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Why is it that the chlorination of 1-chloro-3-nitrobenzene (18) gives 1,4-dichloro-2-nitrobenzene (19) as the major product, 1,2-dichloro-3-nitrobenzene (20) as a minor product and 1,2-dichloro-4-nitrobenzene (21) is not formed at all?

I know $\ce{-NO2}$ is a deactivating, meta-directing group and hence it decreases the electron density at positions 2, 4, 6 while position 5 is unaffected. On the other hand, $\ce{-Cl}$ is a deactivating, o-p directing group and hence it directs the incoming group to positions 2,4 or 6. Taking into account the fact that $\ce{-NO2}$ is a more powerful deactivating group than $\ce{-Cl}$, the incoming $\ce{Cl+}$ should be attached at position 5 as it has more electron-density than 6,2 or 4 position. But the product with $\ce{Cl}$ at position 5 is not formed. Why so? I am unable to rationalise this.

Source: page 583 of "March's Advanced Organic Chemistry, 7th ed." under the heading "Orientation in benzene rings with more than one substituent".

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I quote March's Advanced Organic Chemistry, Reactions, Mechanisms and Structure, 6th edition, page 666.

Some substituents have a pair of electrons (usually unshared) that may be contributed toward the ring. The three arenium ions would then look like this:enter image description here

The stability of these two ions is increased by the extra form not only because it is another canonical form, but because it is more stable than the others and makes a greater contribution to the hybrid. Every atom (except of course hydrogen) in these forms (C and D) has a complete octet, while all the other forms have one carbon atom with a sextet. No corresponding form can be drawn for the meta isomer.The inclusion of this form in the hybrid lowers the energy , but also because it spreads the positive charge over a larger area—out onto the group Z. Groups with a pair of electrons (e.g., as the halogens) to contribute would be expected, then, in the absence of field effects, not only to direct ortho and para, but also to activate these positions for electrophilic attack. (my emphasis)

In your example, if chlorine electrophile attacks at C-4 or C-6, the resulting intermediate is stabilized by an extra resonating structure, lowering the energy of this intermediate.Hence electrophillic aromatic substitution takes place.

enter image description here

enter image description here

If chlorine electrophile attacks at C-5 , the resulting intermediate is not stabilized compared to attack at C-6 .Hence the resulting electrophillic aromatic substitution is not seen.

enter image description here

This is the possible reason that the product with Cl at position 5 is not formed.


Quoting from chemistry.msu.edu:

The influence a substituent exerts on the reactivity of a benzene ring may be explained by the interaction of two effects:

The first is the inductive effect of the substituent. Most elements other than metals and carbon have a significantly greater electronegativity than hydrogen. Consequently, substituents in which nitrogen, oxygen and halogen atoms form sigma-bonds to the aromatic ring exert an inductive electron withdrawal, which deactivates the ring.

The second effect is the result of conjugation of a substituent function with the aromatic ring. This conjugative interaction facilitates electron pair donation or withdrawal, to or from the benzene ring, in a manner different from the inductive shift. If the atom bonded to the ring has one or more non-bonding valence shell electron pairs, as do nitrogen, oxygen and the halogens, electrons may flow into the aromatic ring by p-π conjugation (resonance). enter image description here

The second factor that becomes important in reactions of substituted benzenes concerns the site at which electrophilic substitution occurs.

enter image description here

The exact influence of a given substituent is best seen by looking at its interactions with the delocalized positive charge on the benzenonium intermediates generated by bonding to the electrophile at each of the three substitution sites.

enter image description here

Therefore chlorine is an ortho para directing group with minor meta product.

References:

  • Advanced Organic Chemistry 5th edition Part A: Structure and Mechanisms, Francis A. Carey and Richard J. Sundberg
  • March's Advanced Organic Chemistry, Reactions, Mechanisms and Structure, 6th edition
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  • $\begingroup$ According to your answer, the attack at "your"(as I noticed that you numbered the carbons in a different way than I did) position 6 is favorable but my question itself says that pdt is not formed at all. And you used Cl to stabilize the +ve charge through $+M$ effect but I suppose $-I$ of halogens is greater than their $+M$, so Cl should destabilize it. $\endgroup$ – Carrick Jan 28 '18 at 12:13
  • $\begingroup$ @Carrick first numbering was done as per IUPAC numbering system.Secondly you are correct in saying -I effect of Cl is evident.How ever +M effect of Cl cannot be discounted.Going by your arguments chlorbenzene should not give ortho and para products as major products.Note one more point, -I effect is through a sigma bond while +M effect is through a Pi bond . $\endgroup$ – Chakravarthy Kalyan Jan 28 '18 at 12:20
  • $\begingroup$ btw I took that question from jerry march, and it said $21$ is not formed at all which I think is indeed true because if you delocalize the +ve charge further in the benzene ring, you shall notice that the +ve charge lands on the carbon right beneath the $NO_2$ group. So, don't you think that it should really destabilize that?? $\endgroup$ – Carrick Jan 28 '18 at 12:28
  • $\begingroup$ @Carrick i have added NOTES regarding chlorine's ortho para directing nature.But you also get an extra resonance structure in which +ve charge is stabilized by lone pair on chlorine. 3 to be exat. $\endgroup$ – Chakravarthy Kalyan Jan 28 '18 at 12:47
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    $\begingroup$ why haven't you taken the deactivating effect of $NO_2$ gp into account in any of your proposed explanations. $\endgroup$ – Carrick Jan 28 '18 at 17:09

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