Optical activity and chirality

Chirality and Optical Activity. A compound is chiral (the term dissymmetric was formerly used) if it is not superimposable on its mirror image. A chiral compound does not have a plane of symmetry. Each chiral compound possesses one (or more) of three types of chiral element, namely, a chiral center, a chiral axis, or a chiral plane. 

Since chirality is a property of a molecule as a whole, the specific juxtaposition of two or more stereogenic centers in a molecule may result in an achiral molecule. For example, there are three stereoisomers of tartaric acid (2,3-dihydroxybutanedioic acid). Two of these are chiral and optically active but the third is not. 

Suitably Substituted Adamantanes. Adamantanes bearing four different substituents at the bridgehead positions are chiral and optically active, and 14, for example, has been resolved. This type of molecule is a kind of expanded tetrahedron and has the same symmetry properties as any other tetrahedron. 

For a monograph, see Sokolov, V.I. Chirality and Optical Activity in Organometallic Compounds, Gordon and Breach NY, 1990. 

Hund, one of the pioneers in quantum mechanics, had a fundamental question of relation between the molecular chirality and optical activity [78]. He proposed that all chiral molecules in a double well potential are energetically inequivalent due to a mixed parity state between symmetric and antisymmetric forms. If the quantum tunnelling barrier is sufficiently small, such chiral molecules oscillate between one enantiomer and the other enantiomer with time through spatial inversion and exist in a superposed structure, as exemplified in Figs. 19 and 24. Hund s theory may be responsible for dynamic helicity, dynamic racemization, and epimerization. 

The relationship between chirality and optical activity is conveniently demonstrated by a series of substituted methanes. 

Before the relationship between chirality and optical activity was known, enantiomers were called optical isomers because they seemed identical except for their opposite optical activity. The term was loosely applied to more than one type of isomerism among optically active compounds, however, and this ambiguous term has been replaced by the well-defined term enantiomers. 

The elusive relationship between molecular chirality and optical activity, as well as the Faraday effect, likewise reduce to the minimization of orbital angular momentum as a function of molecular symmetry. 

Chiral nitramines — derivatives of piperidine show an optical activity according to Fcrber and Richardson [14]. Symmetrical compound II is non-chiral, asymmetrical compounds IV and V arc chiral and optically active ... 

All four of these compounds are chiral and optically active. 

Optical isomers. Optical isomers exist for octahedral complexes that do not possess a center of inversion or a mirror plane of symmetry. The complex and its mirror image are not superimposable. One isomer will rotate the plane of polarized light to the left, the other will rotate polarized hght to the right. The complexes are said to be chiral and optically active. Some examples are [Co(ox)3] cis [Rh(en)2Cl2] and cis, cis, cis [PtCl2Br2(NH3)2]. 

Another type of specificity that can occur is the chirality. Isotactic poly(triphenylmethyl methacrylate) is the first known case in which the helicity of the polymer leads to chirality and optical activity [51,52]. A conformational analysis of this polymer has been reported by Cavallo et al. [53]. 

---