Class 12 — Aldehydes, Ketones and Carboxylic Acids

Chapter Overview & Weightage

This is the highest-weightage organic chapter in Class 12 Chemistry. CBSE allots 8-10 marks to it every year, spread across reaction-prediction questions, distinguishing tests, and IUPAC naming. The chapter pays back study effort like no other in organic.

Year	Marks	Question Type
2024	9	Reaction + distinction + naming
2023	8	Mechanism + acidity comparison
2022	10	Aldol + Cannizzaro
2021	8	Reactivity + tests
2020	9	Mixed reactions

The chapter is essentially “reaction memorisation”. A two-week focused revision before boards puts 8-9 marks within easy reach.

Key Concepts You Must Know

Carbonyl group: $\text{C=O}$ , polar, electrophilic at carbon.
Aldehydes vs ketones: aldehydes have at least one H on the carbonyl carbon; ketones have two carbons attached.
Distinction tests: Tollens’ (silver mirror), Fehling’s (red Cu $_2$ O), iodoform (CHI $_3$ ).
Nucleophilic addition: HCN, NaHSO $_3$ , alcohols, amines.
Aldol condensation: $\alpha$ -H + base $\to$ $\beta$ -hydroxy carbonyl, then dehydration.
Cannizzaro reaction: aldehydes without $\alpha$ -H undergo disproportionation in strong base.
Carboxylic acid acidity: pKa $\sim$ 4-5, increases with EWG, decreases with EDG.
Reactions of $-\text{COOH}$ : salt formation, ester formation (Fischer), reduction.

Important Formulas

For carboxylic acids: $-I$ groups (Cl, NO $_2$ ) increase acidity. $+I$ groups (CH $_3$ , alkyl) decrease acidity.

Order: HCOOH > CH $_3$ COOH > CH $_3$ CH $_2$ COOH (alkyl chain lengthening).

ClCH $_2$ COOH > CH $_3$ COOH (Cl is strongly $-I$ ).

This kind of comparison is a 2-mark gimme on every CBSE paper.

Positive iodoform test: contains $\text{CH}_3\text{CO}-$ or $\text{CH}_3\text{CH(OH)}-$ group.

Examples: acetaldehyde, acetone, ethanol, isopropanol.

Solved Previous Year Questions

PYQ 1 (CBSE 2024)

Distinguish between propanone (acetone) and propanal using a chemical test.

Use Tollens’ reagent. Propanal gives a silver mirror; acetone does not. Aldehydes are oxidised by mild Tollens’; ketones are not.

Alternatively, Fehling’s solution gives red Cu $_2$ O with propanal but no reaction with acetone.

PYQ 2 (CBSE 2023)

Arrange in increasing order of acidic strength: $\text{CH}_3\text{COOH}$ , $\text{ClCH}_2\text{COOH}$ , $\text{Cl}_2\text{CHCOOH}$ , $\text{CH}_3\text{CH}_2\text{COOH}$ .

More chlorines = more $-I$ effect = greater stabilisation of the conjugate base = stronger acid.

Order: $\text{CH}_3\text{CH}_2\text{COOH} < \text{CH}_3\text{COOH} < \text{ClCH}_2\text{COOH} < \text{Cl}_2\text{CHCOOH}$ .

PYQ 3 (CBSE 2022)

Write the product of aldol condensation between two molecules of acetaldehyde.

Acetaldehyde has $\alpha$ -H. In dilute base, two molecules combine: one’s $\alpha$ -carbon attacks the other’s carbonyl, forming 3-hydroxybutanal ( $\text{CH}_3\text{CH(OH)CH}_2\text{CHO}$ ), which dehydrates on heating to but-2-enal ( $\text{CH}_3\text{CH=CHCHO}$ ).

Difficulty Distribution

Sub-topic	Easy	Medium	Hard
Reactions	30%	50%	20%
Distinction tests	60%	30%	10%
Acidity ordering	50%	40%	10%
Mechanisms	20%	50%	30%

Mechanisms (aldol, Cannizzaro) form the harder portion of this chapter.

Expert Strategy

Make a one-page reaction map: aldehyde/ketone in the centre, arrows to all products with reagents on each arrow. Five revisions of this map in the last week before boards = 90% retention.

For acidity questions, always cite the inductive or resonance reason on the side. CBSE awards a mark for the explanation even if the order is partially right.

Practice writing IUPAC names from structures and vice versa daily for two weeks. This is a 2-mark question every year and easy to lose if rusty.

Common Traps

Drawing the aldol product without dehydration. The CBSE keyword “ $\alpha,\beta$ -unsaturated” implies the dehydrated final product. Stopping at the $\beta$ -hydroxy intermediate loses 1-2 marks.

Confusing Tollens’ (positive for aldehydes including aromatic) with Fehling’s (positive only for aliphatic aldehydes — fails for benzaldehyde). NEET 2023 explicitly tested this.

Predicting a positive iodoform test for any methyl ketone — almost right, but ethanol and isopropanol also give positive (because they oxidise to acetaldehyde and acetone respectively in the basic iodine medium). Watch for primary/secondary alcohols.