NEET Chemistry — Aldehydes Ketones and Carboxylic Acids…

Chapter Overview & Weightage

This is one of the most reaction-dense chapters in Organic Chemistry. Nucleophilic addition to the carbonyl group is the central theme. Named reactions are heavily tested — Aldol, Cannizzaro, Clemmensen, Wolff-Kishner.

This chapter carries 4-5% weightage in NEET with 2-3 questions. Named reactions and distinction between aldehydes/ketones are the most reliable scoring areas.

Year	NEET Q Count	Key Topics Tested
2025	3	Aldol condensation, Tollens test
2024	2	Cannizzaro reaction, acidity of acids
2023	3	Nucleophilic addition, named reactions
2022	2	Clemmensen reduction, carboxylic acid reactions
2021	2	Aldehyde vs ketone tests, Hell-Volhard-Zelinsky

graph TD
    A[Aldehydes Ketones Acids] --> B[Nucleophilic Addition]
    A --> C[Named Reactions]
    A --> D[Distinction Tests]
    A --> E[Carboxylic Acids]
    B --> F[HCN, NaHSO3, Grignard]
    C --> G[Aldol, Cannizzaro]
    C --> H[Clemmensen, Wolff-Kishner]
    C --> I[Cross Aldol, Perkin]
    D --> J[Tollens: Silver Mirror]
    D --> K[Fehling: Red ppt]
    D --> L[Iodoform Test]
    E --> M[Acidity and Substituent Effects]
    E --> N[HVZ Reaction]

Key Concepts You Must Know

Tier 1 (Always asked)

Nucleophilic addition mechanism to C=O
Aldol condensation (requires alpha-hydrogen)
Cannizzaro reaction (NO alpha-hydrogen: disproportionation)
Tollens test (silver mirror for aldehydes) and Fehling test (red precipitate)
Iodoform test (methyl ketones and acetaldehyde)

Tier 2 (Frequently asked)

Clemmensen (Zn-Hg/HCl) vs Wolff-Kishner (NH $_2$ NH $_2$ /KOH) reduction
Carboxylic acid acidity: effect of EWG and EDG
Hell-Volhard-Zelinsky reaction (alpha-halogenation of acids)
Rosenmund reduction (acid chloride to aldehyde)

Tier 3 (Occasional)

Cross Aldol condensation
Perkin reaction, Knoevenagel condensation
Tollen’s reagent: [Ag(NH $_3$ ) $_2$ ] $^+$

Important Formulas

Reaction	Reagent	Product
Aldol	Dilute NaOH	Beta-hydroxy aldehyde/ketone
Cannizzaro	Conc. NaOH	Alcohol + Carboxylate salt
Clemmensen	Zn-Hg/conc. HCl	-CH $_2$ - (reduces C=O to CH $_2$ )
Wolff-Kishner	NH $_2$ NH $_2$ /KOH	-CH $_2$ - (same product, basic conditions)
Rosenmund	H $_2$ /Pd-BaSO $_4$	R-CHO (from R-COCl)
HVZ	Br $_2$ /P, then H $_2$ O	Alpha-bromo acid

Test	Reagent	Positive for	Observation
Tollens	[Ag(NH $_3$ ) $_2$ ] $^+$	Aldehydes	Silver mirror
Fehling	Cu $^{2+}$ in tartrate	Aliphatic aldehydes	Red ppt (Cu $_2$ O)
Iodoform	NaOH + I $_2$	CH $_3$ CO- or CH $_3$ CH(OH)-	Yellow ppt (CHI $_3$ )
2,4-DNP	2,4-dinitrophenylhydrazine	All C=O	Orange/yellow ppt

Aldol needs alpha-H. Cannizzaro happens when there is NO alpha-H. So HCHO (no alpha-H) undergoes Cannizzaro, while CH $_3$ CHO (has alpha-H) undergoes Aldol. This is the single most important distinction in this chapter.

Solved Previous Year Questions

PYQ 1 — NEET 2024

Problem: Which of the following undergoes Cannizzaro reaction? (a) CH $_3$ CHO (b) HCHO (c) CH $_3$ COCH $_3$ (d) C $_2$ H $_5$ CHO

Solution:

Cannizzaro requires no alpha-hydrogen. Only HCHO has no alpha-H.

CH $_3$ CHO and C $_2$ H $_5$ CHO have alpha-H (on the carbon adjacent to C=O) — they undergo Aldol instead.

CH $_3$ COCH $_3$ also has alpha-H.

Answer: (b) HCHO

PYQ 2 — NEET 2023

Problem: Arrange in decreasing order of acidity: CH $_3$ COOH, ClCH $_2$ COOH, FCH $_2$ COOH, HCOOH

Solution:

Acidity depends on stability of carboxylate ion. EWG (electron-withdrawing groups) stabilise the conjugate base:

FCH $_2$ COOH > ClCH $_2$ COOH > HCOOH > CH $_3$ COOH

F is more electronegative than Cl, so fluoroacetic acid is the strongest. CH $_3$ is electron-donating, making acetic acid the weakest.

Difficulty Distribution

Difficulty	% of Questions	What to Expect
Easy	35%	Named reaction identification, test results
Medium	50%	Acidity order, reaction conditions, mechanism
Hard	15%	Cross Aldol, multi-step synthesis

Expert Strategy

Week 1: Named reactions — Aldol, Cannizzaro, Clemmensen, Wolff-Kishner, Rosenmund. Know the reagent, condition, and product for each.

Week 2: Distinction tests — Tollens, Fehling, iodoform, 2,4-DNP. Practise “identify the compound” problems.

Week 3: Carboxylic acid chemistry — acidity comparisons (substituent effects) and HVZ reaction.

Common Traps

Trap 1 — Benzaldehyde undergoes Cannizzaro, not Aldol. Benzaldehyde has no alpha-hydrogen (the carbons adjacent to C=O are part of the ring). So it undergoes Cannizzaro with conc. NaOH, giving benzyl alcohol + sodium benzoate.

Trap 2 — Fehling’s test does not work for aromatic aldehydes. Benzaldehyde gives a negative Fehling test (no red precipitate) but a positive Tollens test. If a question asks which test distinguishes aromatic aldehydes from ketones — the answer is Tollens, not Fehling.

Trap 3 — Iodoform test is positive for ethanol too. Not just methyl ketones — secondary alcohols with the CH $_3$ CH(OH)- group (like ethanol and 2-propanol) also give iodoform test positive because NaOH/I $_2$ first oxidises them to the corresponding carbonyl.