Answers to Concept Checks
7.1
The α-hydrogens of aldehydes and ketones are acidic, or deprotonate easily, due to both
inductive effects and resonance effects. The electronegative oxygen atom pulls electron density
from the C–H bond, weakening it. Once deprotonated, the resonance stabilization of the negative
charge between the α-carbon, carbonyl carbon, and electron-withdrawing carbonyl oxygen
increases the stability of this form.
The α-hydrogens of aldehydes are slightly more acidic due to the electron-donating
characteristics of the second alkyl group in ketones. This extra alkyl group destabilizes the
carbanion, which slightly disfavors the loss of the α-hydrogens in ketones as compared to
aldehydes.
Steric hindrance is one of the two reasons that aldehydes are slightly more reactive than
ketones. The additional alkyl group gets in the way and makes for a higher-energy, crowded
intermediate.7.2
Tautomers are isomers that can be interconverted by the movement of a hydrogen and a
double bond.
The keto form is thermodynamically favored.
Enolate carbanions act as nucleophiles.
Because the kinetic enolate forms rapidly and can interconvert with the thermodynamic form
if given time, the kinetic form is favored by fast, irreversible reactions, such as with a strong,
sterically hindered base, and lower temperatures. The thermodynamic form, on the other hand,
is favored by slower, reversible reactions, with weaker or smaller bases, and higher temperatures.
7.3
The enolate carbanion acts as the nucleophile (the deprotonated aldehyde or ketone).
The keto form of the aldehyde or ketone acts as the electrophile.
A retro-aldol reaction is the reverse of an aldol reaction and can be favored by the addition of
base and heat. In this reaction, a bond between the α-and β-carbons of a carbonyl is broken.
An aldol condensation is a condensation reaction, in which two molecules are joined to form a