Question

There are four cis, trans isomers of 2-isopropyl-5-methylcyclohexanol: CC1CCC(C(C)C)C(O)C1 2-Isopropyl-5-methylcyclohexanol (a) Using a planar hexagon representation for the cyclohexane ring, draw structural formulas for the four cis, trans isomers. (b) Draw the more stable chair conformation for each of your answers in part (a). (c) Of the four cis, trans isomers, which is most stable? (Hint: If you answered this part correctly, you picked the isomer found in nature and given the name menthol.)

Short answer

Answer: The most stable cis, trans isomer of 2-isopropyl-5-methylcyclohexanol is cis-2-isopropyl-5-methylcyclohexanol, also known as menthol. It is more stable because both substituents (isopropyl and methyl groups) are in equatorial positions, which reduces steric strain in the molecule's chair conformation.

Step-by-step solution

Draw isomers of 2-isopropyl-5-methylcyclohexanol

In this step, substituents (isopropyl and methyl groups) must be placed in cis and trans positions on cyclohexane ring to represent cis and trans isomers. Four possible combinations are: 1. cis isopropyl, cis methyl 2. cis isopropyl, trans methyl 3. trans isopropyl, cis methyl 4. trans isopropyl, trans methyl #b) Draw the more stable chair conformation for each cis, trans isomer#

Draw chair conformations for each isomer

In this step, chair conformations of the cyclohexane ring should be drawn for each isomer, and the more stable one should be determined for each. Keep in mind that more stable conformations have bulky groups in equatorial positions. 1. cis isopropyl, cis methyl: both groups can occupy equatorial positions in the more stable conformation. 2. cis isopropyl, trans methyl: isopropyl group has an equatorial position and methyl group has an axial position in the more stable conformation. 3. trans isopropyl, cis methyl: isopropyl group has an axial position and methyl group has an equatorial position in the more stable conformation. 4. trans isopropyl, trans methyl: both groups have axial positions in the more stable conformation. #c) Identify the most stable cis, trans isomer (menthol)#

Compare stabilities of cis, trans isomers

The most stable isomer will be the one with the least steric strain, meaning that it has more bulky groups in equatorial positions as opposed to axial positions. Comparing the most stable chair conformations of all four isomers, we can conclude that isomer 1 (cis isopropyl, cis methyl) is the most stable because both substituents are in equatorial positions. Hence, the most stable isomer is cis-2-isopropyl-5-methylcyclohexanol, which is also known as menthol.