summarize, we cannot say that what the textbook mentions is wrong;
however, it is more accurate to say that aniline (see structure on
left) is diazotised. When aniline is diazotised, anilinium
chloride is made so that aniline can exist homogeneously in water
solution. But it is aniline that actually reacts.
When –Ph stands for the part of benzene (phenyl group) and aniline is expressed as
Ph-NH2, the reaction of aniline with hydrochloric acid is shown as the following equilibrium state:
Ph-NH2 + H+ + Cl- «
Ph-N+H3 + Cl-
Almost all of aniline becomes anilinium chloride, but a slight
amount of aniline still remains. It is thought that this aniline, not
anilinium chloride, reacts with nitrosonium ion (+N=O, which is generated in the reaction
system), and changes into benzenediazonium after going though several
intermediates. Since benzenediazonium makes a pair with chloride ion (Cl-), it is called “benzenediazonium chloride”. This is similar to naming salt (sodium chloride, NaCl).
Because nitrous acid has a high toxicity and is difficult to handle,
sodium nitrate solution is added to benzenediazonium chloride at 0 to 5
°C (see picture on left). Sodium nitrate reacts with an excess amount
of hydrochloric acid, which generates nitrous acid. Moreover, the
nitrous acid reacts with hydrochloric acid, generating +N=O, which reacts with free aniline at once.
NaNO2 + HCl
HNO + NaCl
HNO2 + HCl
+N=O + Cl- + H2O
The process after the reaction of aniline with +N=O is complicated (shown as follows).
Ph-NH2 + +N=O «
Ph-N2+ (benzenediazonium) + H2O
In this reaction, at first, the nitrogen atom in aniline, which
has an unshared electron pair, attacks the nitrogen atom of +N=O.
Diazotisation of aniline might be shown apparently as follows; however, this expression is not really correct.
Ph-NH2 + HNO2 + HCl «
For details, please refer the section on “reactions of amines” in organic chemistry textbooks.
We would like to thank an anonymous professor for his careful teaching and kind support for this answer.
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