Axial Chirality

How to Manipulate JSmol Structures

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Introduction:

Axial chirality is generally applied to chiral allenes and biphenyls which lack the more common asymmetric carbon. The IUPAC 2013 BlueBook recommends the use of M (minus) and P (plus) descriptors. Older CIP (1966) rules utilize Ra and Sa , respectively. The priority orders are a>b>c>d. Figs. 1a & 1b illustrate the counterclockwise nature of the M-configuration and the right hand rule applied to the axial chirality Ra.

Figs. 1c & 1d illustrate the clockwise assignment of the P-configuration and the left hand rule applied to Sa.

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Fig. 2

 

 

2.3-Pentadiene

(1,3-Dimethylallene)

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 C1-C3 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; "Ra" (CIP 1966)]. Following the rotation of the righthand structure, the same groups rotate clockwise and are defined as P [plus; "Sa" (CIP 1966)].

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Fig. 3

 




Fig. 4

(2Z,5E)-4-((P)-2-Chlorovinylidene)hepta-2,5-diene

This allene employs CIP rules 1 and 3 where Cl>H and Z>E, respectively. Orient the chlorine atom at noon in the foreground. The (Z)-double bond should be at 3 o'clock. This is M or Sa chirality.

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Fig. 5

(2R,4S)-3-((m)-2-Chlorovinylidene)pentane-2,4-diol

Because one end of the allene has enantiomorphic groups, the priorities are Cl>H and R>S invoking CIP rule 1 and rule 5, respectively. Owing to the enantiomorphic groups, CIP rule 4c dictates an m-axial chirality for this allene.

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Fig. 6

(2S,4S)-3-(2-Chlorovinylidene)pentane-2,4-diol

The allenic portion (C1-C2-C3) of the diol is chirotopic and non-stereogenic. Switching the location of chlorine and hydrogen at C1 produces the same enantiomer.There is no axial chirality assignment.

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Fig. 7

2-Bromo-2',6-dichloro-6'-methyl-1,1'-biphenyl

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. [Note: These positions are numbered 3, 8, 5 and 12, respectively, in the JSmol structures.] 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 or P? What about the structure on the right? Click the "show M/P" button.

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Fig. 8