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Chapter 2: Problem 60

Which would you predict to have the larger (more negative) heat of combustion, cis-1,4-dimethylcyclohexane or trans-1,4-dimethylcyclohexane?

Short Answer

Expert verified
Answer: The cis-1,4-dimethylcyclohexane isomer has a larger (more negative) heat of combustion.

Step by step solution

01

Identify the isomers

We are given two isomers of 1,4-dimethylcyclohexane, cis and trans. The cis isomer has the methyl groups on the same side of the ring, while the trans isomer has them on opposite sides of the ring.
02

Understand the relationship between stability and heat of combustion

In order tocompare the heats of combustion of the cis and trans isomers, we need to understand the relationship between the stability of the molecules and their heat of combustion. A more stable molecule will have a lower (less negative) heat of combustion, while a less stable molecule will have a larger (more negative) heat of combustion. The reason for this is that a more stable molecule has a lower potential energy, and thus requires less energy to be released to reach the same final combustion products (usually carbon dioxide and water).
03

Compare the stability of the cis and trans isomers

The stability of the cis and trans isomers will be mainly determined by the steric interactions between the methyl groups. In the cis isomer, the two methyl groups are on the same side of the ring, leading to increased steric strain (repulsion between the electron clouds). On the other hand, the methyl groups in the trans isomer are on opposite sides of the ring, resulting in decreased steric strain.
04

Determine which isomer has a larger heat of combustion

Since the cis isomer has more steric strain than the trans isomer, it is less stable, and will therefore have a larger (more negative) heat of combustion. Thus, the predicted isomer with the larger heat of combustion is cis-1,4-dimethylcyclohexane.

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Most popular questions from this chapter

Arrange the alkanes in each set in order of increasing boiling point. (a) 2-Methylbutane, 2,2-dimethylpropane, and pentane (b) 3,3-Dimethylheptane, 2,2,4-trimethylhexane, and nonane

\(1,2,3,4,5,6-\) Hexachlorocyclohexane shows cis,trans isomerism. At one time a crude mixture of these isomers was sold as an insecticide. The insecticidal properties of the mixture arise from one isomer, known as lindane, which is cis- \(1,2,4,5\)-trans-3,6hexachlorocyclohexane. (a) Draw a structural formula for \(1,2,3,4,5,6\)-hexachlorocyclohexane disregarding, for the moment, the existence of cis,trans isomerism. What is the molecular formula of this compound? (b) Using a planar hexagon representation for the cyclohexane ring, draw a structural Formula for lindane. (c) Draw a chair conformation for lindane, and label which chlorine atoms are axial and which are equatorial. (d) Draw the alternative chair conformation of lindane, and again label which chlorine atoms are axial and which are equatorial. (e) Which of the alternative chair conformations of lindane is more stable? Explain.

Without consulting tables, arrange these compounds in order of decreasing (less negative) heat of combustion: hexane, 2-methylpentane, and 2 ,2-dimethylbutane.

Using a planar pentagon representation for the cyclopentane ring, draw structural formulas for the cis and trans isomers of the following. (a) 1,2-Dimethylcyclopentane (b) 1,3-Dimethylcyclopentane

Which statements are true about constitutional isomers? (a) They have the same molecular formula. (b) They have the same molecular weight. (c) They have the same order of attachment of atoms. (d) They have the same physical properties.
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