Chlorobenzene, reactivity toward

Halides derived from certain heterocyclic aromatic compounds are often quite reactive toward nucleophiles. 2-Chloropyridine, for exfflnple, reacts with sodium methoxide some 230 million times faster than chlorobenzene at 50°C. 

The relative inertness of unactivated aromatic halides towards nucleophiles, under normal conditions, is in sharp contrast to their marked reactivity towards nucleophiles that are also very strong bases. Thus chlorobenzene is readily converted into aniline by reaction with eNH2 (NaNH2) in liquid ammonia at — 33° ... 

Arrange the following compounds in order of their reactivity towards nitration and give the formulae of the major products of the reactions (a) benzene, benzonitrile, methoxybenzene, tri-fluoromethyObenzene (b) acetanilide, benzoic acid, chlorobenzene, acetophenone (c) phenol, ethyl benzoate, nitrobenzene, bro-mobenzene. 

Anisole (methyl phenyl ether) is extremely reactive towards sulfonation due to the presence of the strongly electron-donating (-fM) methoxy group. It therefore reacts with chlorosulfonic acid (two equivalents) at 0 °C to give mainly p-methoxybenzenesulfonyl chloride. With chlorobenzene, the chlorine atom exerts a strong electron-withdrawing (-1 effect) so sulfonation will be rather less easy than for benzene. Treatment of chlorobenzene with chlorosulfonic acid (four equivalents) at 25 C (3 hours) afforded p-chlorobenzenesulfonyl chloride (84%) and di(/7-chlorophenyl sulfone) (6%)." Chlorobenzene with chlorosulfonic acid (six equivalents) at 150-160 °C (8 hours) gave the 2,4-disulfonyl chloride. 

With 77 % aqueous acetic acid, the rates were found to be more affected by added perchloric acid than by sodium perchlorate (but only at higher concentrations than those used by Stanley and Shorter207, which accounts for the failure of these workers to observe acid catalysis, but their observation of kinetic orders in hypochlorous acid of less than one remains unaccounted for). The difference in the effect of the added electrolyte increased with concentration, and the rates of the acid-catalysed reaction reached a maximum in ca. 50 % aqueous acetic acid, passed through a minimum at ca. 90 % aqueous acetic acid and rose very rapidly thereafter. The faster chlorination in 50% acid than in water was, therefore, considered consistent with chlorination by AcOHCl+, which is subject to an increasing solvent effect in the direction of less aqueous media (hence the minimum in 90 % acid), and a third factor operates, viz. that in pure acetic acid the bulk source of chlorine ischlorineacetate rather than HOC1 and causes the rapid rise in rate towards the anhydrous medium. The relative rates of the acid-catalysed (acidity > 0.49 M) chlorination of some aromatics in 76 % aqueous acetic acid at 25 °C were found to be toluene, 69 benzene, 1 chlorobenzene, 0.097 benzoic acid, 0.004. Some of these kinetic observations were confirmed in a study of the chlorination of diphenylmethane in the presence of 0.030 M perchloric acid, second-order rate coefficients were obtained at 25 °C as follows209 0.161 (98 vol. % aqueous acetic acid) ca. 0.078 (75 vol. % acid), and, in the latter solvent in the presence of 0.50 M perchloric acid, diphenylmethane was approximately 30 times more reactive than benzene. 

A p-PhS02 group enhances the electrophilic reactivity of chlorobenzene towards KOH far more than a p-PhSO group, and the p-isomer is much more reactive than the m-isomer in each group. 

Exercise 14-13 Would you expect 4-chloromethoxybenzene and 4-chlorotrifluoro-methylbenzene to be more, or less, reactive than chlorobenzene toward methoxide ion Explain. 

The reactivities of aryl halides, such as the halobenzenes, are exceedingly low toward nucleophilic reagents that normally effect displacements with alkyl halides and activated aryl halides. Substitutions do occur under forcing conditions of either high temperatures or very strong bases. For example, chlorobenzene reacts with sodium hydroxide solution at temperatures around 340° and this reaction was once an important commercial process for the production of benzenol (phenol) ... 

It follows from all the above-mentioned facts that the direct synthesis of methyl-, ethyl and phenylchlorosilanes is a complex heterophase process which depends on many factors and forms a compex reactive mixture. For example, in the direct synthesis of methylchlorosilanes there are about 130 compounds found and characterised. This does not mean, however, that in this or other definite synthesis all the 130 products are formed. The composition of the mixtures formed and the transformation degree of alkyl-chlorides and chlorobenzene in the synthesis of methyl-, ethyl and phenylchlorosilanes depend on the synthesis conditions, the type of the reactor used and many other factors. In spire of the complexity of the process and the variety of its products, the reaction of direct synthesis can nevertheless be directed (towards a preferential formation of a main product), changing the conditions for the preparation of contact mass, introducing various promoters into contact mass and changing the reaction conditions. 

---