Electrophilic Addition with Aqueous Bromine

In this JC1 webinar we want to learn how to describe Electrophilic Addition mechanism when ethene reacts with aqueous bromine.

If you are not familiar with the mechanism of alkene do check out this video lesson on electrophilic addition.

There are 2 possible products, 2-bromoethanol (major) and 1,2-dibromoethane (minor).

EA aq bromine equation

First we have to deduce the electrophile since there are bromine and water present.

EA aq bromine deduce electrophile

Bromine can act as electrophile since one of the bromine atoms can be partial positive charge.

Oxygen in water is more electronegative than hydrogen and will be partial negative charge, hence cannot be electrophile.

So we know step one will involve the ethene attacking electrophilic bromine to form carbocation intermediate and Br-.

EA aq bromine step 1

Next we have to deduce the nucleophile that can attack the carbocation in step 2.

We have 2 possible nucleophiles - oxygen from H2O and Br- formed from step 1.

EA aq bromine deduce nucleophile

The probability of water attacking the carbocation is much higher since water is the solvent so there will be significantly more water as compared to Br-.

This explains why 2-bromoethanol is the major product and 1,2-dibromothane is the minor product.

Formation of 2-bromoethanol involves water attacking the carbocation followed by deprotonation.

EA aq bromine step 2a adding OH

1,2-dibromoethane is formed when Br- attacks the carbocation.

EA aq bromine step 2b adding Br

Finally, an appreciation and understanding of electrophilic addition mechanism allows us to predict the possible products formed.

EA aq bromine deduce possible products

Ethan-1,2-diol is not formed since both oxygens are nucleophilic and cannot react with alkene as electrophiles.

Topic: Alkenes, Organic Chemistry, A Level Chemistry, Singapore

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