How do primary amines differ from secondary and tertiary amines?

A primary, secondary, or tertiary amine is classified based on the number of hydrogen atoms from an ammonia molecule that have been replaced by organic groups.

The differences are represented by their chemical notation:

• Primary amines: One hydrogen atom is replaced by an organic group, represented as ${\text{RNH}}_2$.1
• Secondary amines: Two hydrogen atoms are replaced by organic groups, represented as ${\text{R}}_2\text{NH}$.1
• Tertiary amines: Three hydrogen atoms are replaced by organic groups, represented as ${\text{R}}_3\text{N}$.1

Beyond this structural classification, amines may also be categorized as aliphatic (having only aliphatic groups) or aromatic (having one or more aryl groups attached). Additionally, they can be open-chain or cyclic, depending on whether the nitrogen atom is part of a ring structure.1