Chirality
المؤلف:
Peter Atkins، Julio de Paula
المصدر:
ATKINS PHYSICAL CHEMISTRY
الجزء والصفحة:
412
2025-12-02
61
Chirality
A chiral molecule (from the Greek word for ‘hand’) is a molecule that cannot be superimposed on its mirror image. An achiral molecule is a molecule that can be superimposed on its mirror image. Chiral molecules are optically active in the sense that they rotate the plane of polarized light (a property discussed in more detail in Appendix 3). A chiral molecule and its mirror-image partner constitute an enantiomeric pair of isomers and rotate the plane of polarization in equal but opposite directions. A molecule may be chiral, and therefore optically active, only if it does not possess an axis of improper rotation, Sn. However, we need to be aware that such an axis may be present under a different name, and be implied by other symmetry elements that are present. For example, molecules belonging to the groups Cnh possess an Sn axis implicitly because they possess both Cn and σh, which are the two components of an improper rotation axis. Any molecule containing a centre of inversion, i, also possesses an S2 axis, because i is equivalent to C2 in conjunction with σh, and that com bination of elements is S2 (Fig. 12.14). It follows that all molecules with centres of inversion are achiral and hence optically inactive. Similarly, because S1 = σ, it follows that any molecule with a mirror plane is achiral. A molecule may be chiral if it does not have a centre of inversion or a mirror plane, which is the case with the amino acid alanine (18), but not with glycine (19). However, a molecule may be achiral even though it does not have a centre of inversion. For example, the S4 species (20) is achiral and optically inactive: though it lacks i (that is, S2) it does have an S4 axis.
Fig. 12.14 Some symmetry elements are implied by the other symmetry elements in a group. Any molecule containing an inversion also possesses at least an S2 element because i and S2 are equivalent.
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