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    Although it's an electrophilic addition reaction, and bromine behaves as an electrophile. Plus I never read in any kind of book that ethene in this reaction behaves as a nucleophile -- that's pretty weird. But as this question states it, so I guess it behaves as a nucleophile because it donates a pair of electrons to a bromine atom. But mark scheme rejects this explanation. It says that it behaves as a nuceophile because of having a \pi bond. Can anyone explain this?
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    It has a double bond- an area of negative charge.
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    On page 199 of the chemistry AQA AS textbook it mentions something about the double bond in ethene being a pi orbital... strange question though.
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    I wouldn't mark your answer as being 'wrong', it's just mark schemes are pretty picky (a nucleophile which they've told you it is, is defined as something that donates a pair of electrons) - say it donates a pair of electrons from it's pi bond

    In the same way NH3 can act as a nucleophile because of the lone pair on the nitrogen.
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    The double bond is an area of high electron density which would attack the  \delta^{+} area of the Br2 that was created in an induced dipole. Or something along those lines I think
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    (Original post by EierVonSatan)
    I wouldn't mark your answer as being 'wrong', it's just mark schemes are pretty picky (a nucleophile which they've told you it is, is defined as something that donates a pair of electrons) - say it donates a pair of electrons from it's pi bond

    In the same way NH3 can act as a nucleophile because of the lone pair on the nitrogen.
    Cool! So I guess that my answer was right, but as it has a pi bond, that's why it's donating a pair of electrons - I should've mentioned that.

    Thanks anyway!
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    The double bond is an area of high electron density.

    When the Br2 molecule gets close, a dipole is induced - The bromine nearest the bond becomes delta positive, the bromine further away becomes delta negative.

    Then you get the standard electrophillic addition reaction from then on.
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    Double bond - area of high negative charge density, so when Br2 molecule moves towards it, the Br closer to the double bond gets a delta + (due to the negativity of the double bond) and hence the other gets a delta -, giving the molecule a dipole, so that it can react.
 
 
 
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