="100%"></stop></linearGradient><linearGradient id="line-m6sn3ns7v480h4r7hp-7" gradientUnits="userSpaceOnUse" x1="104.99999999999366" y1="48.27982142516778" x2="136.4999999999905" y2="48.27983556247493"><stop stop-color="currentColor" offset="20%"></stop><stop stop-color="currentColor" offset="100%"></stop></linearGradient><linearGradient id="line-m6sn3ns7v480h4r7hp-9" gradientUnits="userSpaceOnUse" x1="104.99997551345939" y1="102.83942186358193" x2="136.4999755134562" y2="102.83943600088905"><stop stop-color="currentColor" offset="20%"></stop><stop stop-color="currentColor" offset="100%"></stop></linearGradient><linearGradient id="line-m6sn3ns7v480h4r7hp-11" gradientUnits="userSpaceOnUse" x1="85.91464570712054" y1="26.5629926623048" x2="98.51463591250561" y2="48.386838492593355"><stop stop-color="currentColor" offset="20%"></stop><stop stop-color="currentColor" offset="100%"></stop></linearGradient><linearGradient id="line-m6sn3ns7v480h4r7hp-13" gradientUnits="userSpaceOnUse" x1="89.25001224326233" y1="21.000014137307105" x2="104.99999999999366" y2="48.27982142516778"><stop stop-color="currentColor" offset="20%"></stop><stop stop-color="currentColor" offset="100%"></stop></linearGradient><linearGradient id="line-m6sn3ns7v480h4r7hp-15" gradientUnits="userSpaceOnUse" x1="89.24998775672812" y1="75.55961457572126" x2="104.99999999999366" y2="48.27982142516778"><stop stop-color="currentColor" offset="20%"></stop><stop stop-color="currentColor" offset="100%"></stop></linearGradient><linearGradient id="line-m6sn3ns7v480h4r7hp-17" gradientUnits="userSpaceOnUse" x1="89.24998775672812" y1="75.55961457572126" x2="104.99997551345939" y2="102.83942186358193"><stop stop-color="currentColor" offset="20%"></stop><stop stop-color="currentColor" offset="100%"></stop></linearGradient><linearGradient id="line-m6sn3ns7v480h4r7hp-19" gradientUnits="userSpaceOnUse" x1="57.750012243265516" y1="21" x2="89.25001224326233" y2="21.000014137307105"><stop stop-color="currentColor" offset="20%"></stop><stop stop-color="currentColor" offset="100%"></stop></linearGradient><linearGradient id="line-m6sn3ns7v480h4r7hp-21" gradientUnits="userSpaceOnUse" x1="57.749987756731294" y1="75.55960043841414" x2="89.24998775672812" y2="75.55961457572126"><stop stop-color="currentColor" offset="20%"></stop><stop stop-color="currentColor" offset="100%"></stop></linearGradient><linearGradient id="line-m6sn3ns7v480h4r7hp-23" gradientUnits="userSpaceOnUse" x1="42" y1="48.2797931505535" x2="57.750012243265516" y2="21"><stop stop-color="currentColor" offset="20%"></stop><stop stop-color="currentColor" offset="100%"></stop></linearGradient><linearGradient id="line-m6sn3ns7v480h4r7hp-25" gradientUnits="userSpaceOnUse" x1="42" y1="48.2797931505535" x2="57.749987756731294" y2="75.55960043841414"><stop stop-color="currentColor" offset="20%"></stop><stop stop-color="currentColor" offset="100%"></stop></linearGradient></defs><mask id="text-mask-m6sn3ns7v480h4r7hp"><rect x="0" y="0" width="100%" height="100%" fill="white"></rect></mask><style>
                .element-m6sn3ns7v480h4r7hp {
                    font: 14px Helvetica, Arial, sans-serif;
                    alignment-baseline: 'middle';
                }
                .sub-m6sn3ns7v480h4r7hp {
                    font: 8.4px Helvetica, Arial, sans-serif;
                }
