Diastereomerism is a type of stereoisomerism where molecules have the same connectivity of atoms but differ in their spatial arrangement due to the presence of multiple chiral centers or the presence of geometric isomerism (cis-trans isomerism). Diastereomers are not mirror images of each other and have different physical properties, chemical reactivity, and interactions.

Here are some key points about diastereomerism:

1. Chiral Centers: Diastereomers arise in molecules with more than one chiral center. Chiral centers are carbon atoms bonded to four different substituents.
2. Different Arrangements: Diastereomers have different spatial arrangements of substituents around at least one chiral center, while other chiral centers may remain unchanged.
3. Cis-Trans Isomerism: Diastereomers can also arise from cis-trans isomerism in compounds with restricted rotation around a double bond. Cis and trans isomers are different and non-mirror images of each other.
4. Physical Properties: Diastereomers have different physical properties such as melting points, boiling points, and solubility due to their distinct three-dimensional structures.
5. Chemical Reactivity: Diastereomers can exhibit different chemical reactivity and stereoselectivity in reactions involving specific chiral centers.
6. Optical Activity: Unlike enantiomers, diastereomers do not necessarily exhibit opposite optical rotations. Some may be optically active, while others may be optically inactive.
7. Naming: Diastereomers are often named using descriptors such as “cis” and “trans” for geometric isomerism, or they may be assigned configuration descriptors using the “R/S” system.
8. Number of Diastereomers: The number of possible diastereomers increases with the number of chiral centers in the molecule.
9. Cyclic Compounds: Diastereomers can also arise in cyclic compounds with different arrangements of substituents around the ring.
10. Geometric Isomers: In addition to chiral centers, diastereomers can also arise from geometric isomerism around double bonds, where substituents are positioned differently.

Diastereomerism is important in understanding the diversity of molecular structures and their properties. It has applications in various fields, including organic chemistry, pharmacology, and materials science.