How Are Functional-Group Classifications Useful?

One of the main reasons for classifying compounds by their functional groups is that it also classifies their chemical behavior. By this we mean that the reactions of compounds and, to some extent, their physical properties are influenced profoundly by the nature of the functional groups present. Indeed, many organic reactions involve transformations of the functional group that do not affect the rest of the molecule. For instance, alcohols, R−OH, can be transformed into a number of other compounds, such as organic halides, R−Cl or R−Br; ethers, R−O−R; and amines, R−NH2 without changing the structure of the hydrocarbon group RR. Furthermore, any compound possessing a particular functional group may be expected to exhibit reactions characteristic of that group and, to some extent at least, of inorganic compounds with similar functional groups.

A good example of the use of the functional-group concept is for acid-base properties. Alcohols, ROHROH, are structurally related to water, HOH, in that both possess a hydroxyl function. We may then expect the chemistry of alcohols to be similar to that of water. In fact, both are weak acids because the OH group has a reactive proton that it can donate to a sufficiently strongly basic substance, written as :B here:

Water and alcohols both are weak bases because the oxygens of their OH groups have unshared electron pairs to use in bonding with a proton donated by an acid, HA:

We can carry the analogy further to include carboxylic acids, RCO2H, which also have a hydroxyl function. They also should possess acidic and basic properties. They do have these properties and they are, in fact, stronger acids than either water or alcohols and form salts with bases:

Amines, RNH2, are structurally related to ammonia, NH3, and we therefore may predict that they will have similar properties. A property of ammonia that you probably will have encountered earlier is that it acts as a base and forms salts with acids. Amines behave likewise:

It is with logic of this kind - inferring chemical behavior from structural analogies - that much of organic chemistry can be understood. There are other logical classification schemes, however, and one of these depends more on types of reactions than on functional groups.