12 min readPathway sheet1-mark MCQ on reactivity order; 2-mark 'why aldehydes are more reactive than ketones'; 3-mark mechanism or products with HCN, NaHSO₃, alcohols and ammonia derivatives; also a NEET favourite

Nucleophilic addition and addition-elimination reactions

A nucleophile attacks the δ+ carbonyl carbon, giving a tetrahedral intermediate that picks up H⁺. Aldehydes react faster than ketones, for steric and electronic reasons.

Mechanism in one lineNu⁻ attacks the C=O carbon → tetrahedral alkoxide (sp² becomes sp³) → takes H⁺ → addition product

Reactivity orderHCHO > CH₃CHO > CH₃COCH₃; propanal > benzaldehyde

Cyanohydrin>C=O + HCN → >C(OH)CN, speeded up by a base (OH⁻ makes CN⁻)

2,4-DNP testAldehydes and ketones give yellow, orange or red 2,4-dinitrophenylhydrazones

Mechanism of nucleophilic addition

1
The nucleophile (Nu⁻) approaches the planar, δ+ carbonyl carbon from above or below, at right angles to the plane of its sp² orbitals.
The π electrons of C=O move on to the oxygen
2★
>C=O + Nu⁻ → >C(Nu)-O⁻
A tetrahedral alkoxide intermediate forms; the carbon changes from sp² to sp³
3
>C(Nu)-O⁻ + H⁺ → >C(Nu)-OH
The intermediate takes a proton from the medium and gives the neutral addition product
4
Example: CH₃CHO + HCN → (OH⁻) CH₃CH(OH)CN
Ethanal cyanohydrin, 2-hydroxypropanenitrile

★ key step

Why aldehydes react faster than ketones

Steric reasonA ketone has two alkyl or aryl groups on the C=O carbon; an aldehyde has one group and an H. The bulky groups of a ketone hinder the approach of the nucleophile.

Electronic reasonAlkyl groups push electrons towards the C=O carbon (+I effect). Two alkyl groups make a ketone's carbon less δ+, so less electrophilic.

Benzaldehyde is less reactive than propanal: by resonance, the benzene ring pushes electron density to the C=O carbon and makes it less electrophilic. Learn: HCHO > CH₃CHO > CH₃COCH₃, and CH₃CH₂CHO > C₆H₅CHO.

Addition reactions

1★
HCN: >C=O + HCN ⇌ >C(OH)CN (cyanohydrin)
Slow with pure HCN. A base speeds it up: HCN + OH⁻ ⇌ CN⁻ + H₂O, and CN⁻ is a stronger nucleophile. Cyanohydrins are useful in synthesis
2★
Sodium hydrogensulphite: >C=O + NaHSO₃ ⇌ >C(OH)SO₃Na (addition product)
Equilibrium lies to the right for most aldehydes and to the left for most ketones (steric). Dilute acid or alkali gives back the carbonyl compound, so this separates and purifies aldehydes
3
Grignard reagent: >C=O + RMgX → >C(R)OMgX → (H₃O⁺) >C(R)OH
Methanal gives a primary alcohol, other aldehydes secondary, ketones tertiary alcohols
4
Alcohols (aldehydes): RCHO + R'OH ⇌ (dry HCl) RCH(OH)OR' (hemiacetal); RCH(OH)OR' + R'OH ⇌ (dry HCl) RCH(OR')₂ + H₂O (acetal)
Dry HCl protonates the C=O oxygen and makes the carbon more electrophilic
5
Ethylene glycol (ketones): R₂C=O + HOCH₂CH₂OH ⇌ (dry HCl) cyclic ethylene glycol ketal + H₂O
Aqueous mineral acid hydrolyses acetals and ketals back to the carbonyl compound

★ key step

Addition-elimination with ammonia derivatives (H₂N-Z)

1
>C=O + H₂N-Z → >C(OH)-NH-Z
Addition: the lone pair on N attacks the C=O carbon
2★
>C(OH)-NH-Z → >C=N-Z + H₂O
Elimination: a molecule of water is lost, giving a C=N bond
3
Overall: >C=O + H₂N-Z ⇌ (H⁺) >C=N-Z + H₂O
Reversible and catalysed by acid. Too much acid protonates the N of H₂N-Z so it can no longer attack, so the pH is carefully controlled
4
Example: CH₃CHO + H₂NOH → (H⁺) CH₃CH=NOH + H₂O
Ethanal oxime

★ key step

Ammonia derivatives and their products

Reagent, H₂N-Z	Z	Product, >C=N-Z
Amine, RNH₂	-R	Substituted imine (Schiff's base)
Hydroxylamine, NH₂OH	-OH	Oxime
Hydrazine, NH₂NH₂	-NH₂	Hydrazone
Phenylhydrazine, C₆H₅NHNH₂	-NHC₆H₅	Phenylhydrazone
2,4-Dinitrophenylhydrazine (2,4-DNP)	-NH-C₆H₃(NO₂)₂	2,4-Dinitrophenylhydrazone: yellow, orange or red crystals; used to identify aldehydes and ketones
Semicarbazide, NH₂CONHNH₂	-NHCONH₂	Semicarbazone

In real life

Food scientists can measure the aldehydes and ketones that build up in frying oil reused many times by reacting them with 2,4-DNP. A high 'carbonyl value' shows the oil is breaking down.

Exam angle

2 marks: “Why are aldehydes generally more reactive than ketones in nucleophilic addition reactions?” Steric: two bulky groups in a ketone hinder the nucleophile; an aldehyde has one group and an H. Electronic: two alkyl groups (+I) reduce the δ+ charge on the ketone's carbonyl carbon.

3 marks: “Write the mechanism of addition of HCN to ethanal. Why is the reaction faster when a little base is added?” HCN + OH⁻ ⇌ CN⁻ + H₂O. CN⁻ attacks the C=O carbon → tetrahedral alkoxide (sp² → sp³) → takes H⁺ → CH₃CH(OH)CN. Base raises the concentration of CN⁻, the stronger nucleophile.

3 marks: “Account for: (i) NaHSO₃ is used to purify aldehydes (ii) in semicarbazide, H₂N-CO-NH-NH₂, the NH₂ group bonded to C=O does not attack a carbonyl compound (iii) the pH is controlled when ammonia derivatives react with carbonyl compounds.” (i) Forms a water-soluble addition product; dilute acid or alkali gives back the pure aldehyde. (ii) That NH₂ shares its lone pair with C=O by resonance, so it is not nucleophilic; the NH₂ at the NH-NH₂ end attacks. (iii) Acid activates C=O, but too much acid protonates the N of the reagent.

Remember it

Attack, then add H⁺. Fewer and smaller groups on the C=O carbon means faster attack: HCHO > CH₃CHO > CH₃COCH₃.