Stereochemistry of Cyclic Compounds

Geometrical Isomerism.

Since freedom of rotation about single linkings is abolished by ring-closure, geometrical isomerism is of frequent occurrence among cyclic compounds. Thus any disubstituted polymethylene (in which the substituents are on different carbon atoms) may exist in a cis- and a trans-form as the accompanying diagrams of a cis- and trans-I-3-disubstituted cyclohexane show.

Where, as in the compound represented, the two substituents are alike, the molecule of the cis form has a plane of symmetry (the perpendicular plane through the dotted line in fig. 15) and this modification is therefore non resolvable into optical antipodes; the molecule of the trans-form has, however, only an axis of sym metry (the dotted line in fig. 14) ; it is therefore dissymmetric, and this modification can thus exist in optically active forms. This difference between cis- and trans-modifications provides a very reliable method for determining configuration in those cases (as in many cyclo-paraffin carboxylic acids) where it is applicable.

The phenomena presented by cyclic compounds have a most important bearing on the spatial distribution of the carbon valencies. For whilst any non-planar configuration would account for the observed relationship between molecular dissymmetry and•structure, and for the isomerism due to the combinations of centres of asymmetry, the relative stability and readiness of for mation of 5-membered rings proves that the natural angle be tween two of the carbon valencies must be close to 108° (the angle of the regular pentagon). Since, further, spirocyclic corn pounds of the double 5-ring type are formed with great readiness it would appear that the angle between the other two valencies also approximates to 108°. The chemistry of cyclic carbon corn pounds thus strongly indicates the regular tetrahedral arrange ment in which the intervalency angle is 109.5°.

Mirror-image Isomeris.

The conception of the asym metric carbon atom, which in the open-chain series provides such a ready means of recognising molecular dissymmetry, is much less useful when applied to cyclic compounds. In cyclic compounds molecular dissymmetry is most easily recognised by noting the absence of centre, alternating axis and planes of symmetry from the tridimensional formula, for a molecule must possess at least one such element of symmetry to have the property of super posability upon the mirror-image. Examples of molecules with a plane of symmetry have already been given. Well known ex amples of molecules with a centre of symmetry are those of the trans-forms of the cyclic anhydrides of the a-amino-acids, XXVI. (fig. 17). The alternating axis of symmetry has not yet acquired practical importance in stereochemistry, since no compounds with a molecule having as alternating axis but no plane or centre of symmetry are known ; one of the simplest examples would be a compound of the type XXVII.

Among the cyclic compounds possessing special stereochemical interest are methylcyclohexylideneacetic acid, XXVIII. (fig. 18), resolved by Perkin, Pope and Wallach (I. Chem. Soc., 1909, 95, 1789) and the naturally occurring d- and l-inositols. The latter are cyclic hexahydric alcohols of the structure and although there are 7 geometrical isomers of this formula, the configuration of the active forms can be deduced because only Its optical activity demonstrates the molecular dissymmetry that on the tetrahedral theory such spirocyclic compounds, in spite of the symmetry of their structural formulae, should possess. They have an axis of symmetry and the two forms correspond with the right- and left-handed forms of a two-bladed screw-propeller.