So during my lecture on reaction of Alkenes with Halogens, my lecturer used the example of $\ce{Br2}$ gas reacting with $\ce{CH2CH2}$. He mentioned that the $\ce{C=C}$ launches an electrophilic attack on the electropositive Bromine atom (the one that is nearer to $\ce{CH2CH2}$) and I'm wondering why is the $\ce{C=C}$ launching the electrophilic attack? Shouldn't it be done by the Bromine gas since the $\delta^+ \ce{Br}$ is electrophilic and needs the electrons so it attacks the double bond which is most electron dense?

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    $\begingroup$ That's seems wrong. The bromine molecule is the electrophile and the double bond is the nucleophile and I would normally describe it as an electrophilic attack by the bromine on the double bond. $\endgroup$ – bon Aug 21 '15 at 18:06
  • $\begingroup$ If it just happened once, I am guessing it was a slip of the tongue by your lecturer, it happens... I have said much stupider stuff than that in my lessons :/ In any case, as bon said, $\ce{Br_2}$ is the electrophile and $\ce{C2H4}$ the nucleophile. $\endgroup$ – orthocresol Aug 22 '15 at 12:32

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