electrophillic aromatic substitution and diazo coupling

Azo coupling is the most widely used industrial reaction in the production of dyes, lakes and pigments.

 Aromatic diazonium ions acts as electrophiles in coupling reactions with activated aromatics such as anilines or phenols.

 The substitution normally occurs at the para position, except when this position is already occupied, in which case ortho position is favoured.

 The pH of solution is quite important; it must be mildly acidic or neutral, since no reaction takes place if the pH is too low.Let us take an example of methyl orange.

Due to their positive charge, diazonium cations, which are generated by treatment of aromatic amines with nitrous acid and a stronger mineral acid, may participate in an electrophilic aromatic substitution as an electrophile. 

The electrophilic reaction center is the terminal nitrogen of the -N=N+ group. As a result, two aromatic compounds are coupled by a -N=N- group. This is known as the azo group (diazo group). The corresponding reaction is called diazonium coupling (diazo coupling, azo coupling). However, the electrophilicity of diazonium ions is only relatively weak, as their positive charge is delocalized. The unsubstituted benzenediazonium cation may react only with strongly activated aromatic compounds, such as phenolates and amines.

By introducing electron-withdrawing substituents in ortho or para position regarding the azo group, the diazonium ions' electrophilicity may be increased to such a degree that diazonium coupling also occurs with phenols and phenolic ethers, such as anisole.

Due to the fact that diazonium cations are poor electrophiles and relatively bulky species, mainly para substitution usually takes place in diazonium coupling. In the case of para substitution, steric hindrance is at its weakest, while the positive charge's stabilization is at its largest, in the σ complex (and, thus, in the transition state). If the para position is already occupied by another substituent, ortho substitution occurs.

methyl orange is used in acid base titrations;

Methyl orange Methyl orange is one of the indicators commonly used in titrations. In an alkaline solution, methyl orange is yellow and the structure is: Now, you might think that when you add an acid, the hydrogen ion would be picked up by the negatively charged oxygen. That's the obvious place for it to go. Not so! In fact, the hydrogen ion attaches to one of the nitrogens in the nitrogen-nitrogen double bond to give a structure which might be drawn like this:
	Note:  You may find other structures for this with different arrangements of the bonds (although always with the hydrogen attached to that same nitrogen). The truth is that there is delocalisation over the entire structure, and no simple picture will show it properly. Don't worry about this exact structure - it is just to show a real case where the colour of a compound is drastically changed by the presence or absence of a hydrogen ion.
You have the same sort of equilibrium between the two forms of methyl orange as in the litmus case - but the colours are different. You should be able to work out for yourself why the colour changes when you add an acid or an alkali. The explanation is identical to the litmus case - all that differs are the colours.