Question

Draw the two chair conformations of each compound, and label the substituents as axial and equatorial. In each case, determine which conformation is more stable.

(a)$cis\text{-1-ethyl-2-isopropylcyclohexane}$

(b)$trans\text{-1-ethyl-2-isopropylcyclohexane}$

(c)$cis\text{-1-ethyl-3-methylcyclohexane}$

(d)$trans\text{-1-ethyl-3-methylcyclohexane}$

(e)$cis\text{-1-ethyl-4-methylcyclohexane}$

(f)$trans\text{-1-ethyl-4-methylcyclohexane}$

Short answer

Chair conformation is the most stable conformation of cyclohexane, with one part puckered upward and another part puckered downward.

Step-by-step solution

Chair conformation

Chair conformation is the most stable conformation of cyclohexane, with one part puckered upward and another part puckered downward.

Cis-trans isomer

Cis and transisomerism can be used only for the compounds in which two doubly bonded carbon atoms have similar atoms or groups. When similar atoms or groups lie on the same side, it is called a cis-isomer, and when similar atoms or groups lie on the opposite sides, it is called a trans-isomer. These two geometric isomers cannot interconvert without breaking and forming the bonds again.

Drawing cis and transisomers for cycloalkanes

In the case of cycloalkanes, if the two substituents point toward the same faces, they are known as a cis isomer. Again, if the two substituents point toward the opposite faces, they are known as the trans isomer. These two geometric isomers cannot interconvert without breaking and forming the bonds again.

Axial and equatorial positions of cyclohexane

Axial bonds are directed vertically parallel to the axis of the ring, and the equatorial bonds are directed outward from the equator of the ring. As the carbon atoms are numbered in cyclohexane, it can be seen that the odd-numbered carbons have their upward bonds in the axial position and their downward bonds in an equatorial position. The even-numbered carbons have their downward bondsin the axial position and their upward bonds in an equatorial position.

Different sizes of substituents

The substituents are of different sizes in many substituted cyclohexanes. For a larger (bulkier) group, the energy difference between axial and equatorial positions is greater than that of a smaller group (less bulky).

Identifying the most stable conformation

Larger (bulkier) substituents tend to be in an equatorial position. If both the groups cannot be in equatorial positions, the most stable conformation will have the larger (bulkier) group in the equatorial position and the smaller (less bulky) group in the axial position.

The axial position is shown by a solid blue line, and the equatorial position is shown by a solid red line in the structures drawn.