ABSTRACT
Rotation about C-C bonds occurs in molecules that contain only sp3 atoms, as described in Section 8.1, leading to dierent arrangements of atoms called rotamers. Although there is a different spatial orientation of atoms, rotamers are not isomers, but are the same molecule. A different situation arises when a sp3-hybridized carbon has four dierent atoms or groups attached. e relative positions of these atoms and groups are xed. Once attached to a central carbon atom, the only way to change the relative and specic positions of four atoms or groups is to make or break bonds. If the atoms and groups attached to the sp3 center are dierent one from the other, a new type of stereoisomer is possible, based on generating the mirror image of the original compound. e sp3 atom of interest is called a chiral or stereogenic atom. e sp3 atom is usually carbon in this book, but it does not have to be. ªe main criterion for a stereogenic sp3 carbon atom is that there must be four di–erent atoms or groups attached, and its mirror image must not be superimposable.
