Empirical Relationship between the Conformation and the Direction of Carbonyl Group Pyramidalization of Cyclohexanones

A statistical investigation of precise crystal structures from the Cambridge Structural Database
shows that cyclohexanones have a predictable pyramidalization of the carbonyl group in the chair
conformation that depends on the puckering of the ring. Flattened cyclohexanones are usually
“axially pyramidalized” (see Figure ), whereas more strongly puckered cyclohexanones are usually
“equatorially pyramidalized”. It is conceivable that a cooperation between electronic effects
(hyperconjugation) and steric effects (relief from torsional strain) leads to this unique correlation.
No such conformation/pyramidalization correlations are observed for other ketones.