The hydrocarbons attached to functional groups are called radicals (from "radix", meaning root); CH is the methyl radical, CHCH is the ethyl radical, and so on.

This distinction between radicals and functional groups is an old one in organic chemistry, but a useful one. A radical often is represented simply by R, as in ROH, a general expression for an alcohol.

In this chapter we shall look at several important functional groups, seeing first what chemical properties these groups give to a molecule, and then how these properties are modified by the hydrocarbon radical to which they are attached.