Question

Which of the following statements about primary amines is false?
A) Alkylamines are stronger bases than aryl amines
B) Alkyl amines react with nitrous acid to produce alcohols
C) Aryl amines react with nitrous acid to produce phenols
D) Alkylamines are stronger bases than ammonia

Answer 1

To solve this problem we have to look at multiple concepts like the effect of electron-withdrawing and electron-donating group on amines basicity. Also, we should remember the reaction of nitrous acid with amines as well as the series of basicity of amine, ammonia, secondary amines, and tertiary amines.

Complete Step By Step Answer:
To solve this problem we will explain every option.
A) In aryl amines the $- N{H_2}$ group is directly attached to the benzene ring. The unpaired electron on nitrogen is in conjugation with benzene ring so it is less available for protonation. On the other hand, due to the electron-donating nature of the alkyl group, it pushes electrons towards nitrogen and makes unshared electron pairs available for sharing with the proton of the acid. So we conclude that alkyl amines are stronger bases than aryl amines.
B) Reaction of nitrous acid with alkyl amines as follows-
$R - N{H_2}\xrightarrow{{HN{O_2} + HCl}}R - {\mathop N\limits^ + _2}\mathop {Cl}\limits^ - \xrightarrow{{{H_2}O}}ROH + {N_2} + HCl$
R- Represent the alkyl group
Thus reaction of nitrous acid with alkyl amines produces alcohol.
C) Reaction of nitrous acid with aryl amines as follows-
${C_6}{H_5}N{H_2}\xrightarrow{{HN{O_2} + HCl}}{C_6}{H_5}N = NCl$
We can see that nitrous acid does not produce phenols.
D) Due to the electron-donating nature of the alkyl group, it pushes electrons towards nitrogen and makes unshared electron pairs available for sharing with a proton of the acid. We can conclude that the +I effect of the alkyl group increases the basicity of amines. But in the case of ammonia, there is no +I effect to stabilize the positive charge on nitrogen, so ammonia is less basic than alkyl amines.
Hence the correct answer is an option (C).

Note:
Primary alkyl amines react with nitrous acid to form unstable diazonium salts, so liberate nitrogen gas quantitatively and alcohols. This quantitative evolution of nitrogen is used in the estimation of amino acids and proteins.
Whereas aryl amines react with nitrous acid at low temperature to form aryl Diazonium salt which is a very important organic compound to the synthesis of a variety of aromatic compounds.