General Chemical Properties Of The Aryl Halides

Low reactivity of aryl and vinyl halides

An alkyl halide is 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 bromides, and within five minutes or so with primary and secondary bromides. But bromobenzene or vinyl bromide can be heated with alcoholic AgNO₃ for days without the slightest trace of AgBr being detected.

The typical reaction of alkyl halides, is nucleophilic substitution.

R − X +:Z− R − Z + :X−

where Z = OH−, OR−, NH₃, CN−, , ROH, H₂O etc.

It is typical of aryl halides that they undergo nucleophilic substitution only with extreme difficulty. Except for certain industrial processes where very severe conditions are feasible, one does not ordinarily prepare phenols (ArOH), ethers (ArOR), amines (ArNH₂), on nitriles (ArCN) by nucleophilic attack on aryl halides. The aryl halides cannot be used in the Friedel-Crafts’s alkylation reaction just like alkyl halides, which can be used.

However, aryl halides do undergo nucleophilic substitution readily if the aromatic ring contains, in addition to halogen, certain other properly placed groups. The presence of electron withdrawing groups like −NO₂, −CF₃ at ortho or para position to the halogen atom makes the aryl halides more susceptible to nucleophilic attack.

The reactions of unactivated aryl halides with strong bases or at high temperature proceed via the intermediate benzyne. The Dow’s process used for the manufacture of phenol involves benzyne intermediate.

In aromatic ring to which halogen is attached can of course, undergo the typical electrophilic aromatic substitution reactions like nitration, sulphonation, halogenation, Friedel−Craft alkylation. Halogen is unusual in being deactivating, yet ortho and para-directing.

(I) Formation of Grignard Reagent

ArBr + Mg ArMgBr

ArCl + Mg ArMgCl

(II) Electrophilic Aromatic Substitution

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

For example,

(III) Nucleophilic Aromatic Substitution

• Bimolecular SNAr Mechanism

   

ArX + Z− ArZ + X−

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

Reaction proceeds by carbanion intermediate. The rate of the reaction increases with the increase in number of electron withdrawing groups, since the carbanion formed would be stabilized more. The mechanism of the reaction is of addition−elimination type.

Step: 1

Step: 2

It can be seen that the presence of −NO₂ group at ortho or para position would facilitate to disperse the negative charge of the carbanion, thus stabilizing it more and making the reaction to occur fast.

(IV) Fittig Reaction

It is an extension of wurtz reaction and consists of heating an ethereal solution of bromobenzene with metallic sodium.

(V) Ullmann Biaryl Synthesis

Iodobenzene on heating with copper in a sealed tube forms biphenyl.

Chloro− or bromobenzene fails to undergo this coupling reaction unless some strong electron withdrawing group (e.g. NO₂) is present in the ortho or para position.