Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

358 BIVALENT NITKOGBN
time the air in the other part of the apparatus and the reaction vessel
is driven out by carbon dioxide. The flask is clamped in such a
position that, immediately afterwards, it can be placed on a vigorously
boiling water bath kept ready for the purpose. When all the
air in the first part of the apparatus has been expelled by nitric
oxide, i.e. when the gas in the exit tube of the second wash-bottle
has become quite colourless, the two parts of the apparatus are
rapidly connected by means of the rubber tube, the clip is unscrewed,
and the nitric oxide, accompanied by a gentle current of carbon
dioxide, is passed into the flask. As soon as brown fumes (N02)
begin to escape from its exit tube the flask is placed on the boiling
water bath and a rather rapid stream of nitric oxide is passed into
the toluene solution for half an hour. At the end of this time the
solution is yellow. The flame under the bath is now lowered, the
clip screwed down, the nitric oxide generator disconnected, and all
the nitric oxide which remains in the rest of the apparatus is driven
out by a brisk current of carbon dioxide (test with potassium iodidestarch
paper). Then the toluene is completely removed by distillation
in a vacuum, and the crystalline diphenylnitrosamine which
remains is purified by recrystallisation from a little alcohol or from
petrol ether. Melting point 66°.
Experiments—In order to show that this formation of aromatic
nitrosamines is reversible, boil a small amount of the pure compound
just prepared in xylene and hold a piece of moist potassium iodidestarch
paper over the mouth of the tube.
A close analogy to the carbon radicles is provided by certain nitrogen
compounds, which are likewise free unsaturated complexes partaking
of the character of atoms and having an abnormal valency. With
nitrogen, as with carbon, the existence of radicles also depends on the
presence of aromatic rings. Tetraphenylhydrazine corresponds to hexaphenylethane.
The linkage between the two N-atoms is here firmer than in the
carbon prototype. Dissociation into the diphenylnitrogen radicles in
solution is not perceptible to the eye below about 80° :
(H5C6)2N-N(C6H5)2 —• 2(C^S),N.
However, if positive substituents are introduced into the benzene
rings, hydrazine derivatives result which considerably surpass hexaphenylethane
in their degree of dissociation. Already the colourless
ip-tetra-anisylhydrazine is appreciably dissociated at room temperature
into the green -p-dianisylnitrogen radicles (H3CO.C6H4)2N and the corre-