Download OPTICAL ISOMERISM

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12-Jul-12
• All molecules have a mirror image –
but for most molecules it is the same
molecule.
H
H
H
C
C
H
F
H
F
H
fluoromethane
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• 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
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• Left and right hands are an example
of non-superimposable 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.
b
a
a
C
C
d
c
d
c
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b
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12-Jul-12
• 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 or a
racemate.
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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
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propan-2-ol
CH3
CH
CH3
OH
NOT OPTICALLY ACTIVE
Click here to go back to the optical isomerism task
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2-chlorobutane
CH3
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
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1-chlorobutane
CH2
CH2
CH2
CH3
Cl
NOT OPTICALLY ACTIVE
Click here to go back to the optical isomerism task
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3-methylhexane
CH3 CH2 CH CH2 CH2 CH3
CH3
H
CH2CH2CH3
CH2CH2CH3
C
C
CH3
CH2CH3
CH3
CH3CH2
H
OPTICALLY ACTIVE
Click here to go back to the optical isomerism task
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O
butanone
CH3
C
CH2
CH3
NOT OPTICALLY ACTIVE
Click here to go back to the optical isomerism task
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propan-2-ol
CH3
CH
CH3
OH
NOT OPTICALLY ACTIVE
Click here to go back to the optical isomerism task
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2-methylbutanoic acid
CH3
CH3
CH2
CH3
O
CH
C
OH
CH2CH3
CH2CH3
C
C
H
COOH
H
HOOC
CH3
OPTICALLY ACTIVE
Click here to go back to the optical isomerism task
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OH
butan-2-ol
CH3
CH3
CH2
CH
CH3
CH2CH3
CH2CH3
C
C
H
OH
H
HO
CH3
OPTICALLY ACTIVE
Click here to go back to the optical isomerism task
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CH3
1-chloro-3-methylpentane
CH3
CH3
CH2
CH
Cl
CH2
CH2
CH2CH3
CH2CH3
C
C
H
CH2CH2Cl
H
CH2ClCH2
CH3
OPTICALLY ACTIVE
Click here to go back to the optical isomerism task
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• 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
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• 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)
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plane-polarised light after
clockw ise rotation
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• Optical isomers rotate the plane of
plane polarised light.
(-)-enantiomer
(anticlockw ise rotation)
(+)-enantiomer
(clockw ise rotation)
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(±)-racemate
(no overall effect)
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POLARIMETERS
can be used to analyse the effect optical
isomers have on plane polarised light:
• 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.
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• 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).
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• 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.
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• 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.
The photographs are both from ‘Molecule of the Month’ at Bristol University:
http://www.chm.bris.ac.uk/motm/thalidomide/start.html
O
NH
O
O
O
H2C
NH
O
C
C
N
CH2
N
H
H2C
CH2
H
O
S thalidomide (effective drug)
O
O
R thalidomide (dangerous drug)
The body racemises each
enantiomer, so even pure S is
dangerous as it converts to R in
the body.
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• Thalidomide was banned worldwide
when the effects were discovered.
• However, it is starting to be used
again to treat leprosy and HIV.
• Its use is restricted though and
patients have to have a pregnancy
test first (women!) and use two forms
of contraception (if sexually active).
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CH3
CH3
O
O
H
C
CH2
H3C
S carvone (caraway seed)
Caraway Seed has a warm, pungent,
slightly bitter flavour with aniseed overtones.
H2C
C
H
CH3
R carvone (spearmint)
CH3
CH3
CH2
C
H
H
C
CH2
CH3
H3C
S limonene (lemons)
R limonene (oranges)
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GEOMETRIC ISOMERISM
CH3
