FIGURE 8.4 MECHANISM -

A hydrogen atom on the electrophile $\mathrm{H}_{3} \mathrm{O}^{+}$ is attacked by $\pi$ electrons from the nucleophilic double bond, forming a new $\mathrm{C}-\mathrm{H}$ bond. This leaves the other carbon atom with $\mathrm{a}+$ charge and a vacant $p$ orbital. Simultaneously, two electrons from the $\mathrm{H}-\mathrm{O}$ bond move onto oxygen, giving neutral water.
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"/>

2-Methylpropene
(1)
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FIGURE 8.4 MECHANISM -

2 The nucleophile $\mathrm{H}_{2} \mathrm{O}$ donates an electron pair to the positively charged carbon atom, forming a $\mathrm{C}-\mathrm{O}$ bond and leaving a positive charge on oxygen in the protonated alcohol addition product.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0242.jpg?height=239&width=247&top_left_y=1317&top_left_x=1275)


FIGURE 8.4 MECHANISM -


FIGURE 8.4 MECHANISM -

Most ethanol throughout the world is now made by fermentation of biological precursors, such as corn and sugar, but acid-catalyzed alkene hydration is particularly suited to large-scale industrial procedures, and approximately 90,000 tons of ethanol is manufactured each year in the United States by hydration of ethylene. The reaction is of little value in the laboratory, however, because it requires high temperatures- $250{ }^{\circ} \mathrm{C}$ in the case of ethylene-and strongly acidic conditions.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0242.jpg?height=150&width=720&top_left_y=2267&top_left_x=697)


Ethylene -

Acid-catalyzed hydration of double bonds is also uncommon in biological pathways. Instead, biological
hydrations usually require that the double bond be adjacent to a carbonyl group for reaction to proceed. Fumarate, for instance, is hydrated to give malate as one step in the citric acid cycle of food metabolism. Note that the requirement for an adjacent carbonyl group in the addition of water is the same as in Section 8.1 for the elimination of water. We'll see the reason for this requirement in Section 19.13, but will note for now that the reaction is not an electrophilic addition but instead occurs through a mechanism that involves formation of an anion intermediate followed by protonation by an acid HA.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0243.jpg?height=261&width=1327&top_left_y=477&top_left_x=399)


Ethylene -

When it comes to circumventing problems like those with acid-catalyzed alkene hydrations, laboratory chemists have a great advantage over the cellular "chemists" in living organisms. Laboratory chemists are not constrained to carry out their reactions in water solution; they can choose from any of a large number of solvents. Laboratory reactions don't need to be carried out at a fixed temperature; they can take place over a wide range of temperatures. And laboratory reagents aren't limited to containing carbon, oxygen, nitrogen, and a few other elements; they can contain any element in the periodic table.


Ethylene -

In the laboratory, alkenes are often hydrated by the oxymercuration-demercuration procedure, which involves electrophilic addition of $\mathrm{Hg}^{2+}$ to the alkene on reaction with mercury(II) acetate $\left\left(\mathrm{CH}_{3} \mathrm{CO}_{2}\right)_{2} \mathrm{Hg}\right.$. The intermediate organomercury compound is then treated with sodium borohydride, $\mathrm{NaBH}_{4}$, and demercuration occurs to produce an alcohol. For example:
![


1-Methylcyclopentene -

1-Methylcyclopentanol
(92\%)


1-Methylcyclopentene -

Alkene oxymercuration is closely analogous to halohydrin formation. The reaction is initiated by electrophilic addition of $\mathrm{Hg}^{2+}$ (mercuric) ion to the alkene to give an intermediate mercurinium ion, whose structure resembles that of a bromonium ion (FIGURE 8.5). Nucleophilic addition of water as in halohydrin formation, followed by the loss of a proton, then yields a stable organomercury product. The final step, demercuration of the organomercury compound by reaction with sodium borohydride, is complex and involves radicals. Note that the regiochemistry of the reaction corresponds to Markovnikov addition of water; that is, the - OH group attaches to the more highly substituted carbon atom, and the -H attaches to the less highly substituted carbon. The hydrogen that replaces mercury in the demercuration step can attach from either side of the molecule depending on the exact circumstances.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0243.jpg?height=355&width=1420&top_left_y=1839&top_left_x=355)

FIGURE 8.5 Mechanism of the oxymercuration of an alkene to yield an alcohol. (1) Electrophilic addition of $\mathrm{Hg}^{2+}$ gives a mercurinium ion, which (2) reacts with water as in halohydrin formation. Loss of a proton gives an organomercury product, and (3) reaction with $\mathrm{NaBH}_{4}$ removes the mercury. The product of the reaction is a more highly substituted alcohol, corresponding to Markovnikov regiochemistry.


1-Methylcyclopentene -


1-Methylcyclopentene - 8.5 Hydration of Alkenes: Addition of $\mathrm{H}_{2} \mathrm{O}$ by Hydroboration

In addition to the oxymercuration-demercuration method, which yields the Markovnikov product, a complementary method that yields the non-Markovnikov product is also useful. Discovered in 1959 by H.C. Brown at Purdue University and called hydroboration, the reaction involves addition of a B-H bond of borane, $\mathrm{BH}_{3}$, to an alkene to yield an organoborane intermediate, $\mathrm{RBH}_{2}$. Oxidation of the organoborane by reaction with basic hydrogen peroxide, $\mathrm{H}_{2} \mathrm{O}_{2}$, then gives an alcohol. For example:
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0244.jpg?height=267&width=1313&top_left_y=780&top_left_x=401)


Borane is very reactive as a Lewis acid because the boron atom has only six electrons in its valence shell rather than an octet. In tetrahydrofuran solution, $\mathrm{BH}_{3}$ accepts an electron pair from a solvent molecule in a Lewis acid-base reaction to complete its octet and form a stable $\mathrm{BH}_{3}-$ THF complex.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0244.jpg?height=357&width=1126&top_left_y=1193&top_left_x=494)


