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The chlorine atom in a chlorobenzene has an ortho- and para- directing effect on any further substitution by an electrophile. Going by probability ortho- position has 2/3 chance and para- position has 1/3 chance of getting substituted by 2nd atom of chlorine. Then why substitution at para- position is preferred?

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Symmetry is the key. p-Dichlorobenzene has more symmetric crystalline structure than o-dichlorobenzene, hence chlorination of chlorobenzene favors the para product. But do note that at high temperatures, the reaction favors the meta product1.

The chlorination of chlorobenzene vapour in the presence of pumice as a contact substance has been studied at 500°, 550° and 600°. The reaction proceeds violently; even when a large excess of chlorobenzene is used, carbon is formed, especially in the experiments at 550° and at 600°. Besides dichlorobenzenes a considerable quantity of higher chlorinated benzenes is formed; the yields of dichlorobenzenes are low. The mixture of dichlorobenzenes has been isolated from the reaction product and the proportions of isomerides has been determined by thermal analysis. The main product (50%-60%) of the mixture of dichlorobenzenes formed within the range of 500–600°, is meta-dichlorobenzene.

Although the chlorination of chlorobenzene at high temperatures is not suitable for an exact quantitative investigation, it is proved in this paper, that this substitution reaction belongs to the meta type at 500–600° in the presence of pumice as a contact substance.

Although the standard reaction using $\ce{FeCl3/Cl2}$ as catalysts leads to polychlorination and leaving the reaction pot for too long leads to complete chlorination to dodecachlorocyclohexane, selective catalyst is sometimes employed to yield a specific isomer as major product. Serguchev et. al. employed antimony pentafluoride-sulfur system as catalyst3.

References

  1. Wibaut, J.P., van de Lande, L.M.F. and Wallagh, G. (1937), On the chlorination of chlorobenzene in the gaseous phase at 500-600°; The meta directing influence of the chlorine atom. Recl. Trav. Chim. Pays-Bas, 56: 65-70. DOI: 10.1002/recl.19370560109
  2. Aver'yanov, V A. Competing chlorination of 1,1-dichloroethane and chlorobenzene. United States: N. p., 1988. Web. Link: https://www.osti.gov/biblio/6007897-competing-chlorination-dichloroethane-chlorobenzene
  3. Serguchev, Y.A., Chernobaev, I.I. Active intermediates for the selective chlorination of chlorobenzene in the antimony pentachloride–sulfur-containing compound catalytic system. Theor Exp Chem 46, 102–106 (2010). DOI: 10.1007/s11237-010-9125-2
  4. https://patents.google.com/patent/US3636171A/en
  5. Why can't the electrophile attack the chlorine atom in a halobenzene?
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  • $\begingroup$ Thanks @Nilay Ghosh. Information on further chlorination of chlorobenzene favouring meta- position AT HIGHER TEMPERATURE is a bonus to what I sought originally. Your suggestion that AT ROOM TEMPERATURE , further chlorination of chlorobenzene at para- position being due to greater stability of its CRYSTAL leads me to assume that , with BP of 323 k, para dichlorobenzene gets precipitated down in the solution, thus due to Le Chatelier's principle, as para product is precipitated, more and more para product is formed $\endgroup$ Jun 19 at 5:01
  • $\begingroup$ removal of a product affects the forward reaction only if the reaction is at equilibrium and will not affect a kinetically controlled reaction. The favoring of para over ortho is probably a combo of steric effects and inductive effects in the transition state $\endgroup$
    – jimchmst
    Jun 19 at 5:23

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