A functional group is an atom or group of atoms that is responsible for the characteristic chemical reactions of a compound.The rest of the molecule is mostly C-C and C-H bonds, which are fairly unreactive. The functional group is usually where reactions take place.Common functional groups include C=C in alkenes, -OH in alcohols, C-X in halogenoalkanes, C=O in aldehydes and ketones, and -COOH in carboxylic acids.Compounds with the same functional group react in similar ways. For example, ethanol and propanol both contain the -OH group, so they have similar chemical properties.