Functional groups:

Recognition of Functional Groups

A functional group is a portion of an organic molecule that contains atoms other than carbon and hydrogen, or that consists of bonds other than C-C and C-H bonds. For instance, ethane (Fig. 1a) is an alkane and comprises no functional group. Every atom are carbon and hydrogen and all the bonds are C-C and C-H. Alternatively Ethanoic acid, comprises a portion of the molecule (boxed portion) that consists of atoms except the carbon and hydrogen, and bonds except the C-H and C-C.  This section of the molecule is termed as a functional group - in this case a carboxylic acid.

Figure:  (a) Ethane; (b) ethanoic acid.