Stereoisomerism
AH Chemistry Unit 3(c)
CHAIN ISOMERISM
STRUCTURAL ISOMERISM
Same molecular formula but
different structural formulae
POSITION ISOMERISM
FUNCTIONAL GROUP
ISOMERISM
GEOMETRICAL ISOMERISM
STEREOISOMERISM
Same molecular
formula but atoms
occupy different
positions in space.
Occurs due to the restricted
rotation of C=C double bonds...
two forms… CIS and TRANS
OPTICAL ISOMERISM
Occurs when molecules have a
chiral centre. Get two nonsuperimposable mirror images.
Geometric isomers
In alkenes
CIS
Groups/atoms are on the
SAME SIDE of the double bond
TRANS
Groups/atoms are on OPPOSITE
SIDES across the double bond
RESTRICTED ROTATION OF C=C BONDS
Single covalent bonds can easily rotate. What appears to be a different structure is
not. It looks like it but, due to the way structures are written out, they are the same.
ALL THESE STRUCTURES ARE THE SAME BECAUSE C-C BONDS HAVE ‘FREE’ ROTATION
RESTRICTED ROTATION OF C=C BONDS
C=C bonds have restricted rotation so the groups on either end of the bond are
‘frozen’ in one position; it isn’t easy to flip between the two.
This produces two possibilities. The two structures cannot interchange easily
so the atoms in the two molecules occupy different positions in space.
cis
trans
cis
trans
Physical properties
• Geometric isomers display differences in
some physical properties e.g. melting
point, boiling point
• Geometric isomerism also influences
some chemical properties
Optical isomers
• All molecules have a mirror image – but for many
molecules it is the same molecule.
H
H
H
C
C
H
F
H
F
fluoromethane
H
• For some molecules the mirror image is a different
molecule (the mirror image is non-superimposable).
H
OH
OH
C
C
COOH
CH3
(-) lactic acid
in sour milk
HOOC
H3C
H
(+) lactic acid
in muscles
• Left and right hands are an example of nonsuperimposable mirror images.
• This usually happens when a molecule contains a C atom with
four different groups attached (chiral / asymmetric C).
• Such molecules are said to be chiral or optically active.
• The optical isomers are called enantiomers.
• These are distinguished by +/-, D/L or more correctly
R/S.
• A 50/50 mixture of the two enantiomers is called a
racemic mixture and is optically inactive.
TASK
Some of the following molecules
are optically active.
For each one, click its name below and
decide whether it is optically active or not.
Click again to see if you are correct.
a) propan-2-ol
e) butanone
b) 2-chlorobutane
f) 2-methylbutanoic acid
c) 1-chlorobutane
g) butan-2-ol
d) 3-methylhexane
h) 1-chloro-3-methylpentane
propan-2-ol
CH3
CH
CH3
OH
NOT OPTICALLY ACTIVE
Click here to go back to the optical isomerism task
CH3
2-chlorobutane
CH
CH2
CH3
Cl
H
CH2CH3
CH2CH3
C
C
CH3
Cl
H3C
Cl
H
OPTICALLY ACTIVE
Click here to go back to the optical isomerism task
1-chlorobutane
CH2
CH2
CH2
Cl
NOT OPTICALLY ACTIVE
Click here to go back to the optical isomerism task
CH3
3-methylhexane
CH3 CH2 CH CH2 CH2 CH3
CH3
CH2 CH 2CH 3
C
H
CH3
CH2 CH 3
CH2 CH 2CH 3
CH3
CH3CH2
C
H
OPTICALLY ACTIVE
Click here to go back to the optical isomerism task
O
butanone
CH3
C
CH2
CH3
NOT OPTICALLY ACTIVE
Click here to go back to the optical isomerism task
propan-2-ol
CH3
CH
CH3
OH
NOT OPTICALLY ACTIVE
Click here to go back to the optical isomerism task
2-methylbutanoic acid
CH3
CH3
CH2
CH3
O
CH
C
CH2CH3
CH2CH3
C
C
H
COOH
H
HOOC
CH3
OPTICALLY ACTIVE
Click here to go back to the optical isomerism task
OH
OH
butan-2-ol
CH3
CH3
CH2
CH
CH2CH3
CH2CH3
C
C
H
OH
H
HO
CH3
OPTICALLY ACTIVE
Click here to go back to the optical isomerism task
CH3
CH3
1-chloro-3-methylpentane
CH3
CH2
CH
CH3
H
CH2CH2Cl
CH2
CH2CH3
CH2CH3
C
Cl
H
CH2ClCH2
C
OPTICALLY ACTIVE
Click here to go back to the optical isomerism task
CH3
CH2
• Molecules that are optical isomers are called
enantiomers.
• Enantiomers have identical chemical and physical
properties, except:
• their effect on plane polarised light
• their reaction with other chiral molecules
• Light is a form of electromagnetic radiation.
• The wave vibrations are perpendicular to the
direction of travel of the wave.
normal light
(w aves vibrate in all directions)
plane-polarised light
(vibrates in only one direction)
plane-polarised light after
clockw ise rotation
• Optical isomers rotate the plane of plane polarised
light.
(-)-enantiomer
(anticlockw ise rotation)
(+)-enantiomer
(clockw ise rotation)
(±)-racemate
(no overall effect)
POLARIMETERS can be used to analyse the effect optical
isomers have on plane polarised light:
A
B
C
D
E
F
A
B
C
D
E
F
Light source produces light vibrating in all directions
Polarising filter only allows through light vibrating in one direction
Plane polarised light passes through sample
If substance is optically active it rotates the plane polarised light
Analysing filter is turned so that light reaches a maximum
Direction of rotation is measured coming towards the observer
How optical isomers can be formed
• Chiral molecules often react differently with other
chiral molecules.
• This is like the idea that a right hand does not fit a left
handed glove – the molecule must be the correct
shape to fit the molecule it is reacting with.
• Many natural molecules are chiral and most natural
reactions are affected by optical isomerism.
• For example, most amino acids (and so proteins) are
chiral, along with many other molecules.
• In nature, only one optical isomer occurs (e.g. all
natural amino acids are rotate polarised light to the
left).
• Many drugs are optically active, with one
enantiomer only having the beneficial effect.
• In the case of some drugs, the other enantiomer
can even be harmful, e.g. thalidomide.
• In the 1960’s thalidomide was given to pregnant women to
reduce the effects of morning sickness.
• This led to many disabilities in babies and early deaths in many
cases.
O
NH
O
O
O
H2C
NH
O
C
C
N
CH2
H
O
S thalidomide (effective drug)
The body racemises each enantiomer,
so even pure S is dangerous as it
converts to R in the body.
O
N
H2C
CH2
H
O
R thalidomide (dangerous drug)
S carvone (caraway seed)
R carvone (spearmint)
CH3
CH3
O
O
H
C
CH2
H3C
Caraway Seed has a warm, pungent,
slightly bitter flavour with aniseed overtones.
H2C
C
CH3
H
S limonene (lemons)
R limonene (oranges)
CH3
CH3
H
CH2
C
C
H
CH3
H3C
CH2