There (vii) Copper-catalyzed arylation.The azobenzene coupling reaction via

There are several synthetic
methods for azobenzene derivatives, including:44 (i) Azobenzene
coupling reaction involving
diazonium salts; (ii) Coupling of primary arylamines with nitroso compounds
(Mills reaction); (iii) Oxidation reactions of aromatic primary amines; (iv) Reductive
coupling of aromatic nitro derivatives; (v) Dehydrogenation of arylhydrazines;
(vii) Copper-catalyzed arylation.The azobenzene coupling reaction via diazonium salts is one of the most useful
methods for the preparation of azobenzene derivatives. It is carried out in
acidic media, which involves, firstly, the formation of the diazonium salt 31 by diazotization reaction at low
temperature. Then, this reactive intermediate can subsequently
react with an electron-rich aromatic nucleophile 32 via an electrophilic coupling
reaction (Scheme 1). Finally, it produces the azobenzene derivatives 33.45-53 For coupling reaction, the pH of the
solution acts as a very important factor for both aniline 30 and phenol 32 solutions.
For the first step, the acidic medium is necessary to form the nitrous acid,
with which the free aniline can react. The acidic conditions also stabilize the
resulting diazonium salt product simultaneously. During the second step, the
conditions are switched to become basic. In this new environment, the phenol reactant
32 produces the corresponding phenoxide
ion which is then activated enough to attack the diazonium moiety of 31. The resulting intermediate
re-aromatizes readily to produce the desired azobenzene 33. There is a region-chemical issue during
the attack of the phenolate onto the diazonium salt 31. Only two sites are reactive on 32, they are the para- and
ortho-positions. Most of the time, the
para-position leads to the major
adducts for steric reasons. Obviously, if the para-position is already occupied, the phenolate will attack from
its ortho-position.

This method presents many advantages,
such as the use of simple agents, like cheap acids (eg. HCl, H2SO4)
and bases (eg. NaOH, K2CO3). What’s more, this sequence
of reactions offers an entry to the preparation of symmetrical or asymmetrical azobenzenes. 

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