Primary, secondary, tertiary and quaternary nomenclature

PRIMARY, SECONDARY, TERTIARY AND QUATERNARY NOMENCLATURE

Key Notes

Definition

Carbon centers, as well as some functional groups (alcohols, alkyl halides, amines and amides), can be defined as primary (1°), secondary (2°), tertiary (3°) or quaternary (4°).

Carbon centers

Carbon centers can be identified as primary, secondary, tertiary, or quater-nary depending on the number of bonds leading to other carbon atoms. A methyl group contains a primary carbon center. A methylene group (CH₂) contains a secondary carbon center. The methine group (CH) contains a tertiary carbon center while a carbon atom having four substituents is a quaternary center.

Amines and amides

Amines and amides can be defined as being primary, secondary, tertiary, or quaternary depending on the number of bonds leading from nitrogen to carbon.

Alcohols and alkyl halides

Alcohols and alkyl halides are defined as primary, secondary, or tertiary depending on the carbon to which the alcohol or halide is attached. The assignment depends on the number of bonds from that carbon to other car-bon atoms. It is not possible to get quaternary alcohols or quaternary alkyl halides.

Definition

The primary (1°), secondary (2°), tertiary (3°) and quaternary (4°) nomenclature is used in a variety of situations: to define a carbon center, or to define functional groups such as alcohols, halides, amines and amides. Identifying functional groups in this way can be important since the properties and reactivities of these groups may vary depending on whether they are primary, secondary,tertiary,or quaternary.

Carbon centers

One of the easiest ways of determining whether a carbon center is 1°, 2°, 3° or 4° is to count the number of bonds leading from that carbon center to another carbon atom (Fig. 1). A methyl group (CH₃) is a primary carbon center, a methylene group (CH₂) is a secondary carbon center, a methine group (CH) is a tertiary carbon center, and a carbon center with four alkyl substituents (C) is a quaternary carbon center (Fig. 2).

Amines and amides

Amines and amides can be defined as being primary, secondary, tertiary, or qua- ternary depending on the number of bonds from nitrogen to carbon (Fig. 3). Note that a quaternary amine is positively charged and is therefore called a quaternary ammonium ion. Note also that it is not possible to get a quaternary amide.

Alcohols and alkyl halides

Alcohols and alkyl halides can also be defined as being primary, secondary, or ter- tiary (Fig. 4). However, the definition depends on the carbon to which the alcohol or halide is attached and it ignores the bond to the functional group. Thus, qua- ternary alcohols or alkyl halides are not possible.

The following examples (Fig. 5) illustrate different types of alcohols and alkyl halides.