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Q.28.

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Organic Chemistry
Found in: Page 128

Short Answer

Question: Consider 1,2-dimethylcyclohexane.

  1. Draw structures for the cis and trans isomers using a hexagon for the six-membered ring.
  2. Draw the two possible chair conformations for the cis isomer. Which conformation, if either, is more stable?
  3. Draw the two possible chair conformations for the trans isomer. Which conformation, if either, is more stable?
  4. Which isomer, cis or trans, is more stable and why?

Answer

a. The cis and trans isomers are shown below:

b. Both chair conformations have the same stability.

c. Structure II is more stable.

d. The trans isomer is more stable because both bulky methyl groups are present at the equatorial position.

See the step by step solution

Step by Step Solution

Step 1: Different types of isomers

The compounds made of atoms of the same type but differing from each other only in terms of the arrangement of those atoms are referred to as isomers.

Isomers are further classified as structural and stereoisomers. Stereoisomers have different three-dimensional arrangements of atoms, whereas structural isomers have different structures.

Step 2: The structure of the cis and trans isomers for 1,2-dimethylcyclohexane

a. The cis and trans isomers have the same molecular formula but are different only in terms of the three-dimensional arrangement of the functional groups. The cis and trans isomers of 1,2-dimethylcyclohexane are shown below:

Trans 1,2-dimethylcyclohexane

Cis 1,2-dimethyl cyclohexane

b. The two possible chair conformations for the cis isomer of 1,2-dimethyl cyclohexane is shown below:

Cis 1,2-dimethyl cyclohexane

Both of the above cis conformations of 1,2-dimethyl cyclohexane have the same stability. In both structures, one of the methyl groups is present at the axial position, while the other is at the equatorial position.

c. The two possible chair conformations of the trans isomers of 1,2-dimethyl cyclohexane are shown below:

Trans 1,2-dimethyl cyclohexane

Structure II is a more stable trans conformation. This is because both methyl groups are present at the equatorial positions, resulting in less than 1,3 di-axial repulsions.

d. The trans isomer is more stable as compared to the cis isomer. In the case of the trans isomer, both bulky methyl groups can be present at the equatorial positions.

The equatorial position provides more space to the alkyl groups and reduces the repulsions among the different atoms.

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