            </style><g mask="url(#text-mask-m6sn3ns7v480h4r7hp)"><line x1="133.1646334638487" y1="53.84281408747262" x2="145.76462366923374" y2="75.66665991776115" style="stroke-linecap:round;stroke-dasharray:none;stroke-width:1.26" stroke="url('#line-m6sn3ns7v480h4r7hp-1')"></line><line x1="136.4999999999905" y1="48.27983556247493" x2="152.24998775672177" y2="75.55964285033559" style="stroke-linecap:round;stroke-dasharray:none;stroke-width:1.26" stroke="url('#line-m6sn3ns7v480h4r7hp-3')"></line><line x1="136.4999755134562" y1="102.83943600088905" x2="152.24998775672177" y2="75.55964285033559" style="stroke-linecap:round;stroke-dasharray:none;stroke-width:1.26" stroke="url('#line-m6sn3ns7v480h4r7hp-5')"></line><line x1="104.99999999999366" y1="48.27982142516778" x2="136.4999999999905" y2="48.27983556247493" style="stroke-linecap:round;stroke-dasharray:none;stroke-width:1.26" stroke="url('#line-m6sn3ns7v480h4r7hp-7')"></line><line x1="104.99997551345939" y1="102.83942186358193" x2="136.4999755134562" y2="102.83943600088905" style="stroke-linecap:round;stroke-dasharray:none;stroke-width:1.26" stroke="url('#line-m6sn3ns7v480h4r7hp-9')"></line><line x1="85.91464570712054" y1="26.5629926623048" x2="98.51463591250561" y2="48.386838492593355" style="stroke-linecap:round;stroke-dasharray:none;stroke-width:1.26" stroke="url('#line-m6sn3ns7v480h4r7hp-11')"></line><line x1="89.25001224326233" y1="21.000014137307105" x2="104.99999999999366" y2="48.27982142516778" style="stroke-linecap:round;stroke-dasharray:none;stroke-width:1.26" stroke="url('#line-m6sn3ns7v480h4r7hp-13')"></line><line x1="89.24998775672812" y1="75.55961457572126" x2="104.99999999999366" y2="48.27982142516778" style="stroke-linecap:round;stroke-dasharray:none;stroke-width:1.26" stroke="url('#line-m6sn3ns7v480h4r7hp-15')"></line><line x1="89.24998775672812" y1="75.55961457572126" x2="104.99997551345939" y2="102.83942186358193" style="stroke-linecap:round;stroke-dasharray:none;stroke-width:1.26" stroke="url('#line-m6sn3ns7v480h4r7hp-17')"></line><line x1="57.750012243265516" y1="21" x2="89.25001224326233" y2="21.000014137307105" style="stroke-linecap:round;stroke-dasharray:none;stroke-width:1.26" stroke="url('#line-m6sn3ns7v480h4r7hp-19')"></line><line x1="57.749987756731294" y1="75.55960043841414" x2="89.24998775672812" y2="75.55961457572126" style="stroke-linecap:round;stroke-dasharray:none;stroke-width:1.26" stroke="url('#line-m6sn3ns7v480h4r7hp-21')"></line><line x1="42" y1="48.2797931505535" x2="57.750012243265516" y2="21" style="stroke-linecap:round;stroke-dasharray:none;stroke-width:1.26" stroke="url('#line-m6sn3ns7v480h4r7hp-23')"></line><line x1="42" y1="48.2797931505535" x2="57.749987756731294" y2="75.55960043841414" style="stroke-linecap:round;stroke-dasharray:none;stroke-width:1.26" stroke="url('#line-m6sn3ns7v480h4r7hp-25')"></line></g><g><text x="152.24998775672177" y="75.55964285033559" class="debug" fill="#ff0000" style="
                font: 5px Droid Sans, sans-serif;
            "></text><text x="136.4999999999905" y="48.27983556247493" class="debug" fill="#ff0000" style="
                font: 5px Droid Sans, sans-serif;
            "></text><text x="104.99999999999366" y="48.27982142516778" class="debug" fill="#ff0000" style="
                font: 5px Droid Sans, sans-serif;
            "></text><text x="89.25001224326233" y="21.000014137307105" class="debug" fill="#ff0000" style="
                font: 5px Droid Sans, sans-serif;
            "></text><text x="57.750012243265516" y="21" class="debug" fill="#ff0000" style="
                font: 5px Droid Sans, sans-serif;
            "></text><text x="42" y="48.2797931505535" class="debug" fill="#ff0000" style="
                font: 5px Droid Sans, sans-serif;
            "></text><text x="57.749987756731294" y="75.55960043841414" class="debug" fill="#ff0000" style="
                font: 5px Droid Sans, sans-serif;
            "></text><text x="89.24998775672812" y="75.55961457572126" class="debug" fill="#ff0000" style="
                font: 5px Droid Sans, sans-serif;
            "></text><text x="104.99997551345939" y="102.83942186358193" class="debug" fill="#ff0000" style="
                font: 5px Droid Sans, sans-serif;
            "></text><text x="136.4999755134562" y="102.83943600088905" class="debug" fill="#ff0000" style="
                font: 5px Droid Sans, sans-serif;
            "></text></g></svg>"/>


