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NITRATION AND NITRATING AGENTS 11

veley and Manley [12] suggested it on the basis of the existence of an electric con-
ductivity minimum at a nitric acid concentration of 96-97% HNO 3 and later, in

1903, the same authors found evidence in refraction coefficient measurements, then

Küster and Münch [19] in 1905, published further evidence based on measurements

of the properties of the anhydrous nitric acid.

In his later studies Hantzsch assumed, according to the Brönsted theory of

acids and bases, that nitric acid acts as a base towards sulphuric acid. According

to Brõnsted, an acid is a compound able to give off a proton while a base is a com-

pound able to take up a proton.

The acidic character of a compound At appears in the presence of another compound B 1 , which

itself behaves like a base:.

A 1 -> B 1 + H+

As a free proton cannot exist it must be attached to a base B 2. As a result, an acid A 2 is

formed:

H+ + B 2 -> A 2

The two reactions sum up to give a new one:

A 1 + B 2 <--> A 2 + B 1

in which the proton is transferred from an acid A 1 or A 2 to a base B 2 or B 1.

One of the components of the system may be a solvent. This is the case, for example, with

a solution of nitric acid in sulphuric acid. The sulphuric acid, with a very small affinity for the

proton, plays the role of an acid and nitric acid behaves like a base.

Usanovich [20] and Lewis [20a] have generalized the Brõnsted theory. The latter considered

adds to be compounds which are able to employ a lone pair of electrons from another mole-

cule; bases are compounds having a lone pair of electrons which may be used by another

molecule.

Hantzsch opposed the theory of N 2 O 5 existing in the nitrating mixture HNO 3 +

+ H 2 SO 4 Due to the importance of the Hantzsch theory in the development

of our views on the nature of the nitration process, we shall discuss it in detail.

In a number of works Hantzsch [21-23] developed a theory based on Schäfer’s

experiments [24] on the absorption spectrum in ultra-violet light. Schäfer’s chief

observation was, that the spectrum of anhydrous nitric acid in a non-aqueous

solvent (hexane, sulphuric acid, etc.) is analogous with the spectra of nitric acid

esters.

On the basis of these data Hantzsch assumed that, generally speaking, acids

have two forms:

(1) true acids with a spectrum similar to that of metal salts

(2) pseudo-acids with a spectrum similar to that of esters.

According to Hantzsch, the structure of a true nitric acid is NO 3 - H+ or N(O 3 )-H+.

The hydrogen atom is attached by electrostatic force only and that is why it

dissociates readily, while the structure of concentrated nitric acid is that of the

undissociated pseudo-acid NO 2 .OH or N(O 2 )OH.

Both forms are in equilibrium. The pseudo-acid form acts as a nitrating and