Axial Chirality

How to Manipulate JSmol Structures

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	The most common source of chirality in organic compounds is the asymmetric carbon atom. However, it is possible to have molecular chirality without asymmetric carbons. In 1874, Van't Hoff predicted that 1,3-disubstituted allenes (1,2-propadienes) were capable of resolution, i. e., chiral. In 1935, his conjecture was confirmed. The two adjacent allenes are mirror images of one another. Click the "spin off" button and align the structures for confirmation. The chirality may be described by the Cahn-Ingold-Prelog (CIP) method. Rotate the structures so that you are looking down the C2-C4 axis with the near methyl group pointing to noon. The lefthand structure will have the far methyl group pointing to nine o'clock. The path from methyl to methyl or hydrogen to hydrogen from either end of the axis will be counterclockwise and defined as M [minus; aka aS (axial Sinister)]. Following the rotation of the righthand structure, the same groups rotate clockwise and are defined as P[plus; aka aR (axial Rectus)]. Click here for exercises.

 

	Biphenyls and similar structures that have restricted rotation about a connecting single bond are chiral and capable of resolution. Bulky substituents at four positions (2, 2', 6 and 6') or at three of them are sufficient to restrict rotation. For chirality to exist there can be no planes or centers of symmetry. The two biphenyls have bromine, chlorine and methyl substituents. In one ring bromine has priority over chlorine; the other ring has chlorine of higher priority than a methyl group. View the mirror image on the left down the central bond. Is it M(aS) or P(aR)? What about the structure on the right? Click the "show all" button. If these substituents were at the 3, 3', 5 and 5' positions, the conformations would be chiral but not resolvable owing to rapid rotation about the central bond. Click here for exercises.