Why Water Doesn't Intervene in Ethene Electrophilic Addition

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Discussion Overview

The discussion centers on the mechanism of electrophilic addition of bromine water to ethene, specifically addressing why water does not intervene in the first stage of the reaction when the first electrophile bonds to the carbon atom. Participants explore the nature of the reaction and the roles of bromine and water in the process.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that the reaction is stepwise, with water attacking after the formation of the bromonium ion.
  • Others question why both carbons in ethene cannot bond to hydroxyl groups instead of just one, suggesting a misunderstanding of bonding limitations in carbon.
  • A participant highlights that the bromonium ion is an electrophile and that hydroxide is nucleophilic, which complicates the direct substitution of bromide ions by hydroxide.
  • Concerns are raised about the implications of having too many bonds to carbon if both carbons were to bond with hydroxyl groups.

Areas of Agreement / Disagreement

Participants express differing views on the mechanism of the reaction, particularly regarding the roles of the bromonium ion and hydroxide. There is no consensus on why water does not intervene in the first stage of the reaction.

Contextual Notes

Participants reference the need for activation by bromine and the nature of electrophiles and nucleophiles, but the discussion does not resolve the underlying assumptions about the reaction mechanism.

garytse86
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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?
 
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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?
 
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-?
 
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.
 
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?
 
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.
 

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