Question

Reaction of\[B{r_2}\] on ethylene in presence of NaCl gives
A.\[{\rm{BrC}}{{\rm{H}}_{\rm{2}}}{\rm{ - C}}{{\rm{H}}_{\rm{2}}}{\rm{Br}}\]
B.\[ClC{H_2} - C{H_2}Br\]
C. Both A and B
D. None of these

Answer 1

Hint: Alkenes react with bromine in the cold with pure liquid bromine, or with a solution of bromine in an organic solvent like tetrachloromethane. The reaction of ethylene with molecular bromine is an instance of electrophilic addition.

Complete Step by Step Solution:
Ethylene reacts with bromine in an organic solvent like tetrachloromethane.
The double bond present in ethylene breaks down, and a bromine atom is connected to each carbon.
The bromine which was initially red-brown lost its colour to provide a colourless liquid.
In this reaction, ethylene gets converted to 1,2-dibromoethane.
The reaction is an instance of electrophilic addition.
Bromine is an extremely "polarizable" molecule and when it moves toward the pi bond in the ethene, it causes an induced dipole in the bromine molecule.
Step-1
One of the bromine atoms gets connected to both carbon atoms, with a positive charge on the bromine atom. A bromonium ion is constructed.

Image: Formation of bromonium ion

Step-2
The bromonium ion is then attacked from the back by another bromide ion. 1,2-dibromoethane is formed as a product.


Image: Attack of bromide ion.
So, the product in this reaction is 1,2-dibromoethane.
But NaCl is also present in the medium which will produce chloride ions. This chloride ion will displace one bromide ion giving rise to 1-bromo-2-chloroethene.

Image: Formation of 1-bromo-2-chloroethene
So, both \[ClC{H_2} - C{H_2}Br\] and \[{\rm{BrC}}{{\rm{H}}_{\rm{2}}}{\rm{ - C}}{{\rm{H}}_{\rm{2}}}{\rm{Br}}\]are formed.
So, both A and B will be formed.
So, option C is correct.

Note: We learned that a displacement reaction is the one in which the atom or a group of atoms is replaced by another atom in a molecule. Chlorine is more reactive than bromine. So, chloride ion displaces bromine ion in 1,2-dibromoethane. A less reactive halogen can't replace a more reactive halogen.