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NITRATION THEORIES 61

A still more powerful brominating agent is the free Br+ ion, which is probably

formed in the presence of ferric bromide used as a bromination catalyst:

FeBr 3 + Br 2 -> FeBr 4 - + Br+

By analogy it is to be expected that the NO 2 + is the most powerful nitrating agent.

Gillespie and Millen [31] arranged various nitrating agents according to in-

creasing nitration power :

C 2 H 5 O.NO 2 ethyl nitrate

HO.NO 2 nitric acid

CH 3 COO.NO 2 acetyl nitrate

NO 3 .NO 2 nitric anhydride

Cl.NO 2

H 2 O.NO 2 +

nitryl chloride

nitracidium ion

NO 2 + nitronium ion

This order seems to require some alteration. For example, nitryl chloride has been

found to be a definitely weaker nitrating agent than nitric acid and should have

been placed before it.

On the basis of the numerous physico-chemical investigations referred to above,

we assume that in the nitrating mixture HNO 3 -H 2 SO 4 -H 2 O the nitronium ion

is present, it being the essential nitrating agent. Hence Titov [32] suggested in 1941

the following nitration mechanism where nitronium ion forms intermediate com-

plex structures with aromatic compounds (in brackets):

Ingold and his co-workers [33] and Bennett and co-workers [34-36] believed

that the reaction rate depends on the concentration of the nitronium ions, NO 2 +.

They suggested the following summarized nitration mechanism :

R.H + NO 2 + + B -> R.NO 2 + HB (25)

where B is a proton acceptor, for example HSO 4

• or OH
  
  
  • ions.
    Or the basis of the results of his own experimental studies, Titov [37] in 1947
    accepted this view, but at the same time he pointed out that the nitration reaction
  
  
  
  

occurred in a more complex way than that suggested by Ingold. The nitrating agents