Questions
5–7 MCQs per paper
Difficulty
Medium-Hard
Importance
High yield for JEE Advanced and NEET
Overview
Aldehydes, Ketones, and Carboxylic Acids form the backbone of organic chemistry, focusing on the reactivity of the polar carbonyl group. Mastery of this topic is essential for competitive exams as it integrates mechanistic understanding of nucleophilic addition, resonance-stabilization, and complex name reactions.
Nucleophilic Addition to Carbonyls
The carbonyl carbon is electrophilic due to polarization, making it highly susceptible to nucleophilic attack. Reactivity depends on steric hindrance and electronic effects, where aldehydes are generally more reactive than ketones.
- Reactivity order: HCHO > RCHO > R2CO > Ar2CO
- Electron-withdrawing groups increase reactivity
- Formation of cyanohydrins and acetals
- Addition of Grignard reagents yields alcohols
- Schiff's base formation with primary amines
Named Reactions: Aldol & Cannizzaro
These are the two pillars of carbonyl condensation and redox chemistry. Aldol condensation requires alpha-hydrogens, whereas Cannizzaro reaction occurs in aldehydes lacking alpha-hydrogens under strong basic conditions.
- Aldol: Requires alpha-H and dilute NaOH
- Cross-Aldol: Mixtures lead to four possible products
- Cannizzaro: Disproportionation to alcohol and acid salt
- Intramolecular Aldol used for ring closure
- Claisen-Schmidt condensation for aromatic aldehydes
Reduction Mechanisms
Carbonyl reduction techniques allow for the transformation of aldehydes and ketones into alkanes or alcohols. Clemmensen and Wolff-Kishner reductions are critical for synthetic pathways where oxygen removal is required.
- Clemmensen: Zn-Hg/HCl (acidic conditions)
- Wolff-Kishner: NH2NH2/KOH (basic conditions)
- NaBH4 reduces aldehydes/ketones but not esters
- LiAlH4 is a powerful reducing agent for all carbonyls
- Rosenmund reduction for acid chlorides to aldehydes
Acidity of Carboxylic Acids
The acidity of carboxylic acids is determined by the stability of the carboxylate anion via resonance. Substituent effects significantly alter the pKa value, serving as a common testing ground for inductive and mesomeric effects.
- Electron-withdrawing groups (EWG) increase acidity
- Electron-donating groups (EDG) decrease acidity
- Ortho-effect in benzoic acid increases acidity
- Order: Formic > Benzoic > Aliphatic acids
- Decarboxylation via soda-lime (NaOH + CaO)
Formula Sheet
R-CHO + H2O <-> R-CH(OH)2
R-COOH + R'-OH -> R-COOR' + H2O
pKa = -log[Ka]
R-CH2-CHO -> (Aldol) -> R-CH2-CH(OH)-CH(R)-CHO
Exam Tip
Always identify the alpha-hydrogen count first to instantly eliminate between Aldol and Cannizzaro mechanisms in MCQs.
Common Mistakes
- Confusing the reactivity order of nucleophilic addition with electrophilic substitution.
- Forgetting that Clemmensen reduction is acid-sensitive, making it unsuitable for acid-labile groups.
- Neglecting the ortho-effect while comparing pKa of substituted benzoic acids.
More Revision Notes
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