Work, Energy, and Power
The Chemical Bond
Chemical Thermodynamics and the Equilibrium State
Reactions of Aldehydes and Ketones
|An aldehyde or ketone in aqueous solution exists in an equilibrium with its hydrate (the adduct of the aldehyde or ketone with water). The position of equilibrium depends on the degree of substitution of the aldehyde or ketone. The more highly substituted the aldehyde or ketone, the less hydrate is present at equilibrium. The determination of comparative equilibrium position rests on two influences, both of which operate so that more highly substituted aldehydes and ketones are less likely to form hydrates.|
First, the aliphatic groups donate electrons by induction. The shift of negative electron density toward the positive pole of the carbonyl carbon represents on the molecular level a decrease in electrostatic potential energy for highly substituted aldehydes and ketones, an internal energy decrease. A more substituted aldehyde or ketone possesses less free energy compared to its hydrate, moving equilibrium away from hydration.
The second reason that greater substitution makes the hydrate less favorable is that as an aldehyde or ketone interconverts with its hydrate, its geometry shifts from trigonal planar to tetrahedral, and in the new geometry, steric hindrance is more of a factor in determining the internal energy of the substance. In other words, the more highly substituted the aldehyde or ketone, the higher the internal energy of the hydrate product because of steric hindrance. This factor acts to increase the relative free energy of the product of hydration for highly substituted aldehydes and ketones. In summary, with greater substitution, induction by the carbonyl group works to decrease the free energy of the aldehyde/ketone and steric hindrance works to increase the free energy of the hydrate, both of which push the equilibrium away from the hydrate for highly substituted aldehydes and ketones.