4.
Define Topicity?
Answer: Topicity is based on the idea that the four substituents on a stereocenter can be labeled as either prochiral or chiral. A prochiral substituent is one that, with the addition of a hydrogen atom, would create a chiral center. A chiral substituent is one that is already chiral.
If two substituents on a stereocenter are prochiral, they are said to be diastereotopic. This means that they can be differentiated by a chemical reaction, such as addition of a reagent or solvent, or by the presence of a chiral environment, such as a chiral catalyst.
On the other hand, if two substituents on a stereocenter are chiral, they are said to be enantiotopic. This means that they cannot be differentiated by a chemical reaction, but they can be differentiated by the presence of a chiral environment, such as a chiral solvent or catalyst.