Why Water Doesn't Intervene in Ethene Electrophilic Addition

[garytse86]

When ethene undergoes electrophilic addition with bromine water Br2(aq), at the second stage the water reacts with the second carbon,and a hydroxyl group bonds with the carbon across the double bond instead of the bromide ion. Why doesn't water intervene at the first stage, i.e. when the first electrophile bonds with the first carbon atom?

 

[Chemicalsuperfreak]

Originally posted by garytse86
When ethene undergoes electrophilic addition with bromine water Br2(aq), at the second stage the water reacts with the second carbon,and a hydroxyl group bonds with the carbon across the double bond instead of the bromide ion. Why doesn't water intervene at the first stage, i.e. when the first electrophile bonds with the first carbon atom?

Are you asking why it is a stepwise process? Why water attacks after formation of the bromonium ion? This is because ethylene (not ethene, btw) needs to be activated by the bromine for water to attack.

Or are you asking why there is a bromonium ion instead of a full carbocation?

 

[garytse86]

Yes like what you have said the reaction has to be started by bromine, so why shouldn't both carbon be bonded to OH-, not just one OH-?

 

[Chemicalsuperfreak]

Originally posted by garytse86
Yes like what you have said the reaction has to be started by bromine, so why shouldn't both carbon be bonded to OH-, not just one OH-?

Well, for starters, you can't have five bonds to carbon. Which is what would happen if you have a bridged hydroxide ion, which you appear to be suggesting.

 

[garytse86]

I mean as soon as the bromide ion reacts with the carbon, can't the hydroxide take its place so that no bromide ions join, since the surrounding is all water?

 

[Chemicalsuperfreak]

Originally posted by garytse86
I mean as soon as the bromide ion reacts with the carbon, can't the hydroxide take its place so that no bromide ions join, since the surrounding is all water?

Understand that it's not the bromide ion, but the bromonium ion, it's an electrophile, it's got a positive charge. Hydroxide is negative, it's nucleophilic, you can exchange it with the bromonium ion, too many valence electrons for starters.