1. The problem statement, all variables and given/known data Which would have the most negative heat of combustion? Consider only the best conformer. cis-1,3-dimethylcyclohexane trans-1,4-dimethylcyclohexane 2. Relevant equations The more negative the heat of combustion, the more exothermic the combustion. We are given trans and cis configurations here. This is a sign to pay attention to steric hindrances. 3. The attempt at a solution Okay, I attempted drawing out the cyclohexanes. I know that the chair conformation is the most stable cyclohexane conformation so I drew out two chairs. I know that cis means on the same size so I first drew the chair for cis-1,3-dimethylcyclohexane with two axial methyl substituents both pointing up. This however would create 1,3 diaxial interactions (i.e. crowding with the bulky methyl groups). However, depending on how the ring is numbered, one can have an up equatorial methyl group and an up axial equatorial methyl group. This would eliminate the strain caused by placing methyl groups too close too each other (steric strain). Similarly in trans-1,4-dimethylcyclohexane, we can have an up axial methyl group and a down axial methyl group. Or if we renumber the ring as to get the most stable/best conformer, we can have a down equatorial methyl group and an up equatorial methyl group. This conformation similarly lacks steric strain. So wouldn't the heats of combustions be about the same?