Metal Hydride Reduction (NaBH₄ and LiAlH₄)

Metal Hydride Reduction with NaBH₄ and LiAlH₄ converts carbonyls to alcohols, widely used in organic and medicinal chemistry.

Overview:

• Metal hydrides like sodium borohydride (NaBH₄) and lithium aluminium hydride (LiAlH₄) are used to reduce carbonyl compounds (aldehydes, ketones, esters, carboxylic acids, etc.) to alcohols.

Sodium Borohydride (NaBH₄)

• Structure: Na⁺[BH₄]⁻ (a tetrahedral borohydride ion)
• 
• Solubility & Stability: Stable in water and alcohol; can be used in aqueous or alcoholic solutions.

Reactivity:

• Reduces: Aldehydes and ketones easily.
• Does not reduce: Esters, acids, amides (generally unreactive unless in harsh conditions).

Mechanism:

1. The hydride ion (H⁻) from NaBH₄ attacks the electrophilic carbonyl carbon.
2. This forms a tetrahedral alkoxide intermediate.
3. Protonation of the alkoxide by solvent gives the alcohol.

Example:

• R-CHO (aldehyde) + NaBH₄ → R-CH₂OH (primary alcohol)
• R-CO-R’ (ketone) + NaBH₄ → R-CHOH-R’ (secondary alcohol)

Lithium Aluminium Hydride (LiAlH₄)

• Structure: Li⁺[AlH₄]⁻
• 
• Solubility: Reacts violently with water and alcohol — must be used in dry ethers like diethyl ether or THF.

Reactivity:

• Reduces: Aldehydes, ketones, esters, acids, amides, nitriles.
• Very strong reducing agent.

Mechanism (for ester reduction):

1. LiAlH₄ donates a hydride to the carbonyl carbon.
2. Tetrahedral intermediate collapses, ejecting the alkoxide leaving group.
3. Another hydride adds to the aldehyde intermediate.
4. Protonation gives a primary alcohol.

Example:

• R-COOH +  LiAlH₄  →  R-CH₂OH (primary alcohol)
• R-COOR’ +  LiAlH₄  →  R-CH₂OH  +  R’OH
• R-CONH₂ +  LiAlH₄  →  R-CH₂NH₂ (primary amine)

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