91Ó°ÊÓ

Two or more compounds which differ in the spatial arrangement of their atoms are called as stereoisomers i.e the molecules which are stereoisomers of each other are made from same atoms, connected in the same sequence but the atoms are arranged differently in space.

1. Assign the priority order to the groups on the basis of atomic number. Higher the atomic number get highest priority.

2. Double bond is considered as carbon is attached to 2 carbons.

3. Triple bond is considered as carbon is attached to 3 carbons.

4. Lowest priority group is always on dotted line.

5. If lowest priority group is not on dotted line then exchange the group with that group which is on dotted line and to maintain the stability of the compound we have to exchange other two compounds also.

6. Then according to priority order we have to determine the configuration. We have to go highest priority order to lowest priority order and if we go clockwise then it is R configuration and if we go anticlockwise then it is S configuration.

a) Here only anti addition occurs. Because the reactant is trans so the product is expected to be ethryo pair of enantiomers. In this case each asymmetric carbon atom is attached to same four substituents so it is meso compound and will form one stereoisomer.

b) Only one asymmetric centre is there in product so the product is racemic mixture.

c) Here only anti addition occurs. Because the reactant is cis the product is the pair of enantiomers with the bromine on opposite sides of the ring.

d) Here only anti addition occurs. Because the reactant is cis the product is the pair of enantiomers with the bromine on opposite sides of the ring.