The different kinds of amines

What are amines? The easiest way to think of amines is as near relatives of ammonia, NH3. In amines, the hydrogen atoms in the ammonia have been replaced one at a time by hydrocarbon groups. On this page, we are only looking at cases where the hydrocarbon groups are simple alkyl groups. The different kinds of amines Amines fall into different classes depending on how many of the hydrogen atoms are replaced. Primary amines In primary amines, only one of the hydrogen atoms in the ammonia molecule has been replaced. That means that the formula of the primary amine will be RNH2 where "R" is an alkyl group. Examples include:     Naming amines can be quite confusing because there are so many variations on the names. For example, the simplest amine, CH3NH2, can be called methylamine, methanamine or aminomethane. The commonest name at this level is methylamine and, similarly, the second compound drawn above is usually called ethylamine.     Where there might be confusion about where the -NH2 group is attached to a chain, the simplest way of naming the compound is to use the "amino" form. For example: Secondary amines In a secondary amine, two of the hydrogens in an ammonia molecule have been replaced by hydrocarbon groups. At this level, you are only likely to come across simple ones where both of the hydrocarbon groups are alkyl groups and both are the same. For example:   There are other variants on the names, but this is the commonest and simplest way of naming these small secondary amines. Tertiary amines In a tertiary amine, all of the hydrogens in an ammonia molecule have been replaced by hydrocarbon groups. Again, you are only likely to come across simple ones where all three of the hydrocarbon groups are alkyl groups and all three are the same. The naming is similar to secondary amines. For example: Physical properties of amines Boiling points The table shows the boiling points of some simple amines. type formula boiling point (°C) primary CH3NH2 -6.3 primary CH3CH2NH2 16.6 primary CH3CH2CH2NH2 48.6 secondary (CH3)2NH 7.4 tertiary (CH3)3N 3.5 We will need to look at this with some care to sort out the patterns and reasons. Concentrate first on the primary amines. Primary amines It is useful to compare the boiling point of methylamine, CH3NH2, with that of ethane, CH3CH3. Both molecules contain the same number of electrons and have, as near as makes no difference, the same shape. However, the boiling point of methylamine is -6.3°C, whereas ethane's boiling point is much lower at -88.6°C. The reason for the higher boiling points of the primary amines is that they can form hydrogen bonds with each other as well as van der Waals dispersion forces and dipole-dipole interactions.

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