NaBH4 (Sodium Borohydride): Selective Reduction | Chemca

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Sodium Borohydride ($NaBH_4$): Mechanism & Selectivity

By Chemca Editorial Team • Last Updated: January 2026 • 8 min read

Sodium Borohydride ($NaBH_4$) is a mild and selective reducing agent. Unlike its more aggressive cousin $LiAlH_4$, sodium borohydride is specific for reducing Aldehydes and Ketones to alcohols, leaving most other functional groups intact.

1. General Reactions

$NaBH_4$ reduces aldehydes to primary alcohols and ketones to secondary alcohols.

$$ R-CHO \xrightarrow{1. \ NaBH_4, \ EtOH} \xrightarrow{2. \ H_3O^+} R-CH_2OH $$ $$ R-CO-R' \xrightarrow{1. \ NaBH_4, \ EtOH} \xrightarrow{2. \ H_3O^+} R-CH(OH)-R' $$

Conditions:

• Reagent: $NaBH_4$.
• Solvent: Protic solvents like Methanol, Ethanol, or Water (It is stable in these, unlike $LiAlH_4$).
• Active Species: Borohydride ion ($BH_4^-$).

2. Detailed Mechanism

The reaction involves the nucleophilic addition of a hydride ion ($H^-$) to the carbonyl carbon.

Step 1: Nucleophilic Attack

The Boron-Hydrogen bond breaks, transferring a hydride ion ($H^-$) to the electrophilic carbonyl carbon. The pi-electrons shift to oxygen, which coordinates with the boron.

$$ R_2C=O + H-BH_3^- \rightarrow [R_2CH-O-BH_3]^- $$

Note: One mole of $BH_4^-$ can theoretically reduce 4 moles of carbonyl compound, transferring all 4 hydrogens.

Step 2: Protonation (Workup)

The alkoxyborane intermediate is hydrolyzed by the solvent (alcohol or water) or dilute acid to release the alcohol.

$$ (R_2CH-O)_4B^- + 4H_2O \rightarrow 4R_2CH-OH + B(OH)_4^- $$

3. Chemoselectivity (The "Magic" of NaBH4)

What does it reduce?

• Aldehydes $\to$ Primary Alcohols
• Ketones $\to$ Secondary Alcohols
• Acid Chlorides $\to$ Primary Alcohols (Highly reactive, so they do reduce)

What does it NOT reduce?

• Carboxylic Acids ($RCOOH$)
• Esters ($RCOOR'$)
• Amides ($RCONH_2$)
• Nitriles ($RCN$)
• C=C Double Bonds (Isolated)

4. Comparison: $NaBH_4$ vs. $LiAlH_4$

Feature	Sodium Borohydride ($NaBH_4$)	Lithium Aluminium Hydride ($LiAlH_4$)
Reactivity	Mild	Very Strong
Solvent	Ethanol, Methanol, Water	Dry Ether, THF (Reacts violently with water)
Reduces Esters?	No	Yes
Reduces Acids?	No	Yes
Bond Polarity	B-H bond is less polar (less reactive)	Al-H bond is more polar (more reactive)

5. Solved Example

Reactant: 4-Oxopentanoic acid ester ($CH_3-CO-CH_2-CH_2-COOEt$)

• Reaction with $LiAlH_4$: Reduces BOTH ketone and ester. Product: Pentane-1,4-diol.
• Reaction with $NaBH_4$: Reduces ONLY ketone. Product: Ethyl 4-hydroxypentanoate.

NaBH4 Quiz

Test your concepts on Selective Reduction. 10 MCQs with explanations.

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