Question

Mechanism for reaction of bromine water with ethene (stages explained)

Answer 1

The reaction of bromine water with ethene is a halogen addition reaction where a bromonium ion intermediate is formed, followed by nucleophilic attack by bromide, resulting in 1,2-dibromoethane with trans addition of bromine atoms.

Step-by-step explanation:

The reaction between ethene (CH2=CH2) and bromine water (Br2 in water) is a typical example of a halogen addition reaction, where bromine is added across the double bond of ethene, resulting in the formation of 1,2-dibromoethane (CH2Br-CH2Br).

This reaction proceeds through the following stages:

1. Bromine, which is electrophilic, approaches the electron-rich double bond of ethene.
2. As bromine gets closer, the double bond electrons attack one of the bromine atoms, forming a bromonium ion (a cyclic bromonium bridge) and a bromide ion.
3. The bromide ion acts as a nucleophile and attacks the more substituted carbon of the bromonium ion, opening up the three-membered ring and forming 1,2-dibromoethane.

The addition of halogen to an alkene like ethene is stereospecific, leading to trans addition, which means that the two bromine atoms are added on opposite sides of the original double bond.