$s$-Cis conformation -

A bicyclic diene (rigid $s$-trans diene)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0459.jpg?height=275&width=654&top_left_y=1153&top_left_x=947)
(2Z,4Z)-2,4-Hexadiene (s-trans, more stable)


$s$-Cis conformation -

FIGURE 14.10 Two dienes that can't achieve an $\boldsymbol{s}$-cis conformation and thus can't undergo Diels-Alder reactions.
In contrast to these unreactive dienes that can't achieve an $s$-cis conformation, other dienes are fixed only in the correct $s$-cis geometry and are therefore highly reactive in Diels-Alder cycloaddition. 1,3-Cyclopentadiene, for example, is so reactive that it reacts with itself. At room temperature, 1,3-cyclopentadiene dimerizes. One molecule acts as diene and a second molecule acts as dienophile in a self-Diels-Alder reaction.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0459.jpg?height=367&width=803&top_left_y=1763&top_left_x=661)


$s$-Cis conformation -

Biological Diels-Alder reactions are also known but are uncommon. One example occurs in the biosynthesis of the cholesterol-lowering drug lovastatin (trade name Mevacor) isolated from the bacterium Aspergillus terreus. The key step is the intramolecular Diels-Alder reaction of a triene, in which the diene and dienophile components are within the same molecule.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0460.jpg?height=494&width=1496&top_left_y=230&top_left_x=306)


$s$-Cis conformation -

PROBLEM Predict the product of the following Diels-Alder reaction:
14-10
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0460.jpg?height=483&width=1248&top_left_y=1382&top_left_x=417)


$s$-Cis conformation - 14.6 Diene Polymers: Natural and Synthetic Rubbers

Conjugated dienes can be polymerized just as simple alkenes can (Section 8.10). Diene polymers are structurally more complex than simple alkene polymers, however, because double bonds occur every four carbon atoms along the chain, leading to the possibility of cis-trans isomers. The initiator (In) for the reaction can be either a radical, as occurs in ethylene polymerization, or an acid. Note that the polymerization is a 1,4 addition of the growing chain to a conjugated diene monomer.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0461.jpg?height=375&width=1028&top_left_y=184&top_left_x=543)
trans-Polybutadiene
Rubber is a naturally occurring diene polymer of isoprene (2-methyl-1,3-butadiene) and is produced by more than 400 different plants. The major source is the so-called rubber tree, Hevea brasiliensis, from which the crude material, called latex, is harvested as it drips from a slice made through the bark. The double bonds of rubber have $Z$ stereochemistry, but gutta-percha, the $E$ isomer of rubber, also occurs naturally. Harder and more brittle than rubber, gutta-percha has a variety of applications, including use in dental endodontics and as the covering on some golf balls.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0461.jpg?height=446&width=1044&top_left_y=891&top_left_x=535)


A number of different synthetic rubbers are produced commercially by diene polymerization. Both cis- and trans-polyisoprene can be made, and the synthetic rubber thus produced is similar to the natural material. Chloroprene ( 2 -chloro-1,3-butadiene) is polymerized to yield neoprene, an excellent, although expensive, synthetic rubber with good weather resistance. Neoprene is used in the production of industrial hoses and gloves, among other things.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0461.jpg?height=232&width=1042&top_left_y=1570&top_left_x=539)


Both natural and synthetic rubbers are too soft and tacky to be useful until they are hardened by heating with elemental sulfur, a process called vulcanization. Vulcanization cross-links the rubber chains by forming carbon-sulfur bonds between them, thereby hardening and stiffening the polymer. The exact degree of hardening can be varied, yielding material soft enough for automobile tires or hard enough for bowling balls (ebonite).


The unusual ability of rubber to stretch and then contract to its original shape is due to the irregular structure of the polymer chains caused by the double bonds. These double bonds introduce bends and kinks into the polymer chains, thereby preventing neighboring chains from nestling together. When stretched, the randomly coiled chains straighten out and orient along the direction of the pull but are kept from sliding over one another by the cross-links. When the tension is released, the polymer reverts to its original random state.


