Question

Why is the pKa value of protonated hydroxylamine (6.0) so much lower than the pKa value of a protonated primary amine such as protonated methylamine (10.7)?

Short answer

Answer

The electron-withdrawing hydroxyl group decreases the electron density of ammonium in protonated hydroxylamine, thus making it a better acid.

Step-by-step solution

Acid strength

A compound is considered an acid if it donates protons in solution. The strength of an acid is compared using its acid dissociation constant value. The acid dissociation constant gives the extent of dissociation of a weak acid. It is calculated from the concentration of the acid and the ions formed from it at equilibrium.

A stronger acid will have a higher dissociation constant value. The negative logarithm of the acid dissociation constant gives the pKa value and is inversely proportional to acid strength.

Acidity of ammonium ion

Ammonia is a base with nitrogen having a lone pair of electrons, making it a proton acceptor. The ammonium ion is obtained by adding a proton to ammonia; therefore, the ammonium ion is the conjugate acid of ammonia.

The nitrogen ion is an ammonium ion that bears a positive charge and therefore is stabilized by removing the proton. Therefore, the ammonium ion readily removes the proton and is an acid.

Comparing the acidity of protonated hydroxylamine and protonated methylamine

Protonated hydroxylamine contains a hydroxyl group attached to an ammonium ion. The hydroxyl group is an electron-withdrawing group and therefore, reduces the electron density of the nitrogen atom. This causes an increase in the nitrogen atom's positive charge, making it a better acid.

Protonated hydroxylamine

Protonated methyl amine contains a methyl group attached to the ammonium ion. The Methyl group is electron-donating and therefore reduces the positive charge intensity of nitrogen. This reduces the acidity of ammonium ions.

Therefore, protonated hydroxylamine is a better acid than protonated methylamine.