Why deep brown colour of bromine solution in carbon tetrachloride vanishes when it is treated with excess of ethylene?

When bromine solution in carbon tetrachloride is treated with excess ethylene, a reaction occurs between the bromine and ethylene. Ethylene is a reducing agent, meaning it has the ability to donate electrons to other substances. In this case, ethylene reduces the bromine in the solution, causing it to lose its deep brown color.

The reaction between bromine and ethylene can be represented as follows:

Br2 + C2H4 -> C2H4Br2

In this reaction, the ethylene reacts with the bromine to form 1,2-dibromoethane. This reaction consumes the bromine in the solution, causing the deep brown color to vanish.

The deep brown colour of bromine solution in carbon tetrachloride vanishes when it is treated with excess of ethylene because ethylene undergoes an addition reaction with bromine to form 1,2-dibromoethane. This reaction is a typical electrophilic addition reaction, in which the electrophile is the bromine molecule and the nucleophile is the ethylene molecule. The reaction proceeds via a cyclic intermediate, which is formed by the attack of the ethylene molecule on the bromine molecule. The intermediate then collapses to form 1,2-dibromoethane and a molecule of hydrogen bromide.

The reaction can be represented as follows:

```
Br2 + C2H4 → BrCH2CH2Br + HBr
```

The disappearance of the brown colour of the bromine solution is due to the fact that the bromine molecules are consumed in the reaction with ethylene. As a result, the concentration of bromine in the solution decreases, and the colour of the solution becomes lighter.