PROBLEM Draw a segment of the polymer that might be prepared from 2-phenyl-1,3-butadiene. 14-11


PROBLEM Show the mechanism of the acid-catalyzed polymerization of 1,3-butadiene. 14-12


$s$-Cis conformation - 14.7 Ultraviolet Spectroscopy

As we've seen, mass spectrometry, infrared spectroscopy, and nuclear magnetic resonance spectroscopy are techniques of structure determination applicable to all organic molecules. In addition to these three generally useful methods, there is a fourth-ultraviolet (UV) spectroscopy-that is applicable only to conjugated compounds. UV is less commonly used than the other three spectroscopic techniques because of the specialized information it gives, so we'll only discuss it briefly.


| Mass spectrometry | Molecular size and formula |
| :--- | :--- |
| IR spectroscopy | Functional groups present |
| NMR spectroscopy | Carbon-hydrogen framework |
| UV spectroscopy | Conjugated $\boldsymbol{\pi}$ electron systems |


The ultraviolet region of the electromagnetic spectrum extends from the short-wavelength end of the visible region $\left(4 \times 10^{-7} \mathrm{~m}\right)$ to the long-wavelength end of the X-ray region $\left(10^{-8} \mathrm{~m}\right)$, but the narrow range from $2 \times$ $10^{-7} \mathrm{~m}$ to $4 \times 10^{-7} \mathrm{~m}$ is the part of greatest interest to organic chemists. Absorptions in this region are usually measured in nanometers ( nm ), where $1 \mathrm{~nm}=10^{-9} \mathrm{~m}$. Thus, the ultraviolet range of interest is from 200 to 400 nm (FIGURE 14.11).
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0462.jpg?height=532&width=1383&top_left_y=962&top_left_x=368)


FIGURE 14.11 The ultraviolet (UV) and neighboring regions of the electromagnetic spectrum.
We saw in Section 12.5 that when an organic molecule is irradiated with electromagnetic energy, the radiation either passes through the sample or is absorbed, depending on its energy. With IR irradiation, the energy absorbed corresponds to the amount needed to increase molecular vibrations. With UV radiation, the energy absorbed corresponds to the amount needed to promote an electron from a lower-energy orbital to a higherenergy one in a conjugated molecule. The conjugated diene 1,3-butadiene, for instance, has four $\pi$ molecular orbitals, as shown previously in FIGURE 14.3. The two lower-energy, bonding MOs are occupied in the ground state, and the two higher-energy, antibonding MOs are unoccupied.

On irradiation with ultraviolet light (hv), 1,3-butadiene absorbs energy and a $\pi$ electron is promoted from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). Because the electron is promoted from a bonding $\pi$ molecular orbital to an antibonding $\pi^{*}$ molecular orbital, we call this a $\pi \rightarrow \pi^{*}$ excitation (read as "pi to pi star"). The energy gap between the HOMO and the LUMO of 1,3-butadiene is such that UV light of 217 nm wavelength is required to effect the $\pi \rightarrow \pi^{*}$ electronic transition (FIGURE 14.12).
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0463.jpg?height=575&width=1105&top_left_y=195&top_left_x=510)

FIGURE 14.12 Ultraviolet excitation of 1,3-butadiene results in the promotion of an electron from $\Psi_{2}$, the highest occupied molecular orbital (HOMO), to $\Psi_{3}{ }^{*}$, the lowest unoccupied molecular orbital (LUMO).


An ultraviolet spectrum is recorded by irradiating a sample with UV light of continuously changing wavelength. When the wavelength corresponds to the energy level required to excite an electron to a higher level, energy is absorbed. This absorption is detected and displayed on a chart that plots wavelength versus absorbance (A), defined as


$$
A=\log \frac{I_{0}}{I}
$$


where $I_{0}$ is the intensity of the incident light and $I$ is the intensity of the light transmitted through the sample.
Note that UV spectra differ from IR spectra in how they are presented. For historical reasons, IR spectra are usually displayed so that the baseline corresponding to zero absorption runs across the top of the chart and a valley indicates an absorption, whereas UV spectra are displayed with the baseline at the bottom of the chart so that a peak indicates an absorption (FIGURE 14.13).
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0463.jpg?height=625&width=1313&top_left_y=1406&top_left_x=401)


