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Reduction of Carbonyls

01Reduction of Carbonyls

Reduction products

This section covers the alcohol products formed when aldehydes and ketones are reduced with sodium tetrahydridoborate.

Reducing aldehydes and ketones

Aldehydes and ketones can be reduced to alcohols using sodium tetrahydridoborate, NaBHX4\ce{NaBH4}, in aqueous solution.

The two hydrogen atoms added during the overall reduction are represented by 2 [H]\ce{2[H]} in an equation.

1

Aldehyde → primary alcohol

Reduction converts the terminal carbonyl group of an aldehyde into the −CHX2OH\ce{-CH2OH} group of a primary alcohol.

CHX3CHX2CHO+2 [H]→CHX3CHX2CHX2OH\ce{CH3CH2CHO + 2[H] -> CH3CH2CH2OH}

For example, propanal is reduced to propan-1-ol.

2

Ketone → secondary alcohol

Reduction converts the carbonyl group of a ketone into the −CHOH\ce{-CHOH} group of a secondary alcohol.

CHX3COCHX3+2 [H]→CHX3CH(OH)CHX3\ce{CH3COCH3 + 2[H] -> CH3CH(OH)CH3}

For example, propanone is reduced to propan-2-ol.

Carbonyl compound Reduction product General equation
Aldehyde Primary alcohol RCHO+2 [H]→RCHX2OH\ce{RCHO + 2[H] -> RCH2OH}
Ketone Secondary alcohol RX2CO+2 [H]→RX2CHOH\ce{R2CO + 2[H] -> R2CHOH}

Exam Tip: When writing the overall reduction equation, use 2 [H]\ce{2[H]} as the reductant and make sure an aldehyde gives a primary alcohol while a ketone gives a secondary alcohol.

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