When an alkene reacts with $\mathrm{BH}_{3}$ in THF solution, rapid addition to the double bond occurs three times and a trialkylborane, $\mathrm{R}_{3} \mathrm{~B}$, is formed. For example, 1 molar equivalent of $\mathrm{BH}_{3}$ adds to 3 molar equivalents of cyclohexene to yield tricyclohexylborane. When tricyclohexylborane is then treated with aqueous hydrogen $\mathrm{H}_{2} \mathrm{O}_{2}$ in basic solution, an oxidation takes place. The three $\mathrm{C}-\mathrm{B}$ bonds are broken, -OH groups bond to the three carbons, and 3 equivalents of cyclohexanol are produced. The net effect of the two-step hydroboration-oxidation sequence is hydration of the alkene double bond.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0244.jpg?height=367&width=1349&top_left_y=1833&top_left_x=388)


One of the features that makes the hydroboration reaction so useful is the regiochemistry that results when an unsymmetrical alkene is hydroborated. For example, hydroboration-oxidation of 1-methylcyclopentene yields trans-2-methylcyclopentanol. In this process, boron and hydrogen add to the alkene from the same face of the double bond-that is, with syn stereochemistry, the opposite of anti-with boron attaching to the less highly substituted carbon. During the oxidation step, the boron is replaced by an -OH with the same stereochemistry, resulting in an overall syn non-Markovnikov addition of water. This stereochemical result is particularly useful because it is complementary to the Markovnikov regiochemistry observed for oxymercuration-demercuration.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0245.jpg?height=362&width=1096&top_left_y=182&top_left_x=512)

Why does alkene hydroboration take place with syn, non-Markovnikov regiochemistry to yield the less highly substituted alcohol? Hydroboration differs from many other alkene addition reactions in that it occurs in a single step without a carbocation intermediate (FIGURE 8.6). Because the $\mathrm{C}-\mathrm{H}$ and $\mathrm{C}-\mathrm{B}$ bonds form at the same time and from the same face of the alkene, syn stereochemistry results. Non-Markovnikov regiochemistry occurs because attachment of boron is favored at the less sterically crowded carbon atom of the alkene.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0245.jpg?height=689&width=1332&top_left_y=772&top_left_x=388)

FIGURE 8.6 Mechanism of alkene hydroboration. The reaction occurs in a single step in which the $\mathrm{C}-\mathrm{H}$ and $\mathrm{C}-\mathrm{B}$ bonds form at the same time and on the same face of the double bond. The lower energy, more rapidly formed transition state is the one with less steric crowding, leading to non-Markovnikov regiochemistry.


WORKED EXAMPLE 8.1 -


Predicting the Products of a Hydration Reaction -

What products would you obtain from reaction of 2-methyl-2-pentene with:
(a) $\mathrm{BH}_{3}$, followed by $\mathrm{H}_{2} \mathrm{O}_{2}, \mathrm{OH}^{-}$
(b) $\mathrm{Hg}(\mathrm{OAc})_{2}$, followed by $\mathrm{NaBH}_{4}$


Strategy -

When predicting the product of a reaction, you have to recall what you know about the kind of reaction being carried out and apply that knowledge to the specific case you're dealing with. In the present instance, recall that the two methods of hydration-hydroboration-oxidation and oxymercuration-demercuration-give complementary products. Hydroboration-oxidation occurs with syn stereochemistry and gives the nonMarkovnikov addition product; oxymercuration-demercuration gives the Markovnikov product.


Solution -


Solution -

PROBLEM The following cycloalkene gives a mixture of two alcohols on hydroboration followed by oxidation.
8-11 Draw the structures of both, and explain the result.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0247.jpg?height=286&width=497&top_left_y=540&top_left_x=429)


Solution - 8.6 Reduction of Alkenes: Hydrogenation

Alkenes react with $\mathrm{H}_{2}$ in the presence of a metal catalyst such as palladium or platinum to yield the corresponding saturated alkanes. We describe the result by saying that the double bond has been hydrogenated, or reduced. Note that the word reduction is used somewhat differently in organic chemistry from what you might have learned previously. In general chemistry, a reduction is defined as the gain of one or more electrons by an atom. In organic chemistry, however, a reduction is a reaction that results in a gain of electron density for carbon, caused either by bond formation between carbon and a less electronegative atom-usually hydrogen-or by bond-breaking between carbon and a more electronegative atom-usually oxygen, nitrogen, or a halogen. We'll explore this topic in more detail in Section 10.8.


Reduction Increases electron density on carbon by:
-forming this: $\mathrm{C}-\mathrm{H}$


- or breaking one of these: $\mathrm{C}-\mathrm{O} \quad \mathrm{C}-\mathrm{N} \quad \mathrm{C}-\mathrm{X}$


A reduction: -


A reduction: -

An alkene
An alkane
Platinum and palladium are the most common laboratory catalysts for alkene hydrogenations. Palladium is normally used as a very fine powder "supported" on an inert material such as charcoal ( $\mathrm{Pd} / \mathrm{C}$ ) to maximize surface area. Platinum is normally used as $\mathrm{PtO}_{2}$, a reagent known as Adams' catalyst after its discoverer, Roger Adams at the University of Illinois.


A reduction: -

Catalytic hydrogenation, unlike most other organic reactions, is a heterogeneous process rather than a homogeneous one. That is, the hydrogenation reaction does not occur in a homogeneous solution but instead takes place on the surface of solid catalyst particles. Hydrogenation usually occurs with syn stereochemistry: both hydrogens add to the double bond from the same face.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0247.jpg?height=301&width=644&top_left_y=2148&top_left_x=735)

As shown in FIGURE 8.7, hydrogenation begins with adsorption of $\mathrm{H}_{2}$ onto the catalyst surface. Complexation between catalyst and alkene then occurs as a vacant orbital on the metal interacts with the filled alkene $\pi$ orbital
on the alkene. In the final steps, hydrogen is inserted into the double bond and the saturated product diffuses away from the catalyst. The stereochemistry of hydrogenation is syn because both hydrogens add to the double bond from the same catalyst surface.


