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In the electrophilic addition of aqueous bromine to ethene, the first slow step involves the electron rich (pi bond) of ethene attacking the bromine molecule to form a carbocation and Br-: enter image description here

And, then water will react with the carbocation to form 2-bromoethanol. But, why does bromine react with ethene first and not water(which is of higher concentration)?

I think it's because Br2 is the stronger electrophile than water. But why is that? Oxygen is much more electronegative which makes H more electron deficient than Br and thus a stronger electrophile?

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  • $\begingroup$ $\mathrm{p}K_{\mathrm{a}}$ value of water is $14$ while that of $\ce{HBr}$ is $-9$. Thus, $\ce{Br-}$ is very stable compared to $\ce{OH-}$. $\endgroup$ – Apoorv Potnis Feb 11 '18 at 7:18
  • $\begingroup$ sigh So you're asking why ethene won't get just protonated instead of binding Br+ ? Or what? $\endgroup$ – Mithoron Feb 11 '18 at 22:02
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Short answer: you are correct that oxygen is more electronegative than bromine, but the oxygen's thirst for electrons is fairly well satisfied by 2 hydrogens, whereas the bromine -- aack! -- is attached to another electron-hungry bromine.

Bromine will snap up electrons from moderately active metals (even from water!), but water doesn't (or else does it much more slowly). Consider iron, zinc, magnesium - all react more rapidly with bromine than with water.

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