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Nucleophilic Addition Reactions

The characteristic reactions of Carbonyl Compounds ($>C=O$).

By chemca Team • Updated Jan 2026

Unlike alkenes which undergo electrophilic addition, aldehydes and ketones undergo Nucleophilic Addition. This is driven by the polarity of the carbonyl group, where the carbon atom is electron-deficient (electrophilic).

1. Reaction Mechanism

Step-by-Step

1. Nucleophilic Attack (Slow/RDS): The nucleophile ($Nu^-$) attacks the electrophilic carbon perpendicular to the plane of the $sp^2$ orbitals. The hybridization changes from $sp^2$ to $sp^3$, forming a tetrahedral alkoxide intermediate.
2. Protonation (Fast): The intermediate captures a proton ($H^+$) from the solvent to form the electrically neutral addition product.

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

Stereochemistry: Since the carbonyl carbon is planar, the nucleophile can attack from either face. If the product contains a chiral center, a Racemic Mixture is obtained.

2. Reactivity: Aldehydes vs Ketones

Why are Aldehydes more reactive?

Aldehydes are generally more reactive than ketones towards nucleophilic addition due to two factors:

1. Steric Effect

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

2. Electronic Effect (+I)

Alkyl groups release electrons. Two alkyl groups in ketones reduce the positive charge on the carbonyl carbon more than one group in aldehydes, making the carbon less electrophilic.

Reactivity Order: $HCHO > RCHO > ArCHO > R_2CO > ArCOR > Ar_2CO$

3. Important Addition Reactions

A. Addition of HCN

Yields Cyanohydrins. Reaction is catalyzed by base ($OH^-$) to generate the stronger nucleophile $CN^-$ from weak acid HCN.

$$ >C=O + HCN \xrightleftharpoons{pH \approx 9-10} >C(OH)CN $$

B. Addition of Sodium Bisulphite ($NaHSO_3$)

Yields crystalline Bisulphite Adduct. Used for purification of aldehydes and methyl ketones. (Sterically hindered ketones do not react).

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

C. Addition of Alcohols

Aldehydes: React with 1 eq. alcohol $\to$ Hemiacetal (Unstable) $\to$ Acetal (Stable). Requires Dry HCl gas.

Ketones: React with Ethylene Glycol $\to$ Cyclic Ketal (Protecting group).

$$ RCHO + 2R'OH \xrightarrow{Dry HCl} RCH(OR')_2 + H_2O $$

D. Addition of Grignard Reagent

Nucleophilic addition of carbanion ($R^-$) followed by hydrolysis yields Alcohols ($1^\circ, 2^\circ, 3^\circ$).

4. Addition-Elimination (Ammonia Derivatives)

Condensation Reactions

Reactions with $H_2N-Z$ type compounds occur in weakly acidic medium (pH 3.5). The nucleophilic addition is followed by the elimination of a water molecule.

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

Reagent ($H_2N-Z$)	Name	Product ($>C=N-Z$)
$NH_2OH$	Hydroxylamine	Oxime
$NH_2-NH_2$	Hydrazine	Hydrazone
$NH_2-NH-C_6H_5$	Phenylhydrazine	Phenylhydrazone
$NH_2-R$	Primary Amine	Schiff's Base (Imine)
2,4-DNP	Brady's Reagent	2,4-DNP Hydrazone (Red/Orange ppt)

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