Reduction of Aldehydes and Ketones

The reduction of aldehydes and ketones is a common method of forming alcohols.  While there are many methods available to the chemist to reduce aldehydes and ketones, for sophomore organic you need to know reductions with "metal hydride-reducing reagents".

There is a multitude of metal hydride-reducing reagents, but we will focus on only two of them, NaBH₄ (sodium borohydride) and LiAlH₄ (lithium aluminum hydride - LAH).  These reagents deliver a nucleophilic hydride (H:^-).  A common reaction of aldehydes and ketones is known as "nucleophilic addition".  We will investigate these reactions in more detail later.

For example the treatment of acetone with NaBH₄ followed by an acidic workup (H₃O⁺) affords the alcohol propan-2-ol

Mechanism

Metal hydride reducing reagents have the following structure.

In the first reaction the nucleophilic hydride, H:^-, attacks the aldehyde or ketone carbonyl carbon resulting in an alkoxide intermediate.  Upon treatment of the alkoxide with acidic water, the alcohol is formed.   

NaBH₄ is milder than LiAlH₄ -  it will only reduce aldehydes and ketones.

Reducing formaldehyde simply results in methanol

Reduction of aldehydes always produces secondary alcohols (other than formaldehyde above).

When a ketone is reduced a secondary alcohol is formed.  Tertiary alcohols can not be prepared by this method.

Notice how NaBH₄ does not reduce the ester functional group in this example.

LiAlH₄ is much more powerful than NaBH₄  -  it will reduce aldehydes, ketones, esters, amides, nitriles and -NO₂.