A chiral (from the Greek cheir, hand) molecule has the same relationship with its mirror image as the right hand with the left

If an improper rotation axis Sn is present the molecule cannot be chiral. This holds clearly for S1 (plane of symmetry) and S2 (centre).

Is 12 chiral?

Chiral molecules

poly-3-(2-methylbutyl)thiophene)

PTFE c = 19.5 Å

Are the following species polar? Chiral?

C6H5Cl, CHCl3 , 1- or 2– substituted norbornane, 2-methyl-pentane….

Molecules with more than one chiral centre

In Fischer projections the similar groups are: on the same side erythro pair on opposite sides threo pair

2,3-dichloropentane has two asymmetric C and two diastereoisomers. For each diasteroisomer there is in this case a pair of enantiomers.

R,S S,R S,S R,R

Diastereoisomers are not mirror images and have different physical and chemical properties

mirror mirror

non superimposableenantiomers, erythro

non superimposableenantiomers, threo

R,S S,RS,S R,R

Also 2,3-dichlorobutane two asymmetric C and two optically active diastereoisomers, but the three groups, on each of the two C*, are identical:

non superimposable enantiomers

superimposablemeso compound

mirror mirror

2,3-dichlorobutane

Molecules with more chiral centres (2)

Molecules with more than one chiral centre (3)

Meso compounds are achiral: there is an internal compensation amongthe chiral centers in the molecule.2,3-diclorobutane has an intramolecular plane of symmetry in the eclipsed conformation, a centre in the anti conformation

In Fischer projections:if two of the three identical groups are on the same side of the chainmolecule on the two C’s (as in R,S-dichlorobutane) the two C’s have opposite configurations

Molecules with more than one chiral centre (4)

CH3H ClH Cl

CH3H Cl

CH3H ClCl H

CH3H Cl

CH3H ClCl H

CH3Cl H

CH3Cl HH Cl

CH3H Cl

10.a 10.b 10.c 10.d2,3,4-trichloropentane has 3 possibly asymmetric C’s. The possible stereoisomers should increase to (23): 4 diastereoisomers, each with its enantiomer as for 2,3,4-trichlorohexane. In 2,3,4-trichloropentane all three groups on the terminal C’s are identical and stereoisomers reduce to 4:

In 10.a and 10.b (chiral enantiomers) atoms 2 and 4 are chiral and have the same configuration: S,S in a), and R,R in b). In both a) and b) atom 3, has identical groups and is non-chiral. Interchanging H and Cl of atom 3 and rotating by 180° leaves obviously the molecule unaltered.

Molecules with more than one chiral centre (5)CH3

H ClH Cl

CH3H Cl

CH3H ClCl H

CH3H Cl

CH3H ClCl H

CH3Cl H

CH3Cl HH Cl

CH3H Cl

10.a 10.b 10.c 10.dIn 10.c and 10.d atoms 2 and 4 are chiral with opposite configuration : S,R both in 10.c and 10.d. Atom 3 is pseudoasymmetric: C atoms on a molecular symmetry plane as in 10.c and 10.d are pseudoasymmetric: they are examples of prochiral centers. The two possible configurations of the pseudoasymmetric atom are labelled with the letters r (in d) and s (in c), R groups have higher priority than S. Any sp3 C of the kind CXYH2 is prochiral although not pseudoasymmetric.

10.c, 10.d and similar molecules are achiral. Atoms or groups related by a plane of symmetry on prochiral centres are enantiotopic ( pro-R e pro-S). They react in the same way with non-chiral species, but differently with chiral molecules.

Atom C Pseudokiral

CH3H ClH Cl

CH3H Cl

CH3H ClCl H

CH3H Cl

CH3H ClCl H

CH3Cl H

CH3Cl HH Cl

CH3H Cl

A B C DAtom C Nomor 2 3 4

A S - S

B R - R

C S s R

D S r R

Pada C dan D atom C nomor 3 atom C pseudokiral (prioritas R > S)

Atropoisomerism - biphenyls

Biphenyls with bulky groups in positions a, b, c, d may be optically active and resolvable due to atropoisomerism

Examples of resolvable biphenyls

Atropoisomerism

Examples of resolvable ansa-compounds

Paracyclophanes

m = 2, n = 2 resolvable optically stable m = 3, n = 4 resolvable - racemizes at 160 °C m = 4, n = 4 non resolvable