Hydrogen, Hydrides, Boranes

The addition of hydrogen, hydrides or borane reduces the C=O group of aldehydes and ketones to an alcohol.

Catalytic hydrogenation

The reaction is very similar to that studied for the alkenes.

If there are alkenes or alkynes in the molecule, they will also be reduced. The same happens with other functions: look at the table.

Ease of reduction of some functional groups by catalytic hydrogenation
Functional group	Product	Ease
R–COCl	R–CHO	Extreme
R–NO₂	R–NH₂	Very easy
alkyne	alkene	Very easy
aldehyde	primary alcohol	Easy
alkene	alkane	Easy
ketone	secondary alcohol	Moderate
nitrile	primary amine	Moderate
ester	primary alcohol	Difficult
aromatic	cycloalkane	Very difficult

This table is merely orientative: the ease of reduction depends strongly on the overall structure of the molecule.

Bear in mind this competition when planning a complex synthesis.

Remember that the C=O group of aldehydes and ketones can be "protected".

Click here to recall it.

Hydride addition

This reaction was already introduced in the chapter on alcohols as a method for preparing them.

Click here to recall it.

Sodium borohydride: mild reactivity, selective for aldehydes and ketones, does not reduce NO₂, CN or COOR, and is compatible with water and alcohols

Lithium aluminum hydride: very reactive, reduces NO₂, CN and COOR and reacts violently with water and alcohols

Ease of reduction of some functional groups by LAH
Functional group	Product	Ease
aldehyde	primary alcohol	Extreme
ketone	secondary alcohol	Extreme
R–COCl	primary alcohol	Very easy
epoxide	alcohol	Easy
ester	primary alcohol	Easy
R–COOH	primary alcohol	Moderate
amide	amine	Difficult
nitrile	primary amine	Difficult
alkene	—	Inert
alkyne	—	Inert

This table is merely orientative: the ease of reduction depends strongly on the overall structure of the molecule and on the reaction conditions (solvents, additives, temperature, etc.).

Borane addition

The reaction is very similar to that studied for the alkenes.

Click here to recall it.

Addition of borane to a carbonyl group: boron binds to oxygen and hydrolysis gives the alcohol

In the case of aldehydes and ketones, the boron atom always ends up bonded to oxygen.

Again, bear in mind the competition of other functions for the borane.

Ease of reduction of some functional groups by borane
Functional group	Product	Ease
R–COOH	R–CH₂OH	Extreme
alkene	alcohol	Very easy
aldehyde	primary alcohol	Very easy
ketone	secondary alcohol	Easy
nitrile	primary amine	Moderate
epoxide	alcohol	Difficult
ester	primary alcohol	Very difficult
R–COCl	—	Inert

This table is merely orientative: the ease of reduction depends strongly on the overall structure of the molecule and on the reaction conditions (solvents, additives, temperature, etc.).

Notice that, if you compare the three tables on this page, the order of reactivity of the functional groups is very different. Achieving a chemoselective reduction, i.e. one that affects only the function we want, is an art. But don't worry: it is an art learned with time, practice and patience.