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The reaction of benzene with excess of chlorine in the presence of sunlight produces 1,2,3,4,5,6-hexachlorocyclohexane as the major product. This is also given in the preparation section of the Wikipedia page on lindane.

Also, multiple chlorination of methane is also well-known, specially when chlorine is in excess. For example, it is mentioned at the bottom of this page from MasterOrganicChemistry.

My question: After hexachlorocyclohexane (lindane) is formed, I thought that it is an ordinary haloalkane, comparable to chloromethane, so if chlorine is excess, it should also substitute the remaining six hydrogen atoms from lindane - forming 1,1,2,2,3,3,4,4,5,5,6,6-dodecachlorocyclohexane. Since the reaction is used industrially, what I propose obviously doesn't occur. Why is that so?

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    $\begingroup$ +1 Perhaps they don't use excess chlorine, or control the reaction somehow? $\endgroup$
    – S R Maiti
    Jul 13, 2021 at 14:07
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    $\begingroup$ @ShoubhikRMaiti I thought so too, maybe they use 3:1 ratio of $\ce{Cl2}$ and benzene for the reaction, but I couldn't find it in the two or three very basic textbooks (or on the net) I have currently. If you can find that they indeed use a stoichiometric amount in any textbook or online source you know of, please feel free to post that as an answer :) Thank you. $\endgroup$
    – TRC
    Jul 13, 2021 at 14:12
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    $\begingroup$ Somebody has AGAIN downvoted this question of mine without leaving any reason/comment as to why. Please DO NOT downvote if you cannot give the reason - specially as the question had 6 upvotes, meaning it was certainly not an outright nonsensical question. $\endgroup$
    – TRC
    Jul 15, 2021 at 3:24
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    $\begingroup$ @TRC it looks like that this problem has started for quite some time. Even other users are complaining about their posts getting downvoted without any reason. If this problem persists, feel free to contact support team. $\endgroup$ Jul 15, 2021 at 4:24

1 Answer 1

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Given enough exposure time, the reaction will lead to formation of various products, even the fully chlorinated compound: dodecachlorocyclohexane. Increasing the concentration of chlorine also help in the formation of this product. Doing some literature survey confirms this:

[...] During most of the course of this reaction (photochemical chlorination of benzene), both substitution and addition takes place simultaneously, the final reaction product after long periods approaching the composition of dodecachlorocyclohexane ($\ce{C6Cl12}$)

Photochemical Studies. XVII. The Chlorination of Chlorobenzene; a Comparison with Benzene, Edwin J. Hart and W. Albert Noyes, Journal of the American Chemical Society 1934 56 (6), 1305-1310, DOI: 10.1021/ja01321a016

The photochlorination of aromatic hydrocarbons may result in either addition or substitution or both. The photochlorination of benzene in either liquid phase or gas phase yields mainly hexachlorocyclohexane, but when high chlorine concentration and long exposure time are employed, the product is mainly dodecachlorocyclohexane.

Technique of Organic Chemistry: Catalytic, photochemical, and electrolytic reactions, Arnold Weissberger, Interscience Publishers, 1956

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