Chemguide: Support for CIE A level Chemistry
Learning outcomes 10.7(b) and 10.7(c)
These statements are about the basic nature of primary amines.
Before you go on, you should find and read the statements in your copy of the syllabus.
These statements will only be examined in the final exam of a two year course.
Read the page amines as bases down as far as (but not including) the reactions with copper ions.
What you should know from this is that amines, like ammonia, are basic because the lone pair on the nitrogen can accept hydrogen ions. When amines react with an acid, salts are formed - just as they are when ammonia reacts with acids.
So, ammonia reacts with hydrochloric acid or hydrogen chloride gas to make the salt ammonium chloride, NH4Cl.
Methylamine reacts with hydrochloric acid or hydrogen chloride gas to make the salt methylammonium chloride, CH3NH3Cl.
Think of methylammonium chloride as if it were ammonium chloride in which one of the hydrogens in the ammonium group has been replaced by a methyl group.
Both of these salts are ionic, containing either ammonium ions or methylammonium ions and chloride ions.
This statement compares the strengths of ammonia, ethylamine and phenylamine as bases.
Ethylamine is taken as typical of amines in which the amino group is attached to a carbon chain. Phenylamine is typical of those in which the amino group is attached directly to a benzene ring.
This is explained on the page about organic bases.
You will find that there is some overlap between this page, and the last one you looked at. You will also find that the page mainly talks about methylamine rather than ethylamine. It makes no significant difference to the arguments.
For CIE exam purposes, you won't be expected to know about pKb. For the purposes of this page, all you need to remember is that the smaller the pKb value, the stronger the base.
In an exam question, you are most likely to be expected to know that amines like phenylamine are much weaker bases than ammonia, and those like ethylamine are stronger. You will also be expected to be able to explain why.
Releasing the free amine from its salts
You may remember that you can get ammonia from an ammonium salt by reacting it with a base such as sodium hydroxide. The hydroxide ions remove the extra hydrogen ion from the ammonium ion, leaving ammonia gas.
Similarly, if you react, say, methylammonium chloride with sodium hydroxide solution, the extra hydrogen ion is again removed to leave methylamine.
You will have come across this being done if you have looked at the preparation of phenylamine from nitrobenzene in statement 10.7(a). In the last stage, you have to add sodium hydroxide solution to convert the phenylammonium salt into free phenylamine.
© Jim Clark 2011