Reactivity of aldehydes and ketones

Reactivity of aldehydes and ketones toward nucleophiles traces directly back to the carbonyl-group polarity worked out in 12.7: the more electrophilic the carbonyl carbon, the more reactive the compound. In general, ALDEHYDES are MORE reactive than KETONES toward nucleophilic attack, and this difference is explained by TWO combined factors working in the same direction. (1) Electronic effects: an alkyl group has an electron-DONATING inductive effect (+I). A ketone's carbonyl carbon carries TWO such electron-donating alkyl groups, and their combined +I effect measurably reduces the carbonyl carbon's positive polarity (and so its electrophilicity); an aldehyde's carbonyl carbon, by contrast, carries only ONE electron-donating group (plus a plain hydrogen, which has no comparable +I effect), leaving it distinctly MORE electrophilic than a ketone's. (2) Steric effects: a ketone's two BULKY alkyl groups physically get in the way of an incoming nucleophile trying to approach the carbonyl carbon -- called steric hindrance to nucleophilic attack. An aldehyde's carbonyl carbon, carrying just one alkyl group (and one small hydrogen instead of a second alkyl group), is correspondingly less crowded and so more easily approached and attacked by a nucleophile. A 'Remember' box adds one further, separate comparison: AROMATIC aldehydes are LESS reactive than ALIPHATIC aldehydes in nucleophilic addition reactions specifically, becau …