Why are aldehydes more reactive than ketones in nucleophilic addition?

Two reasons: ketones have an extra alkyl group donating electrons (lowering electrophilicity of the carbonyl C), and the second alkyl group sterically blocks nucleophile approach.

Why doesn't benzaldehyde give a positive Fehling's test?

Aromatic aldehydes are not strong enough reducing agents to reduce Cu2+ in tartrate complex; they do however give positive Tollens' (Ag+ is more easily reduced than Cu2+).

What is the difference between Clemmensen and Wolff-Kishner reduction?

Both reduce C=O to CH2. Clemmensen uses Zn-Hg amalgam in concentrated HCl (acidic); Wolff-Kishner uses hydrazine and base/heat. Use Clemmensen for base-sensitive substrates, Wolff-Kishner for acid-sensitive ones.

Why must a carbonyl have alpha-H for aldol reaction?

The alpha-H is removed by base to generate the enolate (the nucleophile). No alpha-H means no enolate, no aldol; instead, molecules with -OH or with hydride donors like HCHO drive Cannizzaro.

What does 2,4-DNPH test for?

Any aldehyde or ketone — it forms an orange/red 2,4-dinitrophenylhydrazone precipitate. It does not distinguish aldehyde from ketone (use Tollens or Fehling for that).

Which compound gives a positive Tollens' test but a negative iodoform test?

Propanal. Propanal (CH3CH2CHO) is an aldehyde (positive Tollens) but has no CH3-CO- pattern (negative iodoform).

Cannizzaro reaction occurs with:

Benzaldehyde. Benzaldehyde has no alpha-H, so it cannot aldolize and undergoes Cannizzaro disproportionation instead.

The product of aldol condensation of two acetone molecules followed by dehydration is:

Mesityl oxide (4-methylpent-3-en-2-one). Aldol gives diacetone alcohol; dehydration gives mesityl oxide ((CH3)2C=CHCOCH3).

Which reagent reduces an aldehyde to a primary alcohol but does not touch an ester?

NaBH4. NaBH4 is selective for aldehydes/ketones; LiAlH4 reduces both esters and aldehydes.

2-hydroxy-2-methylpropanenitrile. (CH3)2C=O + HCN -> (CH3)2C(OH)CN, IUPAC name 2-hydroxy-2-methylpropanenitrile.

Aldehydes and Ketones MDCAT 2026 — Notes, Key Concepts & MCQs (Chemistry)

Aldehydes and Ketones

Aldehydes and Ketones averages 2 MCQs per MDCAT paper, focused on Tollens, Fehling, Aldol, Cannizzaro, and nucleophilic addition mechanisms.

Aldehydes and Ketones is a Chemistry chapter on the official PMDC MDCAT 2026 syllabus, contributing roughly 2 MCQs to the 45-MCQ Chemistry section. Mastering the core concepts below typically secures the full chapter weightage.

The carbonyl group: structure and polarity

The C=O group is sp², planar, with a strong dipole (μ ≈ 2.4 D) because oxygen's electronegativity polarises the π-bond. The carbon is δ+ and electrophilic; the oxygen is δ− and nucleophilic. Aldehydes RCHO are more reactive than ketones R₂C=O for two reasons: (1) the second alkyl group in ketones donates electrons, decreasing electrophilicity, and (2) two alkyl groups create steric crowding for incoming nucleophiles. Formaldehyde is the most reactive carbonyl. Bond length C=O ≈ 1.22 Å; bond enthalpy ~745 kJ/mol. The FSc Punjab Textbook Chemistry XII Chapter 13 and Clayden Chapters 6, 9-11 are the canonical references.

Nucleophilic addition — the master reaction

Nu⁻ attacks the carbonyl carbon; the π-bond breaks; the oxygen becomes O⁻ (alkoxide), which protonates to give -OH. With HCN: cyanohydrin formation gives R₂C(OH)CN (a chiral product if R₁≠R₂). With NaHSO₃: bisulphite addition gives a crystalline R₂C(OH)SO₃Na — used to purify aldehydes. With Grignard RMgX: 1°/2°/3° alcohols depending on the carbonyl. With NaBH₄ or LiAlH₄: hydride reduction to alcohols (NaBH₄ is selective for carbonyls, LiAlH₄ also reduces esters, acids, amides, nitriles).

Tollens, Fehling, Benedict — distinguishing tests

Tollens' reagent ([Ag(NH₃)₂]⁺): aldehydes reduce Ag⁺ to metallic Ag (silver mirror); ketones do not. Even formic acid gives a positive Tollens. Fehling's reagent (Cu²⁺ in alkaline tartrate): aldehydes (except aromatic ones like benzaldehyde) reduce Cu²⁺ to red Cu₂O. Benedict's solution is a milder version. The 2,4-DNPH test (orange-red precipitate) confirms a carbonyl group (aldehyde or ketone) but does not distinguish them. Iodoform (I₂/NaOH) is positive for methyl ketones (CH₃COR) and acetaldehyde — gives yellow CHI₃.

Aldol and Cannizzaro

Aldol condensation (base-catalysed): the α-C of one aldehyde/ketone is deprotonated to an enolate, which attacks the carbonyl C of another molecule; gives a β-hydroxy carbonyl. On heating, dehydration gives an α,β-unsaturated carbonyl. Two molecules of acetaldehyde → 3-hydroxybutanal (aldol) → but-2-enal (crotonaldehyde) on heating. Carbonyls without α-H (formaldehyde, benzaldehyde, trimethylacetaldehyde) cannot aldolize but undergo Cannizzaro disproportionation: 2 PhCHO + NaOH → PhCH₂OH + PhCOO⁻Na⁺. The crossed Cannizzaro of formaldehyde with another no-α-H aldehyde uses HCHO as the hydride donor (always oxidised to formate).

Other named reactions and oxidation

Clemmensen reduction (Zn-Hg/HCl) and Wolff-Kishner (NH₂NH₂/KOH/heat) both reduce C=O to CH₂. Clemmensen is acidic, Wolff-Kishner basic — choose the one compatible with sensitive groups. Haloform reaction: methyl ketones + X₂/OH⁻ → CHX₃ + carboxylate. Beckmann rearrangement of oximes gives amides. Aldehydes are easily oxidised to carboxylic acids by KMnO₄, K₂Cr₂O₇, or even air; ketones resist oxidation unless C-C bonds are broken under harsh conditions. Morrison & Boyd Chapter 19 collates these mechanisms.

Key Concepts

Worked MCQs

Frequently Asked Questions

How Aldehydes and Ketones Is Tested

MDCAT questions on Aldehydes and Ketones are a mix of recall (definitions, classifications), application (predict outcomes, interpret diagrams), and basic numerical/analytical reasoning. PMDC papers from 2020–2025 emphasized the concepts above; older UHS papers (2008–2019) tested them too, with slight variations in question framing.