Aldehydes and Ketones — Part 2: Reactions, mechanisms and tests: visual lesson, ISC Class 12 Chemistry

Lesson

Reactions of carbonyl compounds

Part 2 — addition to C=O, oxidation and reduction, reactions of α-hydrogens, and tests.

In this lesson you will

Draw the mechanism of nucleophilic addition of HCN and explain why aldehydes are more reactive than ketones
Write additions of NaHSO₃, Grignard reagents and ammonia derivatives
Write oxidation, iodoform and reduction reactions (Clemmensen, Wolff–Kishner, red P/HI)
Draw the aldol mechanism; predict aldol, cross aldol or Cannizzaro; use tests to distinguish carbonyl compounds

Diagram

Nucleophilic addition and reactivity

A nucleophile attacks the carbonyl carbon; carbon becomes sp³ in a tetrahedral alkoxide, which is protonated. Reactivity: HCHO > RCHO > R₂CO: alkyl groups (+I) reduce the δ+ on carbon and sterically hinder attack.

Concept

Mechanism: addition of HCN

A trace of base generates CN⁻: $\ce{HCN + OH- -> CN- + H2O}$ . (1) CN⁻ attacks the carbonyl carbon (slow), giving a tetrahedral alkoxide. (2) The alkoxide takes H⁺ from HCN (fast), regenerating CN⁻. Product: a cyanohydrin, which can be hydrolysed to an α-hydroxy acid.

\ce{CH3CH2CHO + HCN -> CH3CH2CH(OH)CN}

\ce{CH3COCH3 + HCN -> (CH3)2C(OH)CN}

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