Classification

The basis of classification

Every amine can be pictured as ammonia, $\mathrm{NH_3}$, with one or more of its hydrogen atoms swapped out for an alkyl or aryl group. The number of hydrogen atoms that have been replaced is what sorts an amine into primary, secondary, or tertiary.

• Replace one hydrogen atom of $\mathrm{NH_3}$ with an alkyl or aryl group $\rightarrow$ a primary amine (1°), written $\mathrm{R-NH_2}$ or $\mathrm{Ar-NH_2}$.
• Replace two hydrogen atoms $\rightarrow$ a secondary amine (2°), written $\mathrm{R-NHR'}$. Equivalently, this is what you get if you take a primary amine, $\mathrm{R-NH_2}$, and replace its one remaining N–H hydrogen with a second alkyl/aryl group, $\mathrm{R'}$.
• Replace a third hydrogen atom (the last one on nitrogen) $\rightarrow$ a tertiary amine (3°), written $\mathrm{R-NR'R''}$, with no hydrogen left on the nitrogen at all.

So the progression runs:

$\mathrm{NH_3} \;\xrightarrow{\text{one H replaced}}\; \mathrm{R-NH_2} \;\xrightarrow{\text{second H replaced}}\; \mathrm{R-NHR'} \;\xrightarrow{\text{third H replaced}}\; \mathrm{R-NR'R''}$

Not the same test as for alcohols and halides …