Organic chemistry and Biological chemistry for Health Sciences
59-191
Lecture 13
Chemical properties of amines:
When amonia dissolves in water, the following equilibrium is established
NH3 + H2O
NH4+ + OH-
A very similar equilibrium is established when an amine dissolves in water.
RNH2 + H2O
Kb =
[R
RNH3+ + OH[RNH3+][OH-]
[RNH2]
(Kb = base ionization constant)
The higher the value of Kb is, the greater the dissociation of the base. Amines have Kb
values slightly higher than amonia. Thus aliphatic amines are stronger bases than amonia.
Like amonia, compounds with amino groups can also neutralize hydronium ions. This
neutralization occurs very rapidly and essentially goes to completion at room
temperature.
Protonated amine cations can neutralize hydroxide ion and revert to amines.
A combination of protonated amine and an anion make up an organic salt called an
amine salt. Like the salts of amonium ions all amine salts of strong acids are more
soluble in water than amine because the full charge carried by the ions of an amine salt
can be much better hydrated by water molecules than the amine itself.
The amino group is a solubility switch. By simply adding enough strong acids the
solubility of amine can be switched on to give protonated amine (more soluble than
amine). By quickly adding enough strong base you can bring the amine back out of
solution.
So adjusting the pH of the medium can change the solubility of complex compounds
containing amino group almost instantly.
Amides of carboxylic acids:
Amides are neutral nitrogen compounds that can be hydrolyzed to carboxylic acids and
amonia.
Carbonyl nitrogen bond is called the amide bond. Amide bond is broken when an amide
is hydrolyzed.
Amides can be derived from amonia or amines. Simple amides are usually derived from
amonia.