Ammonia and Methylamine

Why methylamine is a stranger base than ammonia??

Answer:

The strengths of weak bases are measured on the pK_b scale. The smaller the number on this scale, the stronger the base is.

Three of the compounds we shall be looking at, together with their pK_b values are:

Remember - the smaller the number the stronger the base. Comparing the other two to ammonia, you will see that methylamine is a stronger base, whereas phenylamine is very much weaker.

Methylamine is typical of aliphatic primary amines - where the -NH₂ group is attached to a carbon chain. All aliphatic primary amines are stronger bases than ammonia.

Phenylamine is typical of aromatic primary amines - where the -NH₂ group is attached directly to a benzene ring. These are very much weaker bases than ammonia.

The only difference between this and ammonia is the presence of the CH₃ group in the methylamine. But that's important! Alkyl groups have a tendency to "push" electrons away from themselves. That means that there will be a small amount of extra negative charge built up on the nitrogen atom. That extra negativity around the nitrogen makes the lone pair even more attractive towards hydrogen ions.

Making the nitrogen more negative helps the lone pair to pick up a hydrogen ion.