Which type of compound is chiral but has a line of symmetry, making it not optically active?

A meso compound is a type of molecule that contains multiple chiral centers but also possesses an internal plane of symmetry, which results in the entire molecule being achiral despite the presence of those chiral centers. This internal symmetry means that the compound can be superimposed on its mirror image, a property that eliminates optical activity.

Optical activity arises when a molecule can rotate plane-polarized light, but meso compounds do not exhibit this behavior due to their symmetrical nature, effectively canceling out any rotation that might occur from individual chiral centers. Thus, while meso compounds have chiral centers, their overall structure leads to a lack of optical activity, distinguishing them clearly in the field of stereochemistry.

In contrast, chiral centers are typically part of compounds that are optically active, as they do not have the necessary symmetry. Achiral compounds, by definition, do not have chiral centers and cannot exhibit optical activity due to their symmetrical nature. Racemates are mixtures of equal amounts of enantiomers, leading to no net optical activity, but they are not the same as meso compounds because they do not feature a line of symmetry within a single compound. Hence, meso compounds are unique in that they combine