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


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.   

NaBHis 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.

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