Hi,

Examcrackers likes to ask about, and refer to, "optically active" molecules, and I'm having trouble figuring out what this means.

Optically active means that a solution of a compound can rotate plane polarized light.

To rotate plane polarized light, a compound must be chiral, and the solution cannot be 50/50 R and S (racemic mixture). Aside from this case of a racemic mixture, any solution of a chiral compound or chiral compounds should rotate plane polarized light.

Also, what makes a compound a "meso" compound.

A meso compound is a compound that contains chiral centres but is not a chiral molecule.

This usually comes about because this type of molecule has an internal mirror plane (there should be an example of this in the post titled EK ICE 3 by Divya in this Ochem forum).

For example, a molecule with four chiral centres arranged like RSSR could be meso as there might be a mirror plane between the two S's.

Please let me know if you need further clarification.