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Why can't the electrophile attack the chlorine atom in a halobenzene? I mean, chlorine is a deactivating group, primarily due to its inductive effect.

If we can say that its ortho and para influence on rise of electron density is why electrophiles occupy those positions, why not they bond with chlorine who does take up those electron density at the same time?

Examples of electrophilic substitution could be chlorination of chlorobenzene, nitration of chlorobenzene, sulphonation of chlorobenzene ...

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  • $\begingroup$ Could you add an image of what you mean? I'm having trouble with the description. $\endgroup$ – jerepierre Oct 27 '17 at 14:07
  • $\begingroup$ Apparently, they do have a reaction although they are supposed to be reacting slowly as compared to benzene, which does not have its electron density drawn away from the ring. $\endgroup$ – Karthik Oct 27 '17 at 14:46
  • $\begingroup$ Do you mean to ask why do the pi electrons attack the electrophile and not the lone pairs of Chlorine? $\endgroup$ – Apoorv Potnis Oct 27 '17 at 14:51
  • $\begingroup$ No . But why not the electrophile attacks the lone pair on chlorine. That is why not the nucleophile form a bond with chlorine instead. $\endgroup$ – Karthik Oct 27 '17 at 14:52
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Given the charge distribution and the shape of the HOMO, I would be very surprised if an electrophile does not attack the chlorine. This is from a calculation on the DF-BP86/def2SVP level of theory.

If you account for the hydrogens effectively shielding the negative charges of the carbons, the only place left to go is the chlorine. (Natural Population Analysis, NBO 6.0)

npa charges

The largest coefficient of the HOMO is at the chlorine (C1 ipso, C4 para):

Alpha occ 29 OE=-0.224 is Cl12-p=0.29 C4-p=0.24 C1-p=0.19 C2-p=0.09 C6-p=0.09

HOMO

Generally I would expect it to be rather unreactive towards electrophiles.

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  • $\begingroup$ In fact, you are right that it is difficult to have a reaction with chlorobenzene; why not is it possible for the electrophile to bond with the chlorine? Is it anything to do with a + charge on chlorine in one of its canonical structures during resonance? $\endgroup$ – Karthik Oct 27 '17 at 15:35
  • $\begingroup$ I did not have had the time to go further into detail, but Queiroz et al. J. Org. Chem. 2006, 71, 6192-6203, DOI: 10.1021/jo0609475 (mirror) make quite an effort explaining the mechanism(s). $\endgroup$ – Martin - マーチン Oct 27 '17 at 15:36
  • $\begingroup$ @V.Karthik Please cite your sources. This statement makes little to no sense to me. The highest electron density is at the chlorine atom, not in the aromatic ring. $\endgroup$ – Martin - マーチン Oct 27 '17 at 15:39
  • $\begingroup$ I am following this from my high school book (CBSE-Indian curriculum). Grade 12 $\endgroup$ – Karthik Oct 27 '17 at 15:41
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    $\begingroup$ @V.Karthik There are many, many factors influencing a reaction, how molecules bind, and what products are formed. At a high school level they cannot sufficiently be discussed. The paper above is very detailed and very specific for only a hand full of molecules and even they come to the conclusion that there is no universal way. However, the claim that elecrophiles do not bind to the chlorine is certainly not entirely correct. $\endgroup$ – Martin - マーチン Oct 27 '17 at 15:56

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