Question

Consider the four isomers of 1-ethyl-4-isopropylcyclohexane in a chair conformation.

Which two chair conformations represent the trans isomers in equilibrium?

Answer 1

Among the various conformations of cyclohexane the chair conformer is one of the stable form. There are various conformations of cyclohexanes which are boat , chair , half-chair and half-boat. Now if we look at the stability order of all these conformer then the order will be as follows-

Chair > twist-boat > boat > half-chair

Now if we consider the chair isomer there are two positions one is the axial position and the other is the equatorial position. Due to 1,3-diaxial interaction and other repulsive interaction the substitution at equatorial is more preffered compared to axial position. Now based on the position of the substituents it can be classified as cis or trans form. Now here in the question we are asked to find the trans-isomers which are in equilibrium for 1-ethyl-4-isopropylcyclehexane. So we can go through the options one by one-

(a) In this isomer both the substituent are in 1,4-positions of which the iso-propyl group is axially down and ethyl group is equatorially down. Thus this is a cis form.

(b) In this isomer both the substituent are in 1,4- positions of which the iso-propyl group is axially upward and ethyl group is equatorially upward. Thus this is a cis isomer.

(c) In this isomer both the substituent are in 1,4- positions of which the iso-propyl group is axially down and ethyl group is axially upward. Thus this is a trans isomer.

(d) In this isomer both the substituent are in 1,4- positions of which the iso-propyl group is equatorially down and ethyl group is equatorially upward. Thus this is a trans isomer.

Now, here in the question we are asked to find the trans conformer of 1-ethyl-4-isopropylcyclehexane which are in equilibrium with each other. Here from the four options the trans conformer which are in equilibrium are (C) and (D).

So the conformers - C and D are the trans forms that are in equilibrium with each other.