A reduction: -

FIGURE 8.7 MECHANISM


A reduction: -

Mechanism of alkene hydrogenation. The reaction takes place with syn stereochemistry on the surface of insoluble catalyst particles.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0248.jpg?height=231&width=537&top_left_y=584&top_left_x=477) catalyst surface and dissociates into hydrogen atoms.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0248.jpg?height=276&width=508&top_left_y=578&top_left_x=1101)
(2) The alkene adsorbs to the catalyst surface, using its $\pi$ bond to complex to the metal atoms.
(3) A hydrogen atom is transferred from the metal to one of the alkene carbon atoms, forming a partially reduced intermediate with a $\mathrm{C}-\mathrm{H}$ bond and carbon-metal $\sigma$ bond.
(4) A second hydrogen is transferred from the metal to the second carbon, giving the alkane product and regenerating the catalyst. Because both hydrogens are transferred to the same face of the alkene, the reduction has syn stereochemistry.


A reduction: -

> Alkane plus regenerated catalyst


A reduction: -

An interesting feature of catalytic hydrogenation is that the reaction is extremely sensitive to the steric environment around the double bond. As a result, the catalyst usually approaches the more accessible face of an alkene, giving rise to a single product. In $\alpha$-pinene, for example, one of the methyl groups attached to the fourmembered ring hangs over the top face of the double bond and blocks approach of the hydrogenation catalyst from that side. Reduction therefore occurs exclusively from the bottom face to yield the product shown.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0249.jpg?height=437&width=1183&top_left_y=191&top_left_x=468)


A reduction: -

Alkenes are much more reactive toward catalytic hydrogenation than most other unsaturated functional groups, and the reaction is therefore quite selective. Other functional groups, such as aldehydes, ketones, esters, and nitriles, often survive alkene hydrogenation conditions unchanged, although reaction with these groups does occur under more vigorous conditions. Note that, particularly in the hydrogenation of methyl 3-phenylpropenoate shown below, the aromatic ring is not reduced by hydrogen and palladium even though it contains apparent double bonds.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0249.jpg?height=156&width=697&top_left_y=925&top_left_x=706)


A reduction: -

Cyclohexylacetonitrile
(nitrile not reduced)


A reduction: -

In addition to its usefulness in the laboratory, catalytic hydrogenation is also important in the food industry, where unsaturated vegetable oils are reduced on a large scale to produce the saturated fats used in margarine and cooking products (FIGURE 8.8). As we'll see in Section 27.1, vegetable oils are triesters of glycerol, $\mathrm{HOCH}_{2} \mathrm{CH}(\mathrm{OH}) \mathrm{CH}_{2} \mathrm{OH}$, with three long-chain carboxylic acids called fatty acids. The fatty acids are generally polyunsaturated, and their double bonds have cis stereochemistry. Complete hydrogenation yields the corresponding saturated fatty acids, but incomplete hydrogenation often results in partial cis-trans isomerization of a remaining double bond. When eaten and digested, the free trans fatty acids are released, raising blood cholesterol levels and potentially contributing to coronary problems.


A reduction: -

A polyunsaturated
fatty acid in
vegetable oil


A saturated fatty -


A saturated fatty -

A trans fatty acid


A saturated fatty -

FIGURE 8.8 Catalytic hydrogenation of polyunsaturated fats leads primarily to saturated products, along with a small amount of isomerized trans fats.


A saturated fatty -

Double-bond reductions are very common in biological pathways, although the mechanism is completely different from that of laboratory catalytic hydrogenation over palladium. As with biological hydrations (Section 8.4), biological reductions usually occur in two steps and require that the double bond be adjacent to a carbonyl group. In the first step, the biological reducing agent NADPH (reduced nicotinamide adenine dinucleotide phosphate), adds a hydride ion ( $\mathrm{H}:^{-}$) to the double bond to give an anion. In the second, the anion is protonated by acid HA, leading to overall addition of $\mathrm{H}_{2}$. An example is the reduction of trans-crotonyl ACP to yield butyryl ACP, a step involved in the biosynthesis of fatty acids (FIGURE 8.9).
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0250.jpg?height=258&width=1177&top_left_y=1362&top_left_x=474)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0250.jpg?height=397&width=1128&top_left_y=1661&top_left_x=496)


A saturated fatty -

NADPH
FIGURE 8.9 Reduction of the carbon-carbon double bond in trans-crotonyl ACP, a step in the biosynthesis of fatty acids. One hydrogen is delivered from NADPH as a hydride ion, $\mathrm{H}^{-}$; the other hydrogen is delivered by protonation of the anion intermediate with an acid, HA.


A saturated fatty - 8.7 Oxidation of Alkenes: Epoxidation and Hydroxylation

Like the word reduction used in the previous section for the addition of hydrogen to a double bond, the word oxidation has a slightly different meaning in organic chemistry than what you might have previously learned. In general chemistry, an oxidation is defined as the loss of one or more electrons by an atom. In organic chemistry, however, an oxidation is a reaction that results in a loss of electron density for carbon, caused either by bond formation between carbon and a more electronegative atom-usually oxygen, nitrogen, or a halogen-or by bond-breaking between carbon and a less electronegative atom-usually hydrogen. Note that an oxidation often adds oxygen, while a reduction often adds hydrogen.


