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enter image description here In the above reaction, we use a non-polar compound (such as CCl$_4$) as a solvent. Then, why does the Br$_2$ polarise on attack by the alkene, even though a non-polar solvent does not encourage polarisation of a compound. Is the mutual polarisation enough for the reaction to occur.

Further, in different books the mechanism is either explained by the above mechanism or by the Molecular Orbital Theory (the alkene's filled $\pi$ orbital (HOMO) will interact with bromine's empty $\sigma$* to give the product).

Which one of these is the more popular and/or the officially accepted explanation.

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    $\begingroup$ Solvent doesn't have much to do here and all else is more or less complementary. $\endgroup$ – Mithoron Sep 27 '17 at 16:44
  • $\begingroup$ @Mithoron Then why does polarisation of bromine occur to such an extent $\endgroup$ – Ayushmaan Sep 27 '17 at 17:03
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    $\begingroup$ sigh To what "extent" (BTW I don't like untangling reasonings which it looks like)? It's not exactly easy to tell how electron density changes during reaction. It's not like alkene makes something a Lewis acid - it's Lewis acid because it reacts like Lewis acid. $\endgroup$ – Mithoron Sep 27 '17 at 17:52
  • $\begingroup$ Just answered your question. At this time what I do is collecting molecular orbital textbooks for organic chemistry. Will you share what textbooks exactly did you use? $\endgroup$ – MEL Science Sep 27 '17 at 20:10
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Alkene double bond is an area of high electron density and hence high (partial) negative charge. Bromine molecule, approaching double bond, shifts its electrons of covalent bond towards farther Br atom, it becomes polarized and an induced dipole.

That's the simplest explanation in terms of electron movement I would use.

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To answer your first question: The out-of-plane areas of an alkene — those areas, where one typically draws the π bond — are areas of high electron density. If a polarisable compound approaches these, the approaching side will be positively polarised.

To answer your second question: You are constructing a dichotomy which does not exist. Both explanations are the same except that one explains it with molecular orbitals and the other with electron-pair arrow-pushing.

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