Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

306 MECHANISM OF THE COUPLING
actually belong to the class of acid dyes. The object of the sulphonic
group is to make the dye soluble in the dye bath.
The most important technical azo-dyes are derived from naphthalene,
and indeed there is hardly any branch of organic chemistry
which has been so thoroughly investigated in all its details as that of
the intermediate products which are here involved. The number of
possible combinations is almost without limit, and we can only refer
to the specialist works, particularly F. Mayer, Chemie der organ.
Farbstqffe, Berlin, 1924 ; Mohlau-Bucherer, Farbenchem. PrakL, Berlin,
1926; Fierz-David, Farbenchemie, Berlin, 1924; Eistenpart, Ghem.
Technol. der organ. Farbstoffe, Leipzig, 1925.
The mechanism of the coupling reaction has been very exhaustively
studied. Summarising first what has already been mentioned, it must
be noted that the reaction is not confined to the aromatic series, for
diazo-compounds condense also with enols and with the very closely
related aliphatic a,ci-nitro-compounds. The final products of these reactions
are not azo-compounds, but the isomeric hydrazones formed from
them by rearrangement.
CH, CH
3
3
KOOC—CH
C—OH
Ethyl acetoacetate
+ H 0 N = N
CH3
C—OH
II
ROOC—C—N=N.C 6H 5
>• CO a-Phenylhydrazone of the ester
I of a/3-Diketobutyric acid
ROOC—C=N—NH.C RH K
CH2 H O N = N r C H — N = N
II + I —M II I
0=N—OH C6HS Lo=N—OH C6]
aci-nitromethane HC=N—NH.C 6H 5
NO,
Phenylhydrazone of
nitroformaldehyde
+ H 2O
Even the doubly unsaturated hydrocarbons, such as butadiene, can
be coupled with suitable diazo-compounds. Finally, not only phenols,
but also phenol ethers, such as anisole, are capable of coupling (K. H.
Meyer l ).
1. The simplest theory, developed by K. H. Meyer, traces the ability
to couple to the activity of the double bond which adds itself to the
i Annalen, 1913, 398, 66 ; Ber., 1914, 47, 1741.