`text(FROM DIAZONIUM SALTS)`

See fig.1.

`text(BY HALOGENATION OF ARENES OR SUBSTITUTED ARENES)`

`ArH + X_2 oversettext(Lewis acid)-> ArX + HX`

where `X_2 = Cl_2` or `Br_2`

Lewis acid `= FeCI_3, AICI_3` etc.

For example,

See fig.2.

`text(FROM ARYL THALLIUM COMPOUNDS)`

`ArH + TI (OOCF_3)_3 overset(-CF_3CO_2H)-> undersettext(trifluoro acetate)(ArTI(OOCCF_3)_2) underset(Delta)overset(KI)-> undersettext(Aryl thallium)(ArI)`

This method is applicable for the preparation of aryl iodides only because `I^(-)` is a better nucleophile than other halide ions in `S_N2` displacements.

For example,

See fig.3.

The preparation of aryl halides from diazonium salts is more useful than direct halogenation for many reasons. First of all, fluorides and iodides, which can seldom be prepared by direct halogenation, can be obtained through diazonium salts. Secondly, where direct halogenation yields a mixture of ortho and para isomers, the ortho isomer, atleast, is difficult to obtain pure.

`text(LOW REACTIVITY OF ARYL AND VINYL HALIDES)`

An alkyl halide can be conveniently detected by the precipitation of insoluble silver halides when it is warmed with alcoholic `AgNO_3` . The reaction occurs instantaneously with tertiary alkyl or benzyl halides and within five minutes or so with primary and secondary halides. But halobenzene or vinyl halides can be heated with alcoholic `AgNO_3` for days without the slightest trace of silver halide being detected.

See fig.1.

The typical reaction of alkyl halides is nucleophilic substitution.

`R - X + : Z -> R-Z + : X^(-)`

where `Z = OH^(-) , OR^(-) , NH_3 , CN^(-) , NH_2^(-) , ROH , H_2O` etc

But aryl halides undergo nucleophilic substitution reactions
only in extreme conditions. Except for certain industrial
processes where very severe conditions are feasible, one
does not ordinarily prepare phenols (ArOH), ethers (ArOR),
amines `(ArNH_2)` or nitriles (ArCN) by nucleophi lic attack on
aryl halides. The aryl halides cannot be used in the FriedelCraft's
alkylation reaction just like alkyl halides.

However, aryl halides do undergo nucleophilic substitution readily if the aromatic ring contains, in addition to halogen, certain other properly placed groups, which can activate the ring towards nucleophilic substitution. The presence of electron withdrawing groups like - `NO_2, - CF_3` at ortho or para position to the halogen atom makes the aryl halides more susceptible to nucleophilic attack.

The reactions of unactivated and deactivated aryl halides with strong bases or at high temperature proceed via the benzyne intermediate. The Dow's process used for the manufacture of phenol involves benzyne intermediate. Aryl halides can also undergo typical electrophilic aromatic substitution reactions like nitration. sulphonation. halogenation. Friedel - Craft's alkylation. Hal09en is unusual in being deactivating but ortho and para - directing.

`text(FORMATION OF GRIGNARD REAGENT)`

`ArBr + Mg oversettext(dry ether)-> ArMgBr`

`ArCI + Mg oversettext(tetrohydrofuran)-> ArMgCl`

`text(ELECTROPHILIC AROMATIC SUBSTITUTION)`

Although halogen is deactivating but it directs the incoming electrophile to ortho and para position.

For example,

See fig.2.

`S_NAr` `text(MECHANISM)`

`Ar -X +Z -> Ar -Z +X^(-)`

For facile reaction, Ar must contain strongly electron withdrawing groups at ortho and/or para position to the halogen atom. The reaction involves formation of carbanion as intermediate.

See fig.1.

Reaction proceeds through carbanion formation as intermediate. The rate of the reaction increases with the increase in the number of electron withdrawing groups at ortho and para positions, since the carbanion formed would be readily stabilized. The mechanism of the reaction is addition in the first step and elimination in the second step.

See fig.2.

It can be seen that the presence of `NO_2` group at ortho or para position would facilitate to disperse the negative charge of the carbanion, thus stabilizing it and allowing the reaction to occur fast.

`text(BENZYNE MECHANISM)`

`Ar -X + : Z^(-) oversettextr(Strong base)-> Ar -Z + : X^(-)`

Those rings, which are not activated towards bimolecular displacement, undergo substitution by benzyne mechanism.

Unactivated and deactivated aryl halides undergo nucleophilic substitution by benzyne mechanism. In benzyne mechanism, first step involves elimination while second step involves addition of nucleophi le. Unactivated aryl halide means either no substituent is present or the presence of electron withdrawing group at meta position. Deactivated aryl halides imply the presence of electron donating groups at any position.

See fig.3.

Chlorobenzene with chlorine bonded to `text( )^(14)C` gives almost 50% aniline having `NH_2` bonded to `text( )^(14)C` and 50% aniline with `NH_2` bonded to normal `text( )^(12)C` atom.

See fig.4.