Strategy - 3.6 Conformations of Ethane

Conformational isomers are represented in two ways, as shown in FIGURE 3.7. A sawhorse representation views the carbon-carbon bond from an oblique angle and indicates spatial orientation by showing all $\mathrm{C}-\mathrm{H}$ bonds. A Newman projection views the carbon-carbon bond directly end-on and represents the two carbon atoms by a circle. Bonds attached to the front carbon are represented by lines to the center of the circle, and bonds attached to the rear carbon are represented by lines to the edge of the circle.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0085.jpg?height=256&width=258&top_left_y=216&top_left_x=478)
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Strategy - 3.6 Conformations of Ethane


Strategy - 3.6 Conformations of Ethane

Newman
projection


Strategy - 3.6 Conformations of Ethane

FIGURE 3.7 A sawhorse representation and a Newman projection of ethane. The sawhorse representation views the molecule from an oblique angle, while the Newman projection views the molecule end-on. Note that the molecular model of the Newman projection appears at first to have six atoms attached to a single carbon. Actually, the front carbon, with three attached green atoms, is directly in front of the rear carbon, with three attached red atoms.


Strategy - 3.6 Conformations of Ethane

Despite what we've just said, we actually don't observe perfectly free rotation in ethane. Experiments show that there is a small $(12 \mathrm{~kJ} / \mathrm{mol} ; 2.9 \mathrm{kcal} / \mathrm{mol})$ barrier to rotation and that some conformations are more stable than others. The lowest-energy, most stable conformation is the one in which all six $\mathrm{C}-\mathrm{H}$ bonds are as far away from one another as possible-staggered when viewed end-on in a Newman projection. The highestenergy, least stable conformation is the one in which the six $\mathrm{C}-\mathrm{H}$ bonds are as close as possible-eclipsed in a Newman projection. At any given instant, about $99 \%$ of ethane molecules have an approximately staggered conformation and only about $1 \%$ are near the eclipsed conformation.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0085.jpg?height=408&width=1330&top_left_y=1021&top_left_x=384)

The extra $12 \mathrm{~kJ} / \mathrm{mol}$ of energy present in the eclipsed conformation of ethane is called torsional strain. Its cause has been the subject of controversy, but the major factor is an interaction between $\mathrm{C}-\mathrm{H}$ bonding orbitals on one carbon and antibonding orbitals on the adjacent carbon, which stabilizes the staggered conformation relative to the eclipsed one. Because a total strain of $12 \mathrm{~kJ} / \mathrm{mol}$ arises from three equal hydrogen-hydrogen eclipsing interactions, we can assign a value of approximately $4.0 \mathrm{~kJ} / \mathrm{mol}(1.0 \mathrm{kcal} / \mathrm{mol})$ to each single interaction. The barrier to rotation that results can be represented on a graph of potential energy versus degree of rotation, in which the angle between $\mathrm{C}-\mathrm{H}$ bonds on the front and back carbons as viewed end-on (the dihedral angle) goes full circle from 0 to $360^{\circ}$. Energy minima occur at staggered conformations, and energy maxima occur at eclipsed conformations, as shown in FIGURE 3.8.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0086.jpg?height=722&width=1091&top_left_y=192&top_left_x=514)

FIGURE 3.8 A graph of potential energy versus bond rotation in ethane. The staggered conformations are $12 \mathrm{~kJ} / \mathrm{mol}$ lower in energy than the eclipsed conformations.


Strategy - 3.7 Conformations of Other Alkanes

Propane, the next-higher member in the alkane series, also has a torsional barrier that results in hindered rotation around the carbon-carbon bonds. The barrier is slightly higher in propane than in ethane-a total of 14 $\mathrm{kJ} / \mathrm{mol}(3.4 \mathrm{kcal} / \mathrm{mol})$ versus $12 \mathrm{~kJ} / \mathrm{mol}$.