Oxidation $\quad \begin{array}{lll}\text { Decreases electron density on carbon by: } \\ & - \text { forming one of these: } \mathrm{C}-\mathrm{O} \quad \mathrm{C}-\mathrm{N} & \mathrm{C}-\mathrm{X} \\ & - \text { or breaking this: } \mathrm{C}-\mathrm{H} & \end{array}$
In the laboratory, alkenes are oxidized to give epoxides on treatment with a peroxyacid, $\mathrm{RCO}_{3} \mathrm{H}$, such as meta-chloroperoxybenzoic acid. An epoxide, also called an oxirane, is a cyclic ether with an oxygen atom in a three-membered ring. For example:
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0251.jpg?height=307&width=1356&top_left_y=909&top_left_x=390)


Peroxyacids transfer an oxygen atom to the alkene with syn stereochemistry-both C-O bonds form on the same face of the double bond-through a one-step mechanism without intermediates. The oxygen atom farthest from the carbonyl group is the one transferred.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0251.jpg?height=261&width=820&top_left_y=1360&top_left_x=650)


Another method for the synthesis of epoxides involves the use of halohydrins, prepared by electrophilic addition of HO-X to alkenes (Section 8.3). When a halohydrin is treated with base, HX is eliminated and an epoxide is produced.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0251.jpg?height=342&width=1192&top_left_y=1756&top_left_x=461)


Epoxides undergo an acid-catalyzed ring-opening reaction with water (a hydrolysis) to give the corresponding 1,2-dialcohol, or diol, also called a glycol. Thus, the net result of the two-step alkene epoxidation/hydrolysis is hydroxylation-the addition of an - OH group to each of the two double-bond carbons. In fact, approximately 204 million tons of ethylene glycol, $\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}$, most of it used for automobile antifreeze, are produced worldwide each year by the epoxidation of ethylene and subsequent hydrolysis.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0251.jpg?height=215&width=873&top_left_y=2321&top_left_x=626)


Acid-catalyzed epoxide opening begins with protonation of the epoxide to increase its reactivity, followed by nucleophilic addition of water. This nucleophilic addition is analogous to the final step of alkene bromination, in which a cyclic bromonium ion is opened by a nucleophile (Section 8.2). That is, a trans-1,2-diol results when an epoxycycloalkane is opened by aqueous acid, just as a trans-1,2-dibromide results when a cycloalkene is brominated. We'll look at epoxide chemistry in more detail in Section 18.6.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0252.jpg?height=787&width=1454&top_left_y=425&top_left_x=338)


Hydroxylation can also be carried out directly (without going through an intermediate epoxide) by treating an alkene with osmium tetroxide, $\mathrm{OsO}_{4}$. The reaction occurs with syn stereochemistry and does not involve a carbocation intermediate. Instead, it takes place through an intermediate cyclic osmate, which is formed in a single step by addition of $\mathrm{OsO}_{4}$ to the alkene. This cyclic osmate is then cleaved using aqueous sodium bisulfite, $\mathrm{NaHSO}_{3}$.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0252.jpg?height=226&width=1042&top_left_y=1443&top_left_x=536)


A cyclic osmate intermediate
cis-1,2-Dimethyl-1,2-cyclopentanediol (87\%)


Because $\mathrm{OSO}_{4}$ is both very expensive and very toxic, the reaction is usually carried out using only a small, catalytic amount of $\mathrm{OsO}_{4}$ in the presence of a stoichiometric amount of a safe and inexpensive co-oxidant such as $N$-methylmorpholine $N$-oxide, abbreviated NMO. The initially formed osmate intermediate reacts rapidly with NMO to yield the product diol plus $N$-methylmorpholine and reoxidized $\mathrm{OsO}_{4}$, which reacts with more alkene in a catalytic cycle.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0253.jpg?height=703&width=1471&top_left_y=169&top_left_x=327)


PROBLEM What product would you expect from reaction of cis-2-butene with meta-chloroperoxybenzoic 8-13 acid? Show the stereochemistry.


A saturated fatty - 8.8 Oxidation of Alkenes: Cleavage to Carbonyl Compounds

In all the alkene addition reactions we've seen thus far, the carbon-carbon double bond has been converted into a single bond but the carbon skeleton has been unchanged. There are, however, powerful oxidizing reagents that will cleave $\mathrm{C}=\mathrm{C}$ bonds and produce two carbonyl-containing fragments.


Ozone $\left(\mathrm{O}_{3}\right)$ is perhaps the most useful double-bond cleavage reagent. Prepared by passing a stream of oxygen through a high-voltage electrical discharge, ozone adds rapidly to a $\mathrm{C}=\mathrm{C}$ bond at low temperature to give a cyclic intermediate called a molozonide. Once formed, the molozonide spontaneously rearranges to form an ozonide. Although we won't study the mechanism of this rearrangement in detail, it involves the molozonide coming apart into two fragments that then recombine in a different way.


$$
3 \mathrm{O}_{2} \xrightarrow[\text { discharge }]{\text { Electric }} 2 \mathrm{O}_{3}
$$


Low-molecular-weight ozonides are explosive and therefore not isolated. Instead, the ozonide is immediately treated with a reducing agent, such as zinc metal in acetic acid, to produce carbonyl compounds. The net result of the ozonolysis/reduction sequence is that the $\mathrm{C}=\mathrm{C}$ bond is cleaved and an oxygen atom becomes doubly bonded to each of the original alkene carbons. If an alkene with a tetrasubstituted double bond is ozonized, two ketone fragments result; if an alkene with a trisubstituted double bond is ozonized, one ketone and one aldehyde result; and so on.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0254.jpg?height=150&width=890&top_left_y=191&top_left_x=645)


Isopropylidenecyclohexane
(tetrasubstituted)


Cyclohexanone Acetone -

84\%; two ketones
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0254.jpg?height=234&width=1232&top_left_y=525&top_left_x=441)


Cyclohexanone Acetone -

Several oxidizing reagents other than ozone also cause double-bond cleavage, although such reactions are not often used. For example, potassium permanganate $\left(\mathrm{KMnO}_{4}\right)$ in neutral or acidic solution cleaves alkenes to give carbonyl-containing products. If hydrogens are present on the double bond, carboxylic acids are produced; if two hydrogens are present on one carbon, $\mathrm{CO}_{2}$ is formed.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0254.jpg?height=134&width=1117&top_left_y=947&top_left_x=504)

