Reduction with sodium borohydride: practice and mechanism
NaBH₄ delivers a hydride to the carbonyl carbon: aldehyde → primary, ketone → secondary alcohol. No carbon is added.
Example
NaBH₄ transfers a hydride ion to the carbonyl carbon. Aldehydes are reduced to primary, ketones to secondary alcohols. The mild NaBH₄ does not reduce esters or carboxylic acids.
Mechanism step by step
- 1
Borohydride transfers a hydride ion (H⁻) to the partially positive carbonyl carbon.
- 2
The C=O bond swings over to the alkoxide (R–O⁻).
- 3
The solvent methanol (or aqueous work-up) protonates the alkoxide to the alcohol.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Background
Mechanism step by step
1. The borohydride ion transfers a hydride (H⁻) to the carbonyl carbon and the C=O bond moves onto oxygen. 2. An alkoxide forms. 3. The solvent (methanol, ethanol or water) or the workup protonates it to the alcohol. Aldehydes give primary alcohols, ketones secondary alcohols.
What NaBH₄ reduces and what it does not
NaBH₄ is a mild reducing agent: aldehydes and ketones yes; esters, carboxylic acids, amides and nitriles not under normal conditions. C=C double bonds are left alone too. So a ketone next to an ester can be reduced selectively. Esters and acids need the much stronger LiAlH₄.
NaBH₄ or LiAlH₄?
NaBH₄ tolerates alcohols and water as solvents and is easy to handle. LiAlH₄ reacts violently with water, needs dry ether or THF and a careful workup, but reduces almost every carbonyl compound.
Typical mistakes
- The C=O group is not removed completely. NaBH₄ only reduces to the alcohol; going down to CH₂ needs Wolff–Kishner or Clemmensen.
Often confused with: Grignard reaction · Wittig reaction
Frequently asked questions
Why doesn't NaBH₄ reduce esters?
Esters are less electrophilic due to resonance with the oxygen; the mild NaBH₄ is not strong enough.
How many hydrides does one NaBH₄ deliver?
Up to four, so one mole of NaBH₄ can in theory reduce four moles of ketone.
What is the mechanism of NaBH₄ reduction?
A hydride is transferred from borohydride to the carbonyl carbon, giving an alkoxide; the alkoxide then picks up a proton from the solvent or during workup and becomes the alcohol.
Does NaBH₄ reduce esters?
Not under normal conditions, or only very slowly. Esters are reduced with LiAlH₄ to the primary alcohol, or with DIBAL-H at −78 °C to the aldehyde.