The amount of UV light absorbed is expressed as the sample's molar absorptivity ( $\boldsymbol{\epsilon}$ ), defined by the equation


$$
\varepsilon=\frac{A}{c \times l}
$$


where
$A=$ Absorbance
$c=$ Concentration in $\mathrm{mol} / \mathrm{L}$
$l=$ Sample pathlength in cm
Molar absorptivity is a physical constant, characteristic of the particular substance being observed and thus characteristic of the particular $\pi$ electron system in the molecule. Typical values for conjugated dienes are in
the range $\varepsilon=10,000$ to 25,000 . The units for molar absorptivity, $\mathrm{L} /(\mathrm{mol} \cdot \mathrm{cm})$, are usually dropped.
A particularly important use of this equation comes from rearranging it to the form $c=A /(\varepsilon \cdot I)$, which lets us measure the concentration of a sample in solution when $A, \varepsilon$, and $l$ are known. As an example, $\beta$-carotene, the pigment responsible for the orange color of carrots, has $\varepsilon=138,000 \mathrm{~L} /(\mathrm{mol} \cdot \mathrm{cm})$. If a sample of $\beta$-carotene is placed in a cell with a pathlength of 1.0 cm and the UV absorbance reads 0.37 , then the concentration of $\beta$-carotene in the sample is


$$
\begin{aligned}
c & =\frac{A}{\varepsilon l}=\frac{0.37}{\left(1.38 \times 10^{5} \frac{\mathrm{~L}}{\mathrm{~mol} \cdot \mathrm{~cm}}\right)(1.00 \mathrm{~cm})} \\
& =2.7 \times 10^{-6} \mathrm{~mol} / \mathrm{L}
\end{aligned}
$$


Unlike IR and NMR spectra, which show many absorptions for a given molecule, UV spectra are usually quite simple-often only a single peak. The peak is usually broad, and we identify its position by noting the wavelength at the top of the peak- $\lambda_{\max }$, read as "lambda max."


PROBLEM Calculate the energy range of electromagnetic radiation in the UV region of the spectrum from 200 14-13 to 400 nm (see Section 12.5). How does this value compare with the values calculated previously for IR and NMR spectroscopy?


PROBLEM If pure vitamin A has $\lambda_{\max }=325(\varepsilon=50,100)$, what is the vitamin A concentration in a sample whose 14-14 absorbance at 325 nm is $A=0.735$ in a cell with a pathlength of 1.00 cm ?


$s$-Cis conformation - 14.8 Interpreting Ultraviolet Spectra: The Effect of Conjugation

The wavelength necessary to effect the $\pi \rightarrow \pi^{*}$ transition in a conjugated molecule depends on the energy gap between HOMO and LUMO, which in turn depends on the nature of the conjugated system. Thus, by measuring the UV spectrum of an unknown, we can derive structural information about the nature of any conjugated $\pi$ electron system present in a molecule.


One of the most important factors affecting the wavelength of UV absorption by a molecule is the extent of conjugation. Molecular orbital calculations show that the energy difference between HOMO and LUMO decreases as the extent of conjugation increases. Thus, 1,3-butadiene absorbs at $\lambda_{\max }=217 \mathrm{~nm}$, 1,3,5-hexatriene absorbs at $\lambda_{\max }=258 \mathrm{~nm}$, and 1,3,5,7-octatetraene absorbs at $\lambda_{\max }=290 \mathrm{~nm}$. (Remember: longer wavelength means lower energy.)


Other kinds of conjugated systems, such as conjugated enones and aromatic rings, also have characteristic UV absorptions that are useful in structure determination. The UV absorption maxima of some representative conjugated molecules are given in TABLE 14.2.


| Name | Structure | $\lambda_{\text {max }}(\mathrm{nm})$ |
| :---: | :---: | :---: |
| 2-Methyl-1,3-butadiene | <smiles>C=CC(=C)C</smiles> | 220 |
| 1,3-Cyclohexadiene | <smiles>C1=CCCC=C1</smiles> | 256 |
| 1,3,5-Hexatriene | $\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}_{2}$ | 258 |
| 1,3,5,7-Octatetraene | $\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}_{2}$ | 290 |


TABLE 14.2 Ultraviolet Absorptions of Some Conjugated Molecules


| Name | Structure | $\lambda_{\text {max }}(\mathrm{nm})$ |
| :---: | :---: | :---: |
| 3-Buten-2-one | <smiles>C=CC(C)=O</smiles> | 219 |
| Benzene | $8$ | 203 |


Indole -


Indole - 14.9 Conjugation, Color, and the Chemistry of Vision

Why are some organic compounds colored while others are not? $\beta$-Carotene, the pigment in carrots, is yelloworange, for instance, while cholesterol is colorless. The answer involves both the chemical structures of colored molecules and the way we perceive light.


