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A question from JEE-Advanced(2017) : enter image description here

Bromination of both Z-but-2ene and E-but-2-ene involve syn and anti addition. So, there would be 4 products as suggested in the question. But, I don't understand the terminology of 'trans-addition', mentioned in the question.

What is actually trans addition?

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    $\begingroup$ Uh, I think that the correct term should be "anti" not "trans" $\endgroup$ Jun 6, 2017 at 2:24
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    $\begingroup$ Why do you think there will be syn addition? $\endgroup$
    – Zhe
    Jun 6, 2017 at 2:29
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    $\begingroup$ A screenshot or picture of written text is problematic as it cannot be indexed by our site search or other search engines. Chemistry.se is about the content, and your question is unlikely to be helpful in the future for other people. Please take a couple of minutes to transcribe the text, so that it can be found easily in the future. $\endgroup$ Jun 6, 2017 at 7:01
  • $\begingroup$ @Martin Well, I am kinda underskilled at mhChem and with my mobile phone jt's hard to draw chemical structures. So, I am sorry. But I will edit the question. $\endgroup$ Jun 6, 2017 at 8:06

1 Answer 1

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Just like @QuantumAMERICCINO has pointed out, it's probably anti addition.

Furthermore, you seem to wonder why a syn compound doesn't form. It all lies in the mechanism of bromination:

The double bond in cis-2-butene polarises the $\ce{Br-Br}$ bond in a nearby $\ce{Br2}$ molecule. A $\ce{Br+}$ ion attacks the double bond and forms a bromonium ion.

enter image description here

The bromonium ion makes the adjacent carbon atoms electrophilic. The bromide may attack the most electrophilic out of them. Here, however, they are equivalent.

enter image description here

You should now be able to understand how the mechanism works.

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  • $\begingroup$ So how are there 4 products? $\endgroup$ Jun 6, 2017 at 4:57
  • $\begingroup$ What 4 products @Mockingbird360 $\endgroup$ Jun 6, 2017 at 5:03
  • $\begingroup$ M,N,O,P as suggested in the question. $\endgroup$ Jun 6, 2017 at 5:04
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    $\begingroup$ @Mockingbird360 Oh that, they are just stereoisomers. I showed in my example, the bromine attacking the left carbon. If it attacked the right one instead, you would get the enantiomer of the above compound. For a trans-2-butene you would get a meso product. $\endgroup$ Jun 6, 2017 at 5:11

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