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Diazotisation of aniline

 

  I thought that diazotisation occurs when aniline is put in hydrochloric acid solution then sodium nitrite solution is put in this solution at less than 5 °C. But in my textbook, it says that “Benzenediazonium chloride is generated when sodium nitrite solution is put in hydrochloric acid solution containing anilinium chloride at less than 5 °C.”
  
  Which is correct? Is it possible to diazotise both aniline and anilinium chloride?
   

Gattu Ippatsu (Jan 2002)

 

Answer:
 
To 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-N+H2-N=O « Ph-NH-N=O+-H « Ph-+NH=N-O-H « Ph-N=N-O+-H2 « 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 « Ph-N2+Cl- + 2H2O
  
  For details, please refer the section on “reactions of amines” in organic chemistry textbooks.
 
 
Acknowledgement
  We would like to thank an anonymous professor for his careful teaching and kind support for this answer.
  
 

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