Structure, Classification and Nomenclature of Amines

Structure. In every amine, nitrogen retains one lone pair of electrons and forms

three $\sigma$ bonds -- to hydrogen and/or carbon -- giving it, like ammonia, a

pyramidal ($sp^3$) shape with the lone pair occupying the fourth tetrahedral

position. This lone pair is the single most important structural feature of an amine: it

is the site of protonation (making amines basic) and of nucleophilic attack (making

amines reactive towards electrophiles such as alkyl halides and acid chlorides). Because

nitrogen inverts its pyramidal shape very rapidly at room temperature (like an umbrella

flipping inside out), a simple amine with three different substituents on nitrogen does

NOT show optical activity even though nitrogen technically has four different groups

around it (three substituents plus the lone pair) -- the rapid inversion continuously

interconverts the two mirror-image pyramidal forms faster than they can be separated.

Classification. Amines are classified by how many of ammonia's three hydrogens have

been replaced by an organic group -- a classification based purely on substitution at

nitrogen, unlike the classification of alcohols or haloalkanes which is based on the

carbon skeleton:

• Primary ($1^\circ$): $\text{R}-\text{NH}_2$ -- one carbon group, e.g. $\text{CH}_3\text{CH}_2\text{NH}_2$ (ethylamine).
• Secondary ($2^\circ$): $\text{R}_2\text{NH}$ -- two carbon groups (which may be the same or different), e.g. $(\text{CH}_3\text{CH}_2)_2\text{NH}$ (diethylamine).
• Tertiary ($3^\circ$): $\text{R}_3\text{N}$ -- three carbon groups, e.g. $(\text{CH}_3\text{CH}_2)_3\text{N}$ (triethylamine).

An amine can also be aliphatic (all $\text{R}$ groups are alkyl) or aromatic (at least

one group is directly attached to the ring, as in aniline).

Nomenclature. IUPAC names a simple amine by adding the suffix '-amine' to the parent

alkane name (ethanamine for $\text{CH}_3\text{CH}_2\text{NH}_2$), while common usage adds

'-amine' after the alkyl group's name (ethylamine). A secondary or tertiary amine with

identical substituents is named with the prefix 'di-'/'tri-' (diethylamine,

triethylamine); with DIFFERENT substituents, the larger group is the parent and the …