NEET Chem — Aldehydes, Ketones and Acids

Chapter Overview & Weightage

Aldehydes, ketones, and carboxylic acids form a high-yield Class 12 organic chapter. NEET pulls $2-3$ questions per year, mostly on named reactions, distinguishing tests, and acidity/reactivity comparisons.

Typical NEET weightage: $2-3$ questions per year.

Year	NEET Qs	Question type
2020	3	Distinguishing tests, acidity order
2021	2	Cannizzaro reaction, aldol condensation
2022	3	Mechanism of nucleophilic addition
2023	2	Carboxylic acid acidity
2024	3	Named reactions (Etard, Stephen, HVZ)

Key Concepts You Must Know

Nucleophilic addition mechanism for aldehydes/ketones
Reactivity order: aldehydes > ketones (steric and electronic)
Acidity of carboxylic acids; effect of substituents
Named reactions: Cannizzaro, aldol, Clemmensen, Wolff-Kishner, HVZ, Rosenmund, Stephen, Etard
Distinguishing tests: Tollens’, Fehling’s, Schiff’s, iodoform
Decarboxylation reactions
$\alpha$ -halogenation of carboxylic acids (HVZ)

Important Formulas

$HCOOH > CH_3COOH > CH_3CH_2COOH > (CH_3)_3CCOOH$

Electron-donating groups decrease acidity; electron-withdrawing groups (Cl, NO $_2$ ) increase acidity.

$HCHO > CH_3CHO > CH_3COCH_3 > (CH_3)_3CCOCH_3$

More alkyl groups = more steric hindrance + less electrophilic carbonyl C.

Test	Positive for
Tollens’ (silver mirror)	Aldehydes
Fehling’s (red ppt)	Aliphatic aldehydes
Schiff’s (pink)	Aldehydes
Iodoform (yellow ppt)	Methyl ketones, acetaldehyde, ethanol, isopropanol
2,4-DNP (orange ppt)	All carbonyls (aldehydes + ketones)

Solved Previous Year Questions

PYQ 1 (NEET 2024)

The Etard reaction converts toluene into:

Etard reagent ( $CrO_2Cl_2$ ) selectively oxidises the methyl group of toluene to an aldehyde, giving benzaldehyde ( $C_6H_5CHO$ ).

PYQ 2 (NEET 2023)

Arrange in increasing order of acidity: $CH_3COOH$ , $ClCH_2COOH$ , $Cl_2CHCOOH$ , $Cl_3CCOOH$ .

More chlorines = more electron-withdrawing = greater acidity.

$CH_3COOH < ClCH_2COOH < Cl_2CHCOOH < Cl_3CCOOH$ .

PYQ 3 (NEET 2022)

Which compound undergoes Cannizzaro reaction?

Aldehydes without $\alpha$ -H undergo Cannizzaro (disproportionation in conc. NaOH). Examples: HCHO, $C_6H_5CHO$ , $(CH_3)_3CCHO$ . Aldehydes with $\alpha$ -H undergo aldol condensation instead.

Difficulty Distribution

Difficulty	% of NEET Qs	Typical type
Easy	$40\%$	Named reaction product identification
Medium	$50\%$	Acidity/reactivity order
Hard	$10\%$	Mechanism reasoning, multi-step synthesis

Expert Strategy

For NEET, memorise 8 named reactions: Cannizzaro, aldol, Clemmensen, Wolff-Kishner, Rosenmund, Stephen, Etard, HVZ. Each appears every 2-3 years.

The $\alpha$ -H rule splits aldehydes cleanly: with $\alpha$ -H → aldol; without $\alpha$ -H → Cannizzaro. This single test answers many MCQs.

For acidity comparisons, the order is: substituted carboxylic acids (with EWG) > formic acid > aliphatic acids (more alkyl = less acidic). Memorise the trend, not individual values.

Common Traps

Saying acetone gives a positive Tollens’ test. Acetone is a ketone — no Tollens’ reaction. Only aldehydes do.

Confusing Clemmensen and Wolff-Kishner. Both reduce $C=O$ to $CH_2$ . Clemmensen uses Zn-Hg/HCl (acidic). Wolff-Kishner uses $NH_2NH_2$ /KOH (basic). Choose based on substrate stability.

Treating Rosenmund and Stephen as the same. Rosenmund: acid chloride + $H_2$ on Pd-BaSO $_4$ → aldehyde. Stephen: nitrile + $SnCl_2$ /HCl → aldehyde. Different starting materials.

Saying iodoform is given by all ketones. Only methyl ketones (with $CH_3CO-$ group) give iodoform.