How Are Aldehydes/Ketones Reduced Back to Alcohols?

Aldehydes and ketones are reduced by adding hydrogen across the carbonyl group, converting the $\text{C}=\text{O}$ group into $\text{C}--\text{OH}$. Aldehydes form primary alcohols, while ketones form secondary alcohols.

This reaction is part of Reactivity 3.2: electron transfer reactions and is required at both SL and HL. Reduction can be represented using $2[\text{H}]$, where $[\text{H}]$ represents hydrogen supplied by a reducing agent such as sodium borohydride, $\text{NaBH}_4$.

For example, ethanal is reduced to ethanol:

$\text{CH}_3\text{CHO} + 2[\text{H}] \rightarrow \text{CH}_3\text{CH}_2\text{OH}$

Propanone is reduced to propan-2-ol:

$\text{CH}_3\text{COCH}_3 + 2[\text{H}] \rightarrow \text{CH}_3\text{CH(OH)CH}_3$

Structurally, reduction changes the carbonyl carbon from trigonal planar to approximately tetrahedral. In an aldehyde, that carbon is bonded to one carbon group and one hydrogen; after reduction, it becomes the terminal $\text{CH}_2\text{OH}$ group. In a ketone, it remains bonded to two carbon groups, giving $\text{CH(OH)}$.

At the particle level, hydrogen is added to the carbonyl carbon and oxygen. The oxidation state of the carbonyl carbon decreases, so the transformation is classified as reduction. With a hydride reducing agent, hydride is transferred to the partially positive carbonyl carbon, and the resulting oxygen-containing species is then protonated.

Do not confuse the overall $2[\text{H}]$ equation with a full reaction mechanism. For SL and HL, the symbolic equation and product classification are central; a question may also require you to recognize reduction from the decrease in oxidation state or gain of hydrogen.

A common misconception is that both aldehydes and ketones produce primary alcohols. This is incorrect: the carbonyl carbon in a ketone is bonded to two carbon groups, so reduction produces a secondary alcohol without changing the carbon skeleton.

Exam technique: When asked to “deduce” the product, replace $\text{C}=\text{O}$ with $\text{CH}--\text{OH}$, preserve the carbon skeleton, and identify the alcohol as primary or secondary. Use $2[\text{H}]$ and ensure every atom is balanced.