Question

When ethene reacts with bromine, it forms
A) Chloroethane
B) Ethylene dibromide
C) 1-bromopropane
D) 1,2-dichloroethene

Answer 1

Hint: We know that ethene is the simplest of alkenes having one double bond. It is present in the gaseous state having a sweet flavour. And we know that bromine is a halogen of red-brown colour present in the liquid state.

Complete Step by Step Answer:
Let’s discuss the reaction of bromine with ethene. Ethene, when a reaction with liquid bromine, or with a bromine solution in tetrachloromethane which is an organic solvent, breaks the double bond of ethene and attaching of one bromine atom at each carbon takes place. Therefore, the product formed in this reaction is dibromo-ethane or ethylene dibromide. The chemical reaction is,
\[{\rm{C}}{{\rm{H}}_{\rm{2}}} = {\rm{C}}{{\rm{H}}_{\rm{2}}} + {\rm{B}}{{\rm{r}}_{\rm{2}}} \to \mathop {{\rm{Br}} - {\rm{C}}{{\rm{H}}_{\rm{2}}} - {\rm{C}}{{\rm{H}}_{\rm{2}}} - {\rm{Br}}}\limits_{{\rm{Ethylene}}\,{\rm{bromide}}} \]

Let’s understand the mechanism of the reaction. In the first step, the attachment of one bromine atom to both carbon atoms. And the bromine atom is positively charged.

Image: Formation of bromonium ion

In the second step, the attack of the bromide ion at the bromonium ion from the back side happens. The resultant product is ethylene dibromide. The reaction is:

Image: Formation of ethylene dibromide
Therefore, the reaction of ethene and bromine gives ethylene dibromide.
Hence, option B is right.

Note: The reaction of ethene and bromine is an electrophilic addition. The electrophilic addition defines an addition reaction of a nucleophile and an electrophile that causes the addition of a double or a triple bond. An electrophile defines a species that has a tendency to react with other molecules having a donatable electron pair.