Stereoisomerizations

Stereoisomerizations are transformations that convert one stereoisomer to another without altering the connectivity of the molecule. They differ from conformational changes in that a stereoisomerization changes the spatial arrangement that defines the stereochemistry (for example, the configuration at a stereocenter or the geometric arrangement around a double bond) and thus alters the identity of the stereoisomer.

Common forms include geometric isomerization around C=C or around ring systems, where a trans (E) isomer becomes cis (Z) or vice versa; and configurational changes at stereocenters, leading to racemization (interconversion of enantiomers) or epimerization (between diastereomers). Atropisomerism, arising from hindered rotation about a bond, can also give isolable stereoisomers that interconvert slowly.

Mechanisms vary: thermal processes may proceed via carbenium or carbanion intermediates, diradical pathways, or pericyclic rearrangements; photochemical processes often involve excitation that alters the allowed path, enabling E/Z isomerization or inversion at stereocenters. In many cases, light-induced cis-trans isomerization is a well-studied example, seen in molecules such as stilbene and azobenzene derivatives, and in biological systems such as retinal isomerization in vision.

Kinetics are governed by energy barriers; high barriers allow separable stereoisomers at ambient conditions, while low barriers lead to rapid equilibration. Stereoisomerization is important in synthesis, chiral resolution, materials science, and photochemistry, where the control of stereochemical state tunes physical properties and biological activity.