Final answer:
Achiral molecules can be superimposed on their mirror images and do not possess a chiral center or exhibit optical activity. They often contain planes of symmetry and are optically inactive, in contrast to chiral molecules that can rotate plane-polarized light.
Step-by-step explanation:
Characteristics of Achiral Molecules
An achiral molecule is one that can be superimposed on its mirror image. These molecules often contain planes of symmetry or other elements of symmetry that make both halves of the molecule mirror images of each other. One key feature of an achiral compound is that it does not have a chiral center, meaning there isn't a carbon atom with four different groups attached to it. In contrast, a molecule with such a carbon atom would be considered chiral and, consequently, its mirror image would be non-superimposable, resulting in enantiomers. Enantiomers exhibit a unique physical property called optical activity, which allows them to rotate the plane of plane-polarized light, a characteristic that achiral molecules lack. Achiral molecules do not show such rotation and are therefore optically inactive.
An example of an achiral molecule could be one with two identical substituents attached to a central carbon atom, resulting in an internal plane of symmetry. Such molecules without stereogenic centers do not lead to stereoisomerism like enantiomers.