3,7-Dimethyl-1-octene 2,6-Dimethylheptanoic acid (45\%)
In addition to direct cleavage with ozone or $\mathrm{KMnO}_{4}$, an alkene can also be cleaved in a two-step process by initial hydroxylation to a 1,2-diol, as discussed in the previous section, followed by treatment of the diol with periodic acid, $\mathrm{HIO}_{4}$. If the two -OH groups are in an open chain, two carbonyl compounds result. If the two -OH groups are on a ring, a single, open-chain dicarbonyl compound is formed. As indicated in the following examples, the cleavage reaction takes place through a cyclic periodate intermediate.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0254.jpg?height=719&width=1145&top_left_y=1337&top_left_x=490)


WORKED EXAMPLE 8.3 -


Predicting the Reactant in an Ozonolysis Reaction -

What alkene would yield a mixture of cyclopentanone and propanal on treatment with ozone followed by reduction with zinc?
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0254.jpg?height=123&width=725&top_left_y=2340&top_left_x=700)


Strategy -

Reaction of an alkene with ozone, followed by reduction with zinc, cleaves the $\mathrm{C}=\mathrm{C}$ bond and becomes two $\mathrm{C}=\mathrm{O}$ bonds. Working backward from the carbonyl-containing products, the alkene precursor can be found by removing the oxygen from each product and joining the two carbon atoms.


Strategy -

Solution
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0255.jpg?height=123&width=868&top_left_y=459&top_left_x=626)


Strategy -

PROBLEM What products would you expect from reaction of 1-methylcyclohexene with the following 8-15 reagents?
(a) Aqueous acidic $\mathrm{KMnO}_{4}$
(b) $\mathrm{O}_{3}$, followed by $\mathrm{Zn}, \mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}$


Strategy -

PROBLEM Propose structures for alkenes that yield the following products on reaction with ozone followed by 8-16 treatment with Zn :
(a) $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{O}+\mathrm{H}_{2} \mathrm{C}=\mathrm{O}$
(b) 2 equiv $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{O}$


Strategy - 8.9 Addition of Carbenes to Alkenes: Cyclopropane Synthesis

Yet another kind of alkene addition is the reaction with a carbene to yield a cyclopropane. A carbene, $\mathbf{R}_{\mathbf{2}} \mathbf{C}$ :, is a neutral molecule containing a divalent carbon with only six electrons in its valence shell. It is therefore highly reactive and generated only as a reaction intermediate, rather than as an isolable molecule. Because they're electron-deficient, carbenes behave as electrophiles and react with nucleophilic $\mathrm{C}=\mathrm{C}$ bonds. The reaction occurs in a single step without intermediates.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0255.jpg?height=180&width=643&top_left_y=1233&top_left_x=730)


An alkene A carbene A cyclopropane -

One of the simplest methods for generating a substituted carbene is by treatment of chloroform, $\mathrm{CHCl}_{3}$, with a strong base such as KOH . As shown in FIGURE 8.10, the loss of a proton from $\mathrm{CHCl}_{3}$ gives trichloromethanide anion, ${ }^{-}: \mathrm{CCl}_{2}$, which spontaneously expels a $\mathrm{Cl}^{-}$ion to yield dichlorocarbene, $: \mathrm{CCl}_{2}$.


FIGURE 8.10 MECHANISM -


FIGURE 8.10 MECHANISM -

1. Base abstracts the hydrogen from chloroform, leaving behind the electron pair from the $\mathrm{C}-\mathrm{H}$ bond and forming the trichloromethanide anion.


FIGURE 8.10 MECHANISM -

Chloroform
(1)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0256.jpg?height=193&width=313&top_left_y=806&top_left_x=1291)
richloromethanide
anion
Spontaneous loss of chloride ion then yields the neutral dichlorocarbene.


FIGURE 8.10 MECHANISM -

๑]
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0256.jpg?height=131&width=221&top_left_y=1214&top_left_x=1348)


FIGURE 8.10 MECHANISM -

Dichlorocarbene


FIGURE 8.10 MECHANISM -

The carbon atom in dichlorocarbene is $s p^{2}$-hybridized, with a vacant $p$ orbital extending above and below the plane of the three atoms and with an unshared pair of electrons occupying the third $s p^{2}$ lobe. Note that this electronic description of dichlorocarbene is similar to that of a carbocation (Section 7.9) with respect to both the $s p^{2}$ hybridization of carbon and the vacant $p$ orbital. Electrostatic potential maps further illustrate the similarity (FIGURE 8.11).
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0256.jpg?height=423&width=1427&top_left_y=1713&top_left_x=349)

FIGURE 8.11 The structure of dichlorocarbene. Electrostatic potential maps show how the positive region coincides with the empty $p$ orbital in both dichlorocarbene and a carbocation $\left(\mathrm{CH}_{3}{ }^{+}\right)$. The negative region in the dichlorocarbene map coincides with the lone-pair electrons.


FIGURE 8.10 MECHANISM -

If dichlorocarbene is generated in the presence of an alkene, addition to the double bond occurs and a dichlorocyclopropane is formed. As the reaction of dichlorocarbene with cis-2-pentene demonstrates, the addition is stereospecific, meaning that only a single stereoisomer is formed as product. Starting from a cis alkene, for instance, only cis-disubstituted cyclopropane is produced; starting from a trans alkene, only transdisubstituted cyclopropane is produced.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0257.jpg?height=210&width=1018&top_left_y=188&top_left_x=559)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0257.jpg?height=229&width=822&top_left_y=468&top_left_x=641)


FIGURE 8.10 MECHANISM -

The best method for preparing nonhalogenated cyclopropanes is by a process called the Simmons-Smith reaction. First investigated at the DuPont company, this reaction does not involve a free carbene. Rather, it utilizes a carbenoid-a metal-complexed reagent with carbene-like reactivity. When diiodomethane is treated with a specially prepared zinc-copper mix, (iodomethyl)zinc iodide, $\mathrm{ICH}_{2} \mathrm{ZnI}$, is formed. In the presence of an alkene, $\mathrm{ICH}_{2} \mathrm{ZnI}$ transfers a $\mathrm{CH}_{2}$ group to the double bond to yield cyclopropane. For example, cyclohexene reacts cleanly and with good yield to give the corresponding cyclopropane. Although we won't discuss the mechanistic details, carbene addition to an alkene is one of a general class of reactions called cycloadditions, which we'll study more carefully in Chapter 30.


