ChemistryMediumClass 12

Relative Reactivity — Aldehydes vs ketones

Aldehydes, Ketones & Carboxylic Acids

JEE Qs

Hard

min

Always analyze both steric hindrance and electronic effects (especially inductive and resonance) on the carbonyl carbon's electrophilicity to predict relative reactivity accurately for nucleophilic addition.

🧮 Key Formulas

Reactivity (towards Nucleophilic Addition) α Electrophilicity of Carbonyl Carbon

Electrophilicity of Carbonyl Carbon α 1 / (Steric Hindrance at Carbonyl Carbon)

Electrophilicity of Carbonyl Carbon α 1 / (Electron-donating effects by attached groups)

Aldehyde: R-CHO (one alkyl/aryl group, one hydrogen)

Ketone: R-CO-R' (two alkyl/aryl groups)

✅ Key Points for JEE

1Aldehydes are generally more reactive than ketones towards nucleophilic addition reactions.
2Steric Hindrance: Ketones (R-CO-R') have two bulkier alkyl/aryl groups hindering the approach of a nucleophile to the carbonyl carbon more than aldehydes (R-CHO) which have only one alkyl/aryl group and a smaller hydrogen atom.
3Electronic Effects: Alkyl groups are electron-donating (+I effect). Ketones have two such groups, which collectively reduce the partial positive charge on the carbonyl carbon more effectively than the single alkyl group in aldehydes, thereby decreasing its electrophilicity.
4Formaldehyde (HCHO) is the most reactive carbonyl compound due to minimal steric hindrance (two hydrogens) and absence of electron-donating alkyl groups.
5Electron-withdrawing groups on the R/R' groups (especially in aromatic systems) increase the electrophilicity of the carbonyl carbon and thus increase reactivity, while electron-donating groups decrease it.

⚠️ Common Mistakes

✕Incorrectly attributing increased reactivity to increased steric hindrance, or confusing the roles of steric and electronic factors.
✕Failing to consider both steric and electronic effects simultaneously when comparing reactivity; sometimes one effect might be dominant but both contribute.
✕Not recognizing that reactivity comparisons are primarily for nucleophilic addition reactions at the carbonyl carbon and not other types of reactions.

NCERT Chapters

Class 12 Chemistry Part II Ch 12: Aldehydes, Ketones and Carboxylic Acids