Uses of Aldehydes and Ketones

A carbonyl group can be reduced to two different end points depending on the reagent chosen: partial reduction gives an alcohol (the $\text{C=O}$ becomes $\text{C-OH}$), while complete reduction removes the oxygen entirely and gives a methylene group (the $\text{C=O}$ becomes $\text{CH}_2$).

Reduction to an alcohol. Complex metal hydrides -- sodium borohydride, $\text{NaBH}_4$, and lithium aluminium hydride, $\text{LiAlH}_4$ -- both deliver a hydride ion ($\text{H}^-$) as a nucleophile to the carbonyl carbon, exactly as in §8.5, giving an alkoxide that is protonated on work-up to the alcohol: an aldehyde is reduced to a primary alcohol, and a ketone to a secondary alcohol. $\text{NaBH}_4$ is the milder of the two reagents, selective enough to be used even in protic solvents like methanol without being destroyed, and it reduces only the carbonyl group, leaving an isolated carbon-carbon double bond elsewhere in the same molecule untouched. $\text{LiAlH}_4$ is a considerably more powerful and reactive hydride donor (and must be used in anhydrous, aprotic conditions such as dry ether, since it reacts violently with water), but for a simple, isolated carbonyl group with no other easily-reduced functional group nearby, it gives the same product as $\text{NaBH}_4$ -- the difference between the two reagents becomes important mainly for more complex, multi-functional substrates. Catalytic hydrogenation, $\text{H}_2$ over $\text{Ni}$, $\text{Pd}$ or $\text{Pt}$, also reduces a carbonyl to an alcohol by direct addition of hydrogen across the $\text{C=O}$ bond, but is far less selective than the hydride reagents, since it will also reduce any $\text{C=C}$ double bond present in the same molecule. …