FIGURE 8.10 MECHANISM -

$$
\mathrm{CH}_{2} \mathrm{I}_{2}+\mathrm{Zn}(\mathrm{Cu}) \longrightarrow \mathrm{ICH} 2-\mathrm{ZnI} \quad\left[": \mathrm{CH}_{2}^{\prime \prime \prime}\right]
$$


FIGURE 8.10 MECHANISM -

Diiodomethane
(Iodomethyl)zinc iodide
(a carbenoid)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0257.jpg?height=218&width=852&top_left_y=1254&top_left_x=639)


FIGURE 8.10 MECHANISM -

Cyclohexene


FIGURE 8.10 MECHANISM -

PROBLEM What products would you expect from the following reactions?


FIGURE 8.10 MECHANISM - 8.10 Radical Additions to Alkenes: Chain-Growth Polymers

In our brief introduction to radical reactions in Section 6.6 , we said that radicals can add to $\mathrm{C}=\mathrm{C}$ bonds, taking one electron from the double bond and leaving one behind to yield a new radical. Let's now look at the process in more detail, focusing on the industrial synthesis of alkene polymers. A polymer is a large-sometimes very large-molecule, built up by repetitive joining together of many smaller molecules, called monomers.


Nature makes wide use of biological polymers. Cellulose, for instance, is a polymer built of repeating glucose monomer units; proteins are polymers built of repeating amino acid monomers; and nucleic acids are polymers built of repeating nucleotide monomers.


Cellulose-a glucose polymer -


Cellulose-a glucose polymer -

Protein-an amino acid polymer
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0258.jpg?height=199&width=890&top_left_y=600&top_left_x=623)

An amino acid
Nucleic acid-a nucleotide polymer
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0258.jpg?height=557&width=1093&top_left_y=909&top_left_x=538)


Cellulose-a glucose polymer -

A nucleic acid
Synthetic polymers, such as polyethylene, are much simpler chemically than biopolymers, but there is still a great diversity to their structures and properties, depending on the identity of the monomers and on the reaction conditions used for polymerization. The simplest synthetic polymers are those that result when an alkene is treated with a small amount of a suitable catalyst. Ethylene, for example, yields polyethylene, an enormous alkane that may have a molecular weight up to 6 million $u$ and may contain as many as 200,000 monomer units. Worldwide production of polyethylene is approximately 88 million tons per year.


Polyethylene-a synthetic alkene polymer -


Polyethylene and other simple alkene polymers are called chain-growth polymers because they are formed in a chain-reaction process in which an initiator adds to a carbon-carbon double bond to yield a reactive intermediate. The intermediate then reacts with a second molecule of monomer to yield a new intermediate, which reacts with a third monomer unit, and so on.


Historically, ethylene polymerization was carried out at high pressure (1000-3000 atm) and high temperature $\left(100-250{ }^{\circ} \mathrm{C}\right)$ in the presence of a radical initiator such as benzoyl peroxide. Like many radical reactions, the mechanism of ethylene polymerization occurs in three steps: initiation, propagation, and termination:

- Initiation The polymerization reaction is initiated when a few radicals are generated on heating a small
amount of benzoyl peroxide catalyst to break the weak $\mathrm{O}-\mathrm{O}$ bond. The initially formed benzoyloxy radical loses $\mathrm{CO}_{2}$ and gives a phenyl radical ( $\mathrm{Ph} \cdot$ ), which adds to the $\mathrm{C}=\mathrm{C}$ bond of ethylene to start the polymerization process. One electron from the ethylene double bond pairs up with the odd electron on the phenyl radical to form a new $\mathrm{C}-\mathrm{C}$ bond, and the other electron remains on carbon.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0259.jpg?height=310&width=1391&top_left_y=365&top_left_x=394)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0259.jpg?height=85&width=581&top_left_y=695&top_left_x=796)
- Propagation Polymerization occurs when the carbon radical formed in the initiation step adds to another ethylene molecule to yield another radical. Repetition of the process for hundreds or thousands of times builds the polymer chain.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0259.jpg?height=104&width=1402&top_left_y=905&top_left_x=394)
- Termination The chain process is eventually ended by a reaction that consumes the radical. The combination of two growing chains is one possible chain-terminating reaction.


$$
2 \mathrm{R}-\mathrm{CH}_{2} \mathrm{CH}_{2} \cdot \longrightarrow \mathrm{R}-\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2}-\mathrm{R}
$$


Ethylene is not unique in its ability to form a polymer. Many substituted ethylenes, called vinyl monomers, also undergo polymerization to yield polymers with substituent groups regularly spaced on alternating carbon atoms along the chain. Propylene, for example, yields polypropylene, and styrene yields polystyrene.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0259.jpg?height=681&width=925&top_left_y=1318&top_left_x=600)


When an unsymmetrically substituted vinyl monomer such as propylene or styrene is polymerized, the radical addition steps can take place at either end of the double bond to yield either a primary radical intermediate $\left(\mathrm{RCH}_{2} \cdot\right)$ or a secondary radical $\left(\mathrm{R}_{2} \mathrm{CH} \cdot\right)$. Just as in electrophilic addition reactions, however, we find that only the more highly substituted, secondary radical is formed.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0259.jpg?height=204&width=911&top_left_y=2191&top_left_x=607)


TABLE 8.1 shows some commercially important alkene polymers, their uses, and the monomers from which they are made.