The eclipsed conformation of propane has three interactions-two ethane-type hydrogen-hydrogen interactions and one additional hydrogen-methyl interaction. Since each eclipsing $\mathrm{H} \longleftrightarrow \mathrm{H}$ interaction is the same as that in ethane and thus has an energy "cost" of $4.0 \mathrm{~kJ} / \mathrm{mol}$, we can assign a value of $14-(2 \times 4.0)=6.0$ $\mathrm{kJ} / \mathrm{mol}(1.4 \mathrm{kcal} / \mathrm{mol})$ to the eclipsing $\mathrm{H} \longleftrightarrow \mathrm{CH}_{3}$ interaction (FIGURE 3.9).
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0086.jpg?height=375&width=1365&top_left_y=1406&top_left_x=369)


FIGURE 3.9 Newman projections of propane showing staggered and eclipsed conformations. The staggered conformer is lower in energy by $14 \mathrm{~kJ} / \mathrm{mol}$.


The conformational situation becomes more complex for larger alkanes because not all staggered conformations have the same energy and not all eclipsed conformations have the same energy. In butane, for instance, the lowest-energy arrangement, called the anti conformation, is the one in which the two methyl groups are as far apart as possible $-180^{\circ}$ away from each other. As rotation around the $\mathrm{C} 2-\mathrm{C} 3$ bond occurs, an eclipsed conformation is reached where there are two $\mathrm{CH}_{3} \longleftrightarrow \mathrm{H}$ interactions and one $\mathrm{H} \leftrightarrows \mathrm{H}$ interaction. Using the energy values derived previously from ethane and propane, this eclipsed conformation is more strained than the anti conformation by $2 \times 6.0 \mathrm{~kJ} / \mathrm{mol}+4.0 \mathrm{~kJ} / \mathrm{mol}$ (two $\mathrm{CH}_{3} \longleftrightarrow \mathrm{H}$ interactions plus one $\mathrm{H} \longleftrightarrow \mathrm{H}$ interaction), for a total of $16 \mathrm{~kJ} / \mathrm{mol}(3.8 \mathrm{kcal} / \mathrm{mol})$.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0087.jpg?height=307&width=313&top_left_y=215&top_left_x=353)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0087.jpg?height=248&width=402&top_left_y=272&top_left_x=664)


Butane-anti conformation ( $0 \mathrm{~kJ} / \mathrm{mol}$ )


Butane-eclipsed
conformation
$(16 \mathrm{~kJ} / \mathrm{mol})$
As bond rotation continues, an energy minimum is reached at the staggered conformation where the methyl groups are $60^{\circ}$ apart. Called the gauche conformation, it lies $3.8 \mathrm{~kJ} / \mathrm{mol}(0.9 \mathrm{kcal} / \mathrm{mol})$ higher in energy than the anti conformation even though it has no eclipsing interactions. This energy difference occurs because the hydrogen atoms of the methyl groups are near one another in the gauche conformation, resulting in what is called steric strain. Steric strain is the repulsive interaction that occurs when atoms are forced closer together than their atomic radii allow. It's the result of trying to force two atoms to occupy the same space.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0087.jpg?height=439&width=1382&top_left_y=946&top_left_x=374)

As the dihedral angle between the methyl groups approaches zero, an energy maximum is reached at a second eclipsed conformation. Because the methyl groups are forced even closer together than in the gauche conformation, both torsional strain and steric strain are present. A total strain energy of $19 \mathrm{~kJ} / \mathrm{mol}(4.5 \mathrm{kcal} /$ mol ) has been estimated for this conformation, making it possible to calculate a value of $11 \mathrm{~kJ} / \mathrm{mol}(2.6 \mathrm{kcal} /$ mol ) for the $\mathrm{CH}_{3} \longleftrightarrow \mathrm{CH}_{3}$ eclipsing interaction: total strain of $19 \mathrm{~kJ} / \mathrm{mol}$ minus the strain of two $\mathrm{H} \longleftrightarrow \mathrm{H}$ eclipsing interactions ( $2 \times 4.0 \mathrm{kcal} / \mathrm{mol}$ ) equals $11 \mathrm{~kJ} / \mathrm{mol}$.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0087.jpg?height=451&width=1427&top_left_y=1669&top_left_x=346)


After $0^{\circ}$, the rotation becomes a mirror image of what we've already seen: another gauche conformation is reached, another eclipsed conformation, and finally a return to the anti conformation. A plot of potential energy versus rotation about the $\mathrm{C} 2-\mathrm{C} 3$ bond is shown in FIGURE 3.10.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0088.jpg?height=1001&width=1383&top_left_y=193&top_left_x=371)


FIGURE 3.10 A plot of potential energy versus rotation for the C2-C3 bond in butane. The energy maximum occurs when the two methyl groups eclipse each other, and the energy minimum occurs when the two methyl groups are $180^{\circ}$ apart (anti).


