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) Only syn addition occurs. Because the reactant is trans so the product is expected to be threo pair of enantiomers.

b) Product doesn,t has asymmetric centre so there is no stereoisomer.

c) Only syn addition occurs. Because the reactant is cis so the product is pair of enantiomers with the hydrogen on the same side of the ring. But here product is meso compound so only one stereoisomer will form.

d) Only syn addition occurs. Because the reactant is cis so the product is pair of enantiomers with the hydrogen on the same side of the ring.