Explain what a chiral centre is and how it can lead to optical isomerism.

A chiral centre is a carbon atom bonded to four different groups, leading to optical isomerism.

Optical isomerism is a type of stereoisomerism where two molecules have the same chemical formula and connectivity, but differ in the way they rotate plane-polarized light. This occurs when a molecule has a chiral centre, which is a carbon atom bonded to four different groups. The four groups can be arranged in two different ways, resulting in two different molecules that are mirror images of each other, known as enantiomers.

Enantiomers have identical physical and chemical properties, except for their interaction with plane-polarized light. One enantiomer will rotate the plane of polarized light clockwise, while the other will rotate it counterclockwise. This property is known as optical activity, and the enantiomers are referred to as being optically active.

Chiral centres are found in many biological molecules, such as amino acids and sugars, and their optical isomerism can have significant biological effects. For example, one enantiomer of a drug may be effective in treating a particular condition, while the other enantiomer may have no effect or even be harmful. Therefore, it is important to separate and identify the enantiomers of drugs to ensure their safety and efficacy.