TABLE 8.1 Some Alkene Polymers and Their Uses


| Monomer |  | Trade or common name of
 polymer |  |
| :--- | :--- | :--- | :--- |
| Ethylene | $\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}_{2}$ | Polyethylene | Uses |
| Propene (propylene) | $\mathrm{H}_{2} \mathrm{C}=\mathrm{CHCH}_{3}$ | Polypropylene | Moldings, rope,
 carpets |
| Chloroethylene (vinyl
 chloride) | $\mathrm{H}_{2} \mathrm{C}=\mathrm{CHCl}$ | Poly(vinyl chloride) | Insulation, films, pipes |
| Styrene | $\mathrm{H}_{2} \mathrm{C}=\mathrm{CHC}_{6} \mathrm{H}_{5}$ | Polystyrene | Foam, moldings |
| Tetrafluoroethylene | $\mathrm{F}_{2} \mathrm{C}=\mathrm{CF}_{2}$ | Teflon | Gaskets, nonstick |
| coatings |  |  |  |


WORKED EXAMPLE 8.4 -


Predicting the Structure of a Polymer -

Show the structure of poly(vinyl chloride), a polymer made from $\mathrm{H}_{2} \mathrm{C}=\mathrm{CHCl}$, by drawing several repeating units.


Strategy -

Mentally break the carbon-carbon double bond in the monomer unit, and form single bonds by connecting numerous units together.


Solution -


Solution -

PROBLEM Show the monomer units you would use to prepare the following polymers:


8-18 (a) -


8-18 (a) -

PROBLEM One of the chain-termination steps that sometimes occurs to interrupt polymerization is the
8-19 following reaction between two radicals. Propose a mechanism for the reaction, using fishhook arrows to indicate electron flow.


8-18 (a) -

$$
\geqslant \mathrm{CH}_{2} \dot{\mathrm{C}} \mathrm{H}_{2} \longrightarrow \quad \geqslant \mathrm{CH}_{2} \mathrm{CH}_{3}+\frac{>}{<} \mathrm{CH}=\mathrm{CH}_{2}
$$


8-18 (a) - 8.11 Biological Additions of Radicals to Alkenes

The same high reactivity of radicals that enables the alkene polymerization we saw in the previous section also makes it difficult to carry out controlled radical reactions on complex molecules. As a result, there are severe limitations on the usefulness of radical addition reactions in the laboratory. In contrast to an electrophilic addition, where reaction occurs once and the reactive cation intermediate is rapidly quenched by a nucleophile, the reactive intermediate in a radical reaction is not usually quenched. Instead, it reacts again and again in a largely uncontrollable way.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0261.jpg?height=294&width=841&top_left_y=623&top_left_x=474)


Radical addition -

(Intermediate is not quenched, so reaction does not stop.)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0261.jpg?height=211&width=931&top_left_y=1076&top_left_x=529)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0261.jpg?height=128&width=158&top_left_y=1085&top_left_x=1482)


Radical addition -

In biological reactions, the situation is different from that in the laboratory. Only one substrate molecule at a time is present in the active site of an enzyme, and that molecule is held in a precise position, with other necessary reacting groups nearby. As a result, biological radical reactions are more controlled and more common than laboratory or industrial radical reactions. A particularly impressive example occurs in the biosynthesis of prostaglandins from arachidonic acid, where a sequence of four radical additions take place. Its reaction mechanism was discussed briefly in Section 6.6.


Radical addition -

As shown in FIGURE 8.12, prostaglandin biosynthesis begins with abstraction of a hydrogen atom from C13 of arachidonic acid by an iron-oxy radical to give a carbon radical that reacts with $\mathrm{O}_{2}$ at C 11 through a resonance form. The oxygen radical that results adds to the $\mathrm{C} 8-\mathrm{C} 9$ double bond to give a carbon radical at C8, which adds to the $\mathrm{C} 12-\mathrm{C} 13$ double bond and gives a carbon radical at C 13 . A resonance form of this carbon radical adds at C 15 to a second $\mathrm{O}_{2}$ molecule, completing the prostaglandin skeleton. Reduction of the $\mathrm{O}-\mathrm{O}$ bond then gives prostaglandin $\mathrm{H}_{2}$, called $\mathrm{PGH}_{2}$. The pathway looks complicated, but the entire process is catalyzed with exquisite control by a single enzyme.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0262.jpg?height=348&width=1110&top_left_y=189&top_left_x=383)


Radical addition -


Radical addition -

FIGURE 8.12 Pathway for the biosynthesis of prostaglandins from arachidonic acid. Steps 2 and 5 are radical addition reactions to $\mathrm{O}_{2}$; steps 3 and 4 are radical additions to carbon-carbon double bonds.


Radical addition - 8.12 Reaction Stereochemistry: Addition of $\mathrm{H}_{2} \mathrm{O}$ to an Achiral Alkene

Most of the biochemical reactions that take place in the body, as well as many organic reactions in the laboratory, yield products with chirality centers. For example, acid-catalyzed addition of $\mathrm{H}_{2} \mathrm{O}$ to 1-butene in the laboratory yields 2 -butanol, a chiral alcohol. What is the stereochemistry of this chiral product? If a single enantiomer is formed, is it $R$ or $S$ ? If a mixture of enantiomers is formed, how much of each? In fact, the 2 -butanol produced is a racemic mixture of $R$ and $S$ enantiomers. Let's see why.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0262.jpg?height=245&width=958&top_left_y=2214&top_left_x=578)

