Aldehydes and Ketones
Aldehydes (R–CHO) and ketones (R–CO–R′) share the carbonyl (>C=O) group, the most reactive functional group in second-year organic chemistry. The PMDC MDCAT 2026 syllabus expects you to name them, predict their nucleophilic addition products, distinguish them with Tollens and Fehling reagents, and reduce them to alcohols. This is one of the highest-yield organic chapters — expect 2–3 MCQs.
Nomenclature and Structure
The carbonyl carbon is sp2-hybridised, planar, and bears a strong dipole (δ+C=Oδ−). The C=O π bond is polarised: C is electrophilic, O is nucleophilic. This polarisation drives every reaction in the chapter.
Aldehyde nomenclature
- Replace the –e of the parent alkane with –al.
- The –CHO carbon is always C-1.
- Examples: HCHO = methanal (formaldehyde), CH3CHO = ethanal (acetaldehyde), CH3CH2CHO = propanal, C6H5CHO = benzaldehyde.
Ketone nomenclature
- Replace the –e of the parent alkane with –one. Number the chain so that the carbonyl carbon has the lowest locant.
- Examples: CH3COCH3 = propan-2-one (acetone), CH3COCH2CH3 = butan-2-one, C6H5COCH3 = acetophenone (1-phenylethan-1-one).
Preparation
Two of the most reliable preparation routes appear repeatedly in MDCAT MCQs.
1° alcohol + [O] (acidified K2Cr2O7 or PCC) → aldehyde → further oxidation → carboxylic acid.
2° alcohol + [O] → ketone (no further oxidation under mild conditions).
Example: CH3CH2OH ⟶{K2Cr2O7/H+} CH3CHO ⟶{[O]} CH3COOH.
R–CH=CH–R′ ⟶{O3; Zn/H2O} R–CHO + R′–CHO. The position of the double bond is revealed by the carbonyl fragments produced.
R–COCl + H2 ⟶{Pd/BaSO4, quinoline} R–CHO + HCl. The poisoned palladium catalyst stops at the aldehyde stage and prevents over-reduction to the alcohol.
HC≡CH + H2O ⟶{HgSO4/H2SO4} CH3CHO (via vinyl alcohol tautomer). Higher alkynes give ketones according to Markovnikov.
Nucleophilic Addition Reactions
The defining reactivity of carbonyl compounds. A nucleophile attacks the δ+ carbonyl carbon; the π electrons shift to oxygen, which is then protonated. Aldehydes are more reactive than ketones because (i) their carbonyl carbon is less hindered and (ii) only one electron-donating alkyl group is present (so C remains more δ+).
R–CHO + HCN ⟶{CN−} R–CH(OH)CN. The cyanohydrin extends the carbon chain by one and is hydrolysed to an α-hydroxy acid.
R–CHO + NaHSO3 → R–CH(OH)SO3Na (white crystalline solid). Used to purify aldehydes and methyl ketones — reverse the reaction with dilute acid or base.
R′MgX + HCHO → (after H2O) R′CH2OH (1° alcohol).
R′MgX + R–CHO → 2° alcohol.
R′MgX + R–CO–R″ → 3° alcohol.
Carbonyls react with NH2–Z to give imines/oximes/hydrazones with loss of water:
+ NH2OH → oxime.
+ NH2NH2 → hydrazone.
+ 2,4-DNP → orange/yellow 2,4-dinitrophenylhydrazone (test for >C=O).
+ NH2NHCONH2 → semicarbazone.
Oxidation Reactions
The single sharpest distinction between aldehydes and ketones — aldehydes oxidise easily to carboxylic acids, ketones do not (under normal conditions).
Reagent: ammoniacal AgNO3 = [Ag(NH3)2]+.
R–CHO + 2[Ag(NH3)2]+ + 3 OH− → R–COO− + 2 Ag↓ + 4 NH3 + 2 H2O.
A bright silver mirror confirms an aldehyde. Ketones give no mirror.
Reagent: alkaline Cu2+ tartrate complex (Fehling A + B).
R–CHO + 2 Cu2+ + 5 OH− → R–COO− + Cu2O↓ (red) + 3 H2O.
Aliphatic aldehydes give a brick-red Cu2O precipitate; aromatic aldehydes (benzaldehyde) and ketones do not.
