C19: Nitrogenous Compounds

Question 1

describe and explain relevant basicity’s of methyl amine, dimethylamine and trimethylamine in the gas phase. [3]

Answer 1

methyl amine has only one alkyl group attached to N. dimethylamine has 2 alkyl groups attached to N, trimethylamine has 3.

An alkyl group is electron-donating which increases electron density on N, making N’s lone pair of electrons more available to be donated.

Question 2

explain the relative magnitudes of Kb values in the table [3]

Answer 2

ethylamine: N atom attached to an electron-donating ethyl group
phenyl amine: N’s lone pair of electrons delocalises ov er the benzene ring
4-chlorophenylamine: electron-withdrawing Cl atom.

therefore, electron-density on N and thus availability of N’s lone pair of electrons to accept a proton decreases in order ethyl amine > NH3> phenyl amine >4-chlorophenylamine, Kb decreases in same order.

Question 3

explain why amides are neutral not basic

Answer 3

in amides, nitrogen’s lone pair of electrons delocalises into C=O bond, electron-density on N reduced, lone pair of electrons is NOT available for

Question 4

suggest how the basicity of benzylamine might compare to that of ethyl amine, in aqueous medium. give reasoning for your answer.

Answer 4

benzylamine is a weaker base than ethylamine

the conjugate acid of benzylamine (C6H5CH2NH3+) is not as effectively solvated by water molecules, compared to the conjugate acid of ethyl amine (CH3CH2NH3+). the could be because large benzene ring is non-polar and does not form favourable interactions with the polar water molecules, so benzylamine probably ionises water to a small extent