Nucleophilic Addition Reactions of Aldehydes & Ketones | chemca

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Nucleophilic Addition Reactions

The characteristic reaction of the Carbonyl Group ($>C=O$).

By chemca Team • Updated Jan 2026

Unlike alkenes which undergo electrophilic addition, aldehydes and ketones undergo nucleophilic addition. This is because the carbonyl carbon is electron-deficient (electrophilic) due to the polarity of the C-O bond.

1. Mechanism of Nucleophilic Addition

Step-by-Step

1. Attack of Nucleophile: The nucleophile ($Nu^-$) attacks the electrophilic carbon perpendicular to the plane of the $sp^2$ hybridized orbitals.
2. Hybridization Change: The carbon changes from planar $sp^2$ to tetrahedral $sp^3$. The $\pi$-electrons shift to Oxygen.
3. Protonation: The resulting alkoxide intermediate captures a proton from the medium to form the addition product.

$$ \underset{\text{Planar}}{>C=O} + Nu^- \rightleftharpoons \underset{\text{Intermediate}}{>C(O^-)Nu} \xrightarrow{H^+} \underset{\text{Addition Product}}{>C(OH)Nu} $$

2. Reactivity: Aldehydes vs Ketones

Aldehydes > Ketones

Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to two main reasons:

1. Steric Reasons

Ketones have two bulky alkyl groups hindering the approach of the nucleophile. Aldehydes have only one alkyl group and one small hydrogen.

2. Electronic Reasons

Alkyl groups are electron-releasing (+I). Two alkyl groups in ketones reduce the positive charge on Carbon more effectively than one group in aldehydes, making the carbon less electrophilic.

Reactivity Order: $HCHO > RCHO > RCOR > ArCOR$

3. Important Addition Reactions

A. Addition of HCN

Reaction with Hydrogen Cyanide yields Cyanohydrins. This reaction is catalyzed by a base (to generate $CN^-$).

$$ >C=O + HCN \xrightleftharpoons{OH^-} >C(OH)CN $$

B. Addition of Sodium Hydrogen Sulphite

Reaction with $NaHSO_3$ gives a crystalline bisulphite addition compound. This is used for the separation and purification of aldehydes/ketones.

$$ >C=O + NaHSO_3 \rightleftharpoons >C(OH)SO_3Na $$

Note: Sterically hindered ketones (e.g., Diethyl ketone) do not give this reaction.

C. Addition of Grignard Reagents

Forms Alcohols (discussed in Alcohol preparation).

$$ >C=O + RMgX \rightarrow >C(OMgX)R \xrightarrow{H_3O^+} >C(OH)R $$

D. Addition of Alcohols

Aldehydes react with 1 eq. of alcohol to form Hemiacetal (unstable) and with 2 eq. to form Acetal (stable) in the presence of dry HCl.

$$ RCHO \xrightarrow{R'OH, HCl} \text{Hemiacetal} \xrightarrow{R'OH, HCl} \text{Acetal} $$

Ketones: React with diols (like Ethylene Glycol) to form cyclic Ketals.

4. Addition of Ammonia Derivatives

Addition-Elimination

Reaction with compounds of the type $H_2N-Z$ occurs in weakly acidic medium (optimum pH 3.5). Water is eliminated.

$$ >C=O + H_2N-Z \rightleftharpoons [>C(OH)NHZ] \xrightarrow{-H_2O} >C=N-Z $$

Reagent ($H_2N-Z$)	Product Name	Structure ($>C=N-Z$)
Ammonia ($NH_3$)	Imine	$>C=NH$
Amine ($R-NH_2$)	Schiff's Base	$>C=N-R$
Hydroxylamine ($NH_2OH$)	Oxime	$>C=N-OH$
Hydrazine ($NH_2NH_2$)	Hydrazone	$>C=N-NH_2$
Phenylhydrazine	Phenylhydrazone	$>C=N-NHPh$
2,4-DNP	2,4-DNP Hydrazone	Orange Precipitate

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