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As it has restricted rotation due to the presence of a double bond, it should show geometrical isomerism. Then, why doesn't it show cis/trans isomerism?

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    $\begingroup$ 1. The trans form will not be formed because it leads to too much strain in the small ring ring. 2. It will lose aromaticity. $\endgroup$ – King Tut Feb 13 '18 at 3:54
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    $\begingroup$ This doesn't make any sense. Are you sure you have the right molecule? Dichlorobenzene is basically a benzene ring, there is no real isomerism to speak of. Do you mean something like azobenzene? $\endgroup$ – pentavalentcarbon Feb 13 '18 at 5:12
  • $\begingroup$ Geometrical isomerism is shown by linear as well as cyclic chain. So why can't we talk about benzene ring? And yes, the question is correct $\endgroup$ – Isha Roy Feb 13 '18 at 5:58
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    $\begingroup$ Benzene, as an aromat, does NOT contain double bonds like 2-hexene, or any other olefin. Read two pages further in your textbook. $\endgroup$ – Karl Feb 13 '18 at 6:37
  • $\begingroup$ I have read my textbook but benzene ring wasn't mentioned there that's why i asked this question $\endgroup$ – Isha Roy Feb 13 '18 at 8:06
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There are dichlorobenzene isomers but not cis-trans. You can have ortho (where the Cl atoms are close together), para (where the Cl atoms are opposite) and meta where Cl atoms are one C atom apart. Lets examine the ortho-dichlorobenzene which part of it "looks" the most like cis-1,2 dichloroethene (which also has a trans isomer). Is there any way you could make the Cl atoms in ortho-dichlorobenzene to be as far apart as in trans-1,2 dichloroethene without breaking any bonds? The answer should be no and this is why cis-trans isomerism in common substituted aromatic rings makes no sense.

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