To understand why a racemic product results from the reaction of $\mathrm{H}_{2} \mathrm{O}$ with 1-butene, think about the reaction
mechanism. 1-Butene is first protonated to yield an intermediate secondary carbocation. Because the trivalent carbon is $s p^{2}$-hybridized and planar, the cation has a plane of symmetry and is achiral. As a result, it can react with $\mathrm{H}_{2} \mathrm{O}$ equally well from either the top or the bottom. Reaction from the top leads to ( $S$ )-2-butanol through transition state 1 (TS 1) in FIGURE 8.13, and reaction from the bottom leads to ( $R$ )-2-butanol through TS 2. But the two transition states are mirror images, so they have identical energies, form at identical rates, and are equally likely to occur.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0263.jpg?height=736&width=1212&top_left_y=472&top_left_x=451)

FIGURE 8.13 Reaction of $\mathbf{H}_{2} \mathbf{O}$ with the carbocation resulting from protonation of 1-butene. Reaction from the top leads to $S$ product and is the mirror image of reaction from the bottom, which leads to $R$ product. Because they are energetically identical, they are equally likely and lead to a racemic mixture of products. The dotted $\mathrm{C} \cdots \mathrm{O}$ bond in the transition state indicates partial bond formation.


As a general rule, the formation of a new chirality center by achiral reactants always leads to a racemic mixture of enantiomeric products. Put another way, optical activity can't appear from nowhere; an optically active product can only result by starting with an optically active reactant or chiral environment (Section 5.12).


In contrast to laboratory reactions, enzyme-catalyzed biological reactions often give a single enantiomer of a chiral product, even when the substrate is achiral. One step in the citric acid cycle of food metabolism, for instance, is the aconitase-catalyzed addition of water to (Z)-aconitate (usually called cis-aconitate) to give isocitrate.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0263.jpg?height=283&width=861&top_left_y=1677&top_left_x=632)


Even though cis-aconitate is achiral, only the ( $2 R, 3 S$ ) enantiomer of the product is formed. As discussed in Section 5.11 and Section 5.12, cis-aconitate is a prochiral molecule, which is held in a chiral environment by the aconitase enzyme during the reaction. In this environment, the two faces of the double bond are chemically distinct, and addition occurs on only the Re face at C2.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0263.jpg?height=302&width=1013&top_left_y=2153&top_left_x=556)


Radical addition - 8.13 Reaction Stereochemistry: Addition of $\mathrm{H}_{2} \mathrm{O}$ to a Chiral Alkene

The reaction discussed in the previous section involves an addition to an achiral reactant and forms an optically inactive, racemic mixture of two enantiomeric products. What would happen, though, if we were to carry out the reaction on a single enantiomer of a chiral reactant? For example, what stereochemical result would be obtained from addition of $\mathrm{H}_{2} \mathrm{O}$ to a chiral alkene, such as $(R)$-4-methyl-1-hexene? The product of the reaction, 4-methyl-2-hexanol, has two chirality centers and so has four possible stereoisomers.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0264.jpg?height=236&width=747&top_left_y=500&top_left_x=689)


Let's think about the two chirality centers separately. What about the configuration at C4, the methyl-bearing carbon atom? Since C 4 has the $R$ configuration in the starting material and this chirality center is unaffected by the reaction, its configuration is unchanged. Thus, the configuration at C 4 in the product remains $R$ (assuming that the relative rankings of the four attached groups are not changed by the reaction).


What about the configuration at C2, the newly formed chirality center? As shown in FIGURE 8.14, the stereochemistry at C 2 is established by reaction of $\mathrm{H}_{2} \mathrm{O}$ with a carbocation intermediate in the usual manner. But this carbocation doesn't have a plane of symmetry; it is chiral because of the chirality center at C4. Because the carbocation is chiral and has no plane of symmetry, it doesn't react equally well from the top and bottom faces. One of the two faces is likely, for steric reasons, to be a bit more accessible than the other, leading to a mixture of $R$ and $S$ products in some ratio other than $50: 50$. Thus, two diastereomeric products, ( $2 R, 4 R$ )-4-methyl-2-hexanol and ( $2 S, 4 R$ )-4-methyl-2-hexanol, are formed in unequal amounts, and the mixture is optically active.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0264.jpg?height=720&width=760&top_left_y=1288&top_left_x=680)
(2S,4R)-4-Methyl-2-hexanol
(2R,4R)-4-Methyl-2-hexanol
FIGURE 8.14 Stereochemistry of the acid-catalyzed addition of $\mathbf{H}_{\mathbf{2}} \mathbf{O}$ to the chiral alkene, ( $R$ )-4-methyl-1-hexene. A mixture of diastereomeric $2 R, 4 R$ and $2 S, 4 R$ products is formed in unequal amounts because reaction of the chiral carbocation intermediate is not equally likely from top and bottom. The product mixture is optically active.


As a general rule, the formation of a new chirality center by a chiral reactant leads to unequal amounts of diastereomeric products. If the chiral reactant is optically active because only one enantiomer is used rather than a racemic mixture, then the products are also optically active.


PROBLEM What products are formed from acid-catalyzed hydration of racemic ( $\pm$ )-4-methyl-1-hexene? What 8-20 can you say about the relative amounts of the products? Is the product mixture optically active?

PROBLEM What products are formed from hydration of 4-methylcyclopentene? What can you say about the 8-21 relative amounts of the products?


Terpenes: Naturally Occurring Alkenes -

Ever since its discovery in Persia around 1000 A.D., it has been known that steam distillation, the distillation of plant materials together with water, produces a fragrant mixture of liquids called essential oils. The resulting oils have long been used as medicines, spices, and perfumes, and their investigation played a major role in the emergence of organic chemistry as a science during the 19th century.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0265.jpg?height=692&width=825&top_left_y=630&top_left_x=647)


Terpenes: Naturally Occurring Alkenes -

FIGURE 8.15 The wonderful fragrance of leaves from the California bay laurel is due primarily to myrcene, a simple terpene. (credit: "California Bay Umbellularia californica" by Don Loaire/Flickr, CC BY 2.0)