Figure 7.1. Deprotonation of an α-Carbon, Forming a CarbanionThe α-hydrogens of ketones tend to be slightly less acidic than those of aldehydes due to the
electron-donating properties of the additional alkyl group in a ketone. This property is the same
reason that alkyl groups help to stabilize carbocations—but in this case, they destabilize the
carbanion.
KEY CONCEPT
Electron-withdrawing groups like oxygen stabilize organic anions. Electron-donating groups
like alkyl groups destabilize organic anions.STERIC HINDRANCE
In reactions, aldehydes are slightly more reactive to nucleophiles than ketones. This is due in part to
steric hindrance in the ketone, which arises from the additional alkyl group that ketones contain.
When the nucleophile approaches the ketone or aldehyde in order to react, the additional alkyl
groups on the ketone are in the way, more so than the single hydrogen of the aldehyde. This makes
for a higher-energy, more crowded intermediate step. Remember, higher-energy intermediates
mean that the reaction is less likely to proceed.
KEY CONCEPT
Ketones are slightly less likely to react with nucleophiles than aldehydes because the extra
alkyl group destabilizes the carbanion and increases steric hindrance.