Relative Reactivity of Aldehydes and Ketones towards Nucleophilic Addition

Although every carbonyl compound reacts by the same general mechanism (§8.5), aldehydes and ketones do not react at the same rate: aldehydes are consistently and substantially more reactive towards nucleophilic addition than ketones, for two reasons that reinforce each other.

Steric reasoning. In an aldehyde, $\text{R-CHO}$, one of the two groups on the carbonyl carbon is simply a hydrogen atom -- small, and essentially non-blocking. In a ketone, $\text{R-CO-R}'$, that hydrogen is replaced by a second alkyl or aryl group, which is considerably bulkier. As the nucleophile approaches the carbonyl carbon and as the carbon rehybridises from planar $sp^2$ towards tetrahedral $sp^3$ in the transition state, the two flanking substituents are pushed closer together into that more crowded tetrahedral geometry; the larger the two flanking groups, the more this crowding raises the energy of the transition state and slows the reaction. A ketone, with two carbon substituents instead of one, therefore experiences more steric hindrance to addition than an aldehyde does, and a highly branched ketone reacts slower still.

Electronic reasoning. Alkyl groups are weakly electron-donating (through hyperconjugation and a modest inductive effect), so they push electron density towards whatever they are attached to. An aldehyde has only one electron-donating alkyl group feeding density into the carbonyl carbon, while a ketone has two -- so a ketone's carbonyl carbon carries somewhat more electron density, is somewhat less positively polarised ($\delta^+$), and is therefore a less attractive target for an incoming nucleophile than an aldehyde's carbonyl carbon is. Formaldehyde, $\text{HCHO}$, has no alkyl groups on its carbonyl carbon at all (both substituents are hydrogen), so it is the most reactive carbonyl compound of all towards nucleophilic addition, ahead of every other aldehyde. …