CH3CHO and methyl ketones (CH3CO–R) + I2/NaOH → CHI3↓ (yellow crystals) + RCOO−Na+. Diagnostic for the CH3CO– or CH3CH(OH)– fragment.
| Test | Reagent | Aliphatic CHO | Aromatic CHO | Ketones |
|---|---|---|---|---|
| Tollens' | [Ag(NH3)2]+ (ammoniacal AgNO3) | Silver mirror | Silver mirror (still positive) | No reaction |
| Fehling's | Alkaline Cu2+ tartrate | Brick-red Cu2O | No reaction (key distinguisher) | No reaction |
| Benedict's | Alkaline Cu2+ citrate | Brick-red Cu2O | No reaction | No reaction |
| Iodoform | I2 + NaOH | CH3CHO only → yellow CHI3 | No reaction | Only methyl ketones → yellow CHI3 |
| Schiff's | Decolourised fuchsin | Pink colour returns | Pink colour returns | No reaction |
| 2,4-DNP | 2,4-dinitrophenylhydrazine | Yellow/orange precipitate | Yellow/orange precipitate | Yellow/orange precipitate |
Use Tollens to distinguish aldehyde from ketone; use Fehling to distinguish aliphatic from aromatic aldehyde; use 2,4-DNP to confirm any carbonyl group.
Reduction to Alcohols
The reverse of oxidation: hydride is delivered to the carbonyl carbon, giving an alkoxide that is protonated on workup to an alcohol.
R–CHO + NaBH4 ⟶{MeOH/H2O} R–CH2OH (1° alcohol).
R–CO–R′ + NaBH4 → R–CH(OH)–R′ (2° alcohol). Does not reduce esters or carboxylic acids — useful when those groups must be preserved.
Reduces aldehydes, ketones, esters, amides, and carboxylic acids all the way to alcohols (or amines). Used in dry ether under inert atmosphere — reacts violently with water.
R–CHO + H2 ⟶{Ni or Pt, heat/pressure} R–CH2OH. Reduces C=C bonds simultaneously, so it's not selective for the carbonyl.
Reduces >C=O all the way to >CH2:
Clemmensen: Zn(Hg)/conc. HCl (acidic conditions).
Wolff–Kishner: NH2NH2, KOH, ethylene glycol, heat (basic conditions).
Reactivity and Comparison
Aldehydes and ketones share the carbonyl group but differ in steric and electronic environment, leading to four key practical differences.
- Reactivity in nucleophilic addition: Aldehyde > ketone (less steric hindrance; one fewer +I alkyl group).
- Oxidation by Tollens/Fehling/K2Cr2O7: Aldehyde yes; ketone no (under mild conditions).
- Iodoform test: Positive for ethanal and all methyl ketones; negative for other aldehydes/ketones.
- 2,4-DNP: Both give orange/yellow precipitates — confirms a carbonyl group but does not differentiate the two.
- Aldol condensation: Both undergo it if they have an α-H. Compounds without α-H (HCHO, C6H5CHO, (CH3)3CCHO) instead undergo Cannizzaro reaction with strong base.
Worked MCQs
Five MCQs that capture the high-yield testing patterns for this chapter. Read the explanation even when you get the answer right — that's where the deeper concept lives.
Q1. Which of the following gives a silver mirror with Tollens reagent?
Tollens reagent oxidises aldehydes to carboxylates and reduces Ag+ to metallic silver. Propanal (CH3CH2CHO) is an aldehyde; acetone is a ketone, ether is non-reactive, and methanol is an alcohol.
Q2. Reduction of butan-2-one with NaBH4 in methanol gives:
NaBH4 delivers a hydride to the carbonyl carbon; the alkoxide is protonated by methanol on workup. Butan-2-one (CH3COCH2CH3) becomes the secondary alcohol butan-2-ol (CH3CH(OH)CH2CH3).
Q3. Which compound does NOT give a positive iodoform test?
The iodoform test is positive for compounds containing a CH3CO– group or a CH3CH(OH)– group. Propanal (CH3CH2CHO) has neither — its α-carbon is –CH2–, not –CH3.
Q4. Aldehydes are more reactive than ketones in nucleophilic addition because:
Two effects work together: an aldehyde has only one alkyl group donating +I electrons (vs two in a ketone), so its carbonyl C is more δ+; and the H atom takes up less space than a second alkyl group, so the nucleophile approaches more easily.
Q5. Rosenmund reduction converts an acid chloride to an aldehyde using:
Rosenmund uses palladium "poisoned" with sulphur and quinoline so that catalytic hydrogenation halts at the aldehyde stage instead of going on to the alcohol. LiAlH4 would over-reduce to the alcohol; Clemmensen would go all the way to the alkane.
Quick Recap
- Carbonyl C is sp2, planar, δ+; nucleophiles attack here.
- Reactivity order: HCHO > RCHO > RCOR′ (steric + electronic).
- Tollens silver mirror → aldehyde; Fehling red Cu2O → aliphatic aldehyde; iodoform yellow → CH3CO group.
- NaBH4 = mild (carbonyls only); LiAlH4 = strong (all carbonyls including esters/acids).
- Clemmensen (Zn/Hg, HCl) and Wolff–Kishner (NH2NH2, KOH) reduce >C=O all the way to >CH2.
- Cyanohydrin from HCN; bisulphite adduct from NaHSO3 (purification); 2,4-DNP → orange ppt confirms any >C=O.