The visible region of the electromagnetic spectrum is adjacent to the ultraviolet region, extending from approximately 400 to 800 nm . Colored compounds have such elaborate systems of conjugation that their "UV" absorptions extend into the visible region. $\beta$-Carotene, for instance, has 11 double bonds in conjugation, and its absorption occurs at $\lambda_{\max }=455 \mathrm{~nm}$ (FIGURE 14.14).
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0466.jpg?height=746&width=1210&top_left_y=191&top_left_x=452)

FIGURE 14.14 Ultraviolet spectrum of $\boldsymbol{\beta}$-carotene, a conjugated molecule with 11 double bonds. The absorption occurs in the visible region.
"White" light from the sun or from a lamp consists of all wavelengths in the visible region. When white light strikes $\beta$-carotene, the wavelengths from 400 to 500 nm (blue) are absorbed while all other wavelengths are transmitted and can reach our eyes. We therefore see the white light with the blue removed, which we perceive as a yellow-orange color for $\beta$-carotene.


Conjugation is crucial not only for the colors we see in organic molecules but also for the light-sensitive molecules on which our visual system is based. The key substance for vision is dietary $\beta$-carotene, which is converted to vitamin A by enzymes in the liver, oxidized to an aldehyde called 11-trans-retinal, and then isomerized by a change in geometry of the C11-C12 double bond to produce 11-cis-retinal.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0466.jpg?height=681&width=1288&top_left_y=1405&top_left_x=413)


There are two main types of light-sensitive receptor cells in the retina of the human eye: rod cells and cone cells. The 3 million or so rod cells are primarily responsible for seeing dim light and shades of gray, whereas the 100 million cone cells are responsible for seeing bright light and colors. In the rod cells of the eye, 11-cis-retinal is converted into rhodopsin, a light-sensitive substance formed from the protein opsin and 11-cis-retinal. When light strikes the rod cells, isomerization of the C11-C12 double bond occurs and trans-rhodopsin, called metarhodopsin II, is produced. In the absence of light, this cis-trans isomerization takes approximately 1100 years, but in the presence of light, it occurs within $2 \times 10^{-13}$ seconds! Isomerization of rhodopsin is accompanied by a change in molecular geometry, which in turn causes a nerve impulse to be sent through the optic nerve to the brain, where it is perceived as vision.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0467.jpg?height=405&width=1501&top_left_y=182&top_left_x=309)


Rhodopsin
Metarhodopsin II
Metarhodopsin II is then recycled back into rhodopsin by a multistep sequence involving cleavage to all-trans-retinal and cis-trans isomerization back to 11-cis-retinal.


CHEMISTRY MATTERS -


Photolithography -

Fifty years ago, someone interested in owning a computer would have paid approximately $\$ 150,000$ for 16 megabytes of random-access memory that would have occupied a volume the size of a small desk. Today, anyone can buy 60 times as much computer memory for $\$ 10$ and fit the small chip into their shirt pocket. The difference between then and now is due to improvements in photolithography, the process by which integratedcircuit chips are made.


Photolithography -

Photolithography begins by coating a layer of $\mathrm{SiO}_{2}$ onto a silicon wafer and further coating with a thin (0.5-1.0 $\mu \mathrm{m})$ film of a light-sensitive organic polymer called a resist. A mask is then used to cover those parts of the chip that will become a circuit, and the wafer is irradiated with UV light. The nonmasked sections of the polymer undergo a chemical change when irradiated, which makes them more soluble than the masked, unirradiated sections. On washing the irradiated chip with solvent, solubilized polymer is selectively removed from the irradiated areas, exposing the $\mathrm{SiO}_{2}$ underneath. This $\mathrm{SiO}_{2}$ is then chemically etched away by reaction with hydrofluoric acid, leaving behind a pattern of polymer-coated $\mathrm{SiO}_{2}$. Further washing removes the remaining polymer, leaving a positive image of the mask in the form of exposed ridges of $\mathrm{SiO}_{2}$ (FIGURE 14.16). Additional cycles of coating, masking, and etching then produce the completed chips.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0468.jpg?height=676&width=896&top_left_y=1088&top_left_x=612)


Photolithography -

FIGURE 14.15 Manufacturing the ultrathin circuitry on this computer chip depends on the organic chemical reactions of special polymers. (credit: "Integrated circuit on microchip" by Jon Sullivan/Wikimedia Commons, Public Domain)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0468.jpg?height=521&width=1267&top_left_y=1840&top_left_x=429)