# In the presence of an electron-rich species, why is Br2 the stronger electrophile than water? [closed]

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-:

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?

• $\mathrm{p}K_{\mathrm{a}}$ value of water is $14$ while that of $\ce{HBr}$ is $-9$. Thus, $\ce{Br-}$ is very stable compared to $\ce{OH-}$. – Apoorv Potnis Feb 11 '18 at 7:18
• sigh So you're asking why ethene won't get just protonated instead of binding Br+ ? Or what? – Mithoron Feb 11 '18 at 22:02