Statement: A chiral compound with an ‘X’ group attached to the stereocenter has been replaced by a ‘Y’ group. The product obtained rotates the plane of polarized light in the direction opposite to that of the original compound.

What process is taking place?

Inversion or retention

When a chiral compound undergoes a reaction where an 'X' group attached to the stereocenter is replaced by a 'Y' group, two possible processes can occur inversion or retention.

1. Inversion: In this process, the 'Y' group replaces the 'X' group while the configuration of the stereocenter is inverted. This means that the substituents attached to the stereocenter change their positions, resulting in the opposite configuration. In terms of optical activity, if the original compound rotates the plane of polarized light in one direction, the product obtained after inversion will rotate the plane of polarized light in the opposite direction.

2. Retention: In this process, the 'Y' group replaces the 'X' group while maintaining the same configuration at the stereocenter. The substituents attached to the stereocenter do not change their positions during the substitution. In terms of optical activity, if the original compound rotates the plane of polarized light in one direction, the product obtained after retention will also rotate the plane of polarized light in the same direction as the original compound.

Therefore, the process taking place when the product obtained rotates the plane of polarized light in the opposite direction to the original compound could be either inversion or retention.