The notion of assigning definite energy values to specific interactions within a molecule is very useful, and we'll return to it in the next chapter. A summary of what we've seen thus far is given in TABLE 3.5.


The same principles just developed for butane apply to pentane, hexane, and all higher alkanes. The most favorable conformation for any alkane has the carbon-carbon bonds in staggered arrangements, with large substituents arranged anti to one another. A generalized alkane structure is shown in FIGURE 3.11.


| Interaction | Cause | Energy cost |  |
| :---: | :---: | :---: | :---: |
|  |  | ( $\mathrm{kJ} / \mathrm{mol}$ ) | ( $\mathrm{kcal} / \mathrm{mol}$ ) |
| $\mathrm{H} \leftrightarrow \mathrm{H}$ eclipsed | Torsional strain | 4.0 | 1.0 |
| $\mathrm{H} \leftrightarrow \mathrm{CH}_{3}$ eclipsed | Mostly torsional strain | 6.0 | 1.4 |
| $\mathrm{CH}_{3} \longleftrightarrow \mathrm{CH}_{3}$ eclipsed | Torsional and steric strain | 11.0 | 2.6 |
| $\mathrm{CH}_{3} \longleftrightarrow \mathrm{CH}_{3}$ gauche | Steric strain | 3.8 | 0.9 |


FIGURE 3.11 The most stable alkane conformation is the one in which all substituents are staggered and the carbon-carbon bonds are arranged anti, as shown in this model of decane.
One final point: saying that one particular conformer is "more stable" than another doesn't mean the molecule adopts and maintains only the more stable conformation. At room temperature, rotations around $\sigma$ bonds occur so rapidly that all conformers are in equilibrium. At any given instant, however, a larger percentage of molecules will be found in a more stable conformation than in a less stable one.


WORKED EXAMPLE 3.4 -


Newman Projections -

Sight along the C1-C2 bond of 1-chloropropane, and draw Newman projections of the most stable and least stable conformations.


Strategy -

The most stable conformation of a substituted alkane is generally a staggered one in which large groups have an anti relationship. The least stable conformation is generally an eclipsed one in which large groups are as close as possible.


Solution -


Solution -

Least stable (eclipsed)


Solution -

PROBLEM Make a graph of potential energy versus angle of bond rotation for propane, and assign values to the 3-15 energy maxima.


Solution -

PROBLEM Sight along the C2-C1 bond of 2-methylpropane (isobutane).
3-16 (a) Draw a Newman projection of the most stable conformation.
(b) Draw a Newman projection of the least stable conformation.
(c) Make a graph of energy versus angle of rotation around the $\mathrm{C} 2-\mathrm{C} 1$ bond.
(d) Assign relative values to the maxima and minima in your graph, given that an $\mathrm{H} \longleftrightarrow \mathrm{H}$ eclipsing interaction costs $4.0 \mathrm{~kJ} / \mathrm{mol}$ and an $\mathrm{H} \longleftrightarrow \mathrm{CH}_{3}$ eclipsing interaction costs $6.0 \mathrm{~kJ} / \mathrm{mol}$.

PROBLEM Sight along the C2-C3 bond of 2,3-dimethylbutane, and draw a Newman projection of the most
3-17 stable conformation.
PROBLEM Draw a Newman projection along the C2-C3 bond of the following conformation of 3-18 2,3-dimethylbutane, and calculate a total strain energy:
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0090.jpg?height=351&width=445&top_left_y=193&top_left_x=428)


(8.0) CHEMISTRY MATTERS -


Gasoline -

British Foreign Minister Ernest Bevin once said that "The Kingdom of Heaven runs on righteousness, but the Kingdom of Earth runs on alkanes." (Actually, he said "runs on oil" not "runs on alkanes," but they're essentially the same.) By far, the major sources of alkanes are the world's natural gas and petroleum deposits. Laid down eons ago, these deposits are thought to be derived primarily from the decomposition of tiny singlecelled marine organisms called foraminifera. Natural gas consists chiefly of methane but also contains ethane, propane, and butane. Petroleum is a complex mixture of hydrocarbons that must be separated into fractions and then further refined before it can be used.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0091.jpg?height=1096&width=823&top_left_y=761&top_left_x=651)


