Methoxycarbonylcyclohexane

The methoxycarbonyl group -an ester- derives from the carboxyl but it should be bigger because it bears more atoms. However, the methoxycarbonyl has a conformational energy slightly lower (1.29 kcal/mol) than that of the carboxyl (1.41 kcal/mol). This is apparently contradictory... The carboxyl groups of two different molecules tend to form strong hydrogen bonds that is not possible in the corresponding esters because in the latter the OH groups no longer exist. The carboxyls are actually bigger than the alkoxycarbonyls when they are bonded by H-bonding to another molecule.