Aliphatic and aromatic functional groups

ALIPHATIC AND AROMATIC FUNCTIONAL GROUPS

Key Notes

Aliphatic functional groups

Functional groups are defined as aliphatic if there is no aromatic ring directly attached to them. It is possible to have an aromatic molecule con-taining an aliphatic functional group if the aromatic ring is not directly attached to the functional group.

Aromatic functional groups

Functional groups are defined as aromatic if they have an aromatic ring directly attached to them. In the case of esters and amides, the aromatic ring must be attached to the carbonyl side of the functional group. If the aromatic ring is attached to the heteroatom, the functional groups are defined as aliphatic.

Aliphatic functional groups

Functional groups can be classed as aliphatic or aromatic. An aliphatic functional group is one where there is no aromatic ring directly attached to the functional group (Fig. 1a and b).

Aromatic functional groups

An aromatic functional group is one where an aromatic ring is directly attached to the functional  group (Fig. 1c and d).

There is one complication involving esters and amides. These functional groups are defined as aromatic or aliphatic depending on whether the aryl group is directly attached to the carbonyl end of the functional group, that is, Ar–CO–X. If the aromatic ring is attached to the heteroatom instead, then the ester or amide is classed as an aliphatic amide (Fig. 2).