3.1. Refresher and Expansion: Functional Groups

Recall that a functional group is a collection of specific atoms connected in a specific way. Since structure directly dictates reactivity, functional groups behave VERY similarly (often identically) in each molecule in which they are found.

Since we are often only talking about the functional groups, we find it convenient to use the letter ‘R’ to abbreviate part of a molecule. This allows us to be generic (and faster) in our discussion since the extra parts of the molecule are not relevant (Figure 3.1).

Figure 3.1 – Abbreviation of Molecules Using ‘R’.

Confusingly, the letter ‘R’ is sometimes used to mean “any set of atoms is possible here” and sometimes used to mean “a specific set of atoms is possible here”. A safe approach is to always assume ‘R’ groups cannot be, or be connected via, a heteroatom (an atom other than carbon and hydrogen).

There are hundreds of specifically named functional groups. For an introductory text only the most common ones will be important. It is possible that others may be encountered in assignments, labs, discussions, etc. In these cases, searching for the functional group using a resource such as Wikipedia will be a straightforward way of understanding what is being discussed.

Recall that amines are functional groups where a nitrogen is connected to carbons and/or hydrogens (Figure 3.2; ‘R’ is a carbon-based group and hydrogens are indicated directly). Amines are subdivided into primary, secondary, and tertiary amines. The distinction is in the number of carbons attached to the nitrogen. A primary amine has 1 carbon attached to the nitrogen, a secondary has 2, and a tertiary has 3. Technically ammonia, which has 0 carbons attached to nitrogen, is also classed as a primary amine.

Figure 3.2 – Representation and Examples of Amines.

Each of these types of amines has a corresponding ammonium cation where the lone pair is used for another bond (Figure 3.3; ‘R’ is a carbon-based group and hydrogens are indicated directly). The subdivision follows the same pattern. This adds a new category, quaternary ammonium, where the nitrogen has four carbon-based groups attached to it.

Figure 3.3 – Representation of Amines and Corresponding Ammonium Cations.

With the exception of quaternary ammoniums, these ammonium cations are the conjugate acids of the corresponding amine bases. They are frequently encountered in mechanisms and as naturally occurring functional groups.

Recall that functional groups change names when an atom within them changes. For example, if the oxygen of an aldehyde or ketone is replaced by a nitrogen the functional group is properly named as an imine instead (Figure 3.4; ‘R’ is either a carbon-based group or hydrogen). Most functional groups change names when an oxygen is replaced by a nitrogen. Several nitrogen-based analogues of commonly encountered functional groups are themselves common. They are frequently encountered in mechanisms and as naturally occurring functional groups.

Figure 3.4 – Representation and Examples of Select Nitrogen-Based Analogues to Common Functional Groups.