Why Water Doesn't Intervene in Ethene Electrophilic Addition

In summary: Originally posted by garytse86Understand 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.
  • #1
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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  • #2
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?
 
  • #3
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-?
 
  • #4
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.
 
  • #5
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?
 
  • #6
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.
 

1. Why doesn't water intervene in ethene electrophilic addition?

Water does not intervene in ethene electrophilic addition because it is a weak nucleophile. This means that it does not have the ability to attack the carbon-carbon double bond in ethene and form a bond. Additionally, water is a polar molecule, meaning it has a partial positive and negative charge. In electrophilic addition, the electrophile (such as hydrogen chloride) is attracted to the negative end of the water molecule, rather than the ethene molecule.

2. Can water be used as a solvent in ethene electrophilic addition reactions?

Yes, water can be used as a solvent in ethene electrophilic addition reactions. However, it is not a preferred solvent because it is a weak nucleophile and may not efficiently dissolve the reactants. Other solvents, such as polar aprotic solvents, are often used in electrophilic addition reactions instead.

3. How is the reaction mechanism for ethene electrophilic addition affected by the presence of water?

The presence of water does not significantly affect the reaction mechanism for ethene electrophilic addition. Water may act as a solvent, but it does not participate in the reaction itself. The mechanism still involves the electrophilic attack of the electrophile on the carbon-carbon double bond in ethene, followed by the formation of a carbocation intermediate and the addition of a nucleophile.

4. Can water participate in electrophilic addition reactions in other molecules?

Yes, water can participate in electrophilic addition reactions in other molecules. However, it is more likely to act as a nucleophile in these reactions, rather than an electrophile. This is because water has lone pairs of electrons that can attack electrophilic sites in other molecules. In ethene, the presence of the double bond makes it a stronger nucleophile than water.

5. Are there any exceptions where water does intervene in ethene electrophilic addition?

There are some exceptions where water may intervene in ethene electrophilic addition. For example, in the presence of a strong acid, water can act as an electrophile and add to the carbon-carbon double bond in ethene. This is known as acid-catalyzed hydration and is a common reaction in organic chemistry. However, in general, water is not considered a significant participant in ethene electrophilic addition reactions.

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