Chiral molecules
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?
Are the following species polar? Chiral?
C6H5Cl, CHCl3 , 1- or 2– substituted norbornane, 2-methylpentane….
poly-3-(2-methylbutyl)thiophene)
PTFE
c = 19.5 Å
Molecules with more than one chiral centre
2,3-dichloropentane has two asymmetric C and two diastereoisomers. For
each diasteroisomer there is in this case a pair of enantiomers.
Diastereoisomers are not mirror images and have
different physical and chemical properties
mirror
mirror
R,S
S,R
non superimposable
enantiomers, erythro
S,S
R,R
non superimposable
enantiomers, threo
In Fischer projections the similar groups are: on the same side  erythro pair
on opposite sides  threo pair
Molecules with more chiral centres (2)
2,3-dichlorobutane
Also 2,3-dichlorobutane two asymmetric C and two
optically active diastereoisomers, but the three
groups, on each of the two C*, are identical:
mirror
S,S
mirror
R,R
non superimposable
enantiomers
S,R
R,S
superimposable
meso compound
Molecules with more than one chiral centre (3)
Meso compounds are achiral: there is an internal compensation among
the 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 chain
molecule 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)
CH3
H
Cl
Cl
H
Cl
H
CH3
CH3
H
Cl
Cl
H
Cl
H
CH3
CH3
H
Cl
H
Cl
H
Cl
CH3
CH3
H
Cl
Cl
H
H
Cl
CH3
10.a
10.b
10.c
10.d
2,3,4-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
Cl
Cl
H
Cl
H
CH3
CH3
Cl
H
H
Cl
H
Cl
CH3
CH3
H
Cl
H
Cl
H
Cl
CH3
CH3
H
Cl
Cl
H
H
Cl
CH3
10.a
10.b
10.c
10.d
In 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
CH3
H
Cl
Cl
H
Cl
H
CH3
A
Atom C Nomor
CH3
Cl
H
H
Cl
H
Cl
CH3
CH3
H
Cl
H
Cl
H
Cl
CH3
B
CH3
H
Cl
Cl
H
H
Cl
CH3
C
D
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
Paracyclophanes
m = 2, n = 2 resolvable optically stable
m = 3, n = 4 resolvable - racemizes at 160 °C
m = 4, n = 4 non resolvable
Examples of resolvable ansa-compounds