Gasoline -

FIGURE 3.12 Gasoline is a finite resource. It won't be around forever. (credit: "The first oil well" (https://www.loc.gov/item/ 2010649522/) by Unknown/Library of Congress)


Gasoline -

The petroleum era began in August 1859, when the world's first oil well was drilled by Edwin Drake near Titusville, Pennsylvania. The petroleum was distilled into fractions according to boiling point, but it was highboiling kerosene, or lamp oil, rather than gasoline that was primarily sought. Literacy was becoming widespread at the time, and people wanted better light for reading than was available from candles. Gasoline was too volatile for use in lamps and was initially considered a waste by-product. The world has changed greatly since those early days, however, and it is now gasoline rather than lamp oil that is prized.


Gasoline -

Petroleum refining begins by fractional distillation of crude oil into three principal cuts according to boiling point (bp): straight-run gasoline (bp 30-200 ${ }^{\circ} \mathrm{C}$ ), kerosene (bp $175-300^{\circ} \mathrm{C}$ ), and heating oil, or diesel fuel (bp $275-400{ }^{\circ} \mathrm{C}$ ). Further distillation under reduced pressure then yields lubricating oils and waxes and leaves a tarry residue of asphalt. The distillation of crude oil is only the first step in gasoline production, however. Straight-run gasoline turns out to be a poor fuel in automobiles because of engine knock, an uncontrolled combustion that can occur in a hot engine causing potentially serious damage.


Gasoline -

The octane number of a fuel is the measure by which its antiknock properties are judged. It was recognized long ago that straight-chain hydrocarbons are far more prone to inducing engine knock than highly branched compounds. Heptane, a particularly bad fuel, is assigned a base value of 0 octane number, and 2,2,4-trimethylpentane, commonly known as isooctane, has a rating of 100 .
<img class="imgSvg" id = "m6sn3mdb4rrfrcrz0df" 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Heptane
(octane number $=\mathbf{0}$ )
<img class="imgSvg" id = "m6sn3mdeuspbbx1egwj" 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Gasoline -

2,2,4-Trimethylpentane
(octane number $=100$ )


Gasoline -

Because straight-run gasoline burns so poorly in engines, petroleum chemists have devised numerous methods for producing higher-quality fuels. One of these methods, catalytic cracking, involves taking the high-boiling kerosene cut ( $\mathrm{C}_{11}-\mathrm{C}_{14}$ ) and "cracking" it into smaller branched molecules suitable for use in gasoline. Another process, called reforming, is used to convert $\mathrm{C}_{6}-\mathrm{C}_{8}$ alkanes to aromatic compounds such as benzene and toluene, which have substantially higher octane numbers than alkanes. The final product that goes in your tank has an approximate composition of $15 \% \mathrm{C}_{4}-\mathrm{C}_{8}$ straight-chain alkanes, $25 \%$ to $40 \% \mathrm{C}_{4}-\mathrm{C}_{10}$ branched-chain alkanes, $10 \%$ cyclic alkanes, $10 \%$ straight-chain and cyclic alkenes, and $25 \%$ arenes (aromatics).


Key Terms -

```
- alcohol
ester
- aldehyde
- ether
- aliphatic
- functional group
- alkane
- gauche conformation
- alkene
- hydrocarbon
- alkyl group
- isomer
- alkyl halide
- ketone
- alkyne
- Newman projection
- amide
- nitrile
- amine
- R group
- anti conformation
- saturated
- arene
- sawhorse representation
- branched-chain alkane
- staggered conformation
- carbonyl group
- stereochemistry
- carboxylic acid
- steric strain
- conformation
- straight-chain alkane
- conformational isomer
- substituent
- conformer
- sulfide
- constitutional isomer
- thiol
- eclipsed conformation
- torsional strain
```


Key Terms -


Summary -

Alkanes are relatively unreactive and rarely involved in chemical reactions, but they nevertheless provide a useful vehicle for introducing some important general ideas. In this chapter, we've used alkanes to introduce the basic approach to naming organic compounds and to take an initial look at some of the three-dimensional aspects of molecules.


Summary -

A functional group is a group of atoms within a larger molecule that has a characteristic chemical reactivity. Because functional groups behave in approximately the same way in all molecules where they occur, the chemical reactions of an organic molecule are largely determined by its functional groups.


Summary -

[^0]structures are called isomers. More specifically, compounds such as butane and isobutane, which differ in their connections between atoms, are called constitutional isomers.


Summary -

Carbon-carbon single bonds in alkanes are formed by $\sigma$ overlap of carbon $s p^{3}$ hybrid orbitals. Rotation is possible around $\sigma$ bonds because of their cylindrical symmetry, and alkanes therefore exist in a large number of rapidly interconverting conformations. Newman projections make it possible to visualize the spatial consequences of bond rotation by sighting directly along a carbon-carbon bond axis. Not all alkane conformations are equally stable. The staggered conformation of ethane is $12 \mathrm{~kJ} / \mathrm{mol}(2.9 \mathrm{kcal} / \mathrm{mol}) \mathrm{more}$ stable than the eclipsed conformation because of torsional strain. In general, any alkane is most stable when all its bonds are staggered.


Additional Problems -


Visualizing Chemistry -

PROBLEM Identify the functional groups in the following substances, and convert each drawing into a 3-19 molecular formula (red $=0$, blue $=\mathrm{N}$ ).
(a)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0093.jpg?height=264&width=494&top_left_y=868&top_left_x=493)
Phenylalanine
(b)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0093.jpg?height=389&width=529&top_left_y=865&top_left_x=1058)


Visualizing Chemistry -

Lidocaine
PROBLEM Give IUPAC names for the following alkanes, and convert each drawing into a skeletal structure.


Visualizing Chemistry -


Visualizing Chemistry -

PROBLEM Draw a Newman projection along the C2-C3 bond of the following conformation of 2-butanol.
3-21
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0093.jpg?height=297&width=470&top_left_y=2204&top_left_x=421)


Functional Groups -

PROBLEM Locate and identify the functional groups in the following molecules.


Functional Groups -


Functional Groups -

PROBLEM Propose structures that meet the following descriptions:
3-23 (a) A ketone with five carbons (b) A four-carbon amide (c) A five-carbon ester
(d) An aromatic aldehyde
(e) A keto ester
(f) An amino alcohol

PROBLEM Propose structures for the following:
3-24 (a) A ketone, $\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}$ (b) A nitrile, $\mathrm{C}_{5} \mathrm{H}_{9} \mathrm{~N}$ (c) A dialdehyde, $\mathrm{C}_{4} \mathrm{H}_{6} \mathrm{O}_{2}$
(d) A bromoalkene, $\mathrm{C}_{6} \mathrm{H}_{11} \mathrm{Br}$ (e) An alkane, $\mathrm{C}_{6} \mathrm{H}_{14}$ (f) cyclic saturated hydrocarbon, $\mathrm{C}_{6} \mathrm{H}_{12}$
(g) A diene (dialkene), $\mathrm{C}_{5} \mathrm{H}_{8}$
(h) A keto alkene, $\mathrm{C}_{5} \mathrm{H}_{8} \mathrm{O}$


Functional Groups -

PROBLEM Predict the hybridization of the carbon atom in each of the following functional groups:
3-25 (a) Ketone
(b) Nitrile
(c) Carboxylic acid


Functional Groups -

PROBLEM Draw the structures of the following molecules:
3-26 (a) Biacetyl, $\mathrm{C}_{4} \mathrm{H}_{6} \mathrm{O}_{2}$, a substance with the aroma of butter; it contains no rings or carbon-carbon multiple bonds.
(b) Ethylenimine, $\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{~N}$, a substance used in the synthesis of melamine polymers; it contains no multiple bonds.
(c) Glycerol, $\mathrm{C}_{3} \mathrm{H}_{8} \mathrm{O}_{3}$, a substance isolated from fat and used in cosmetics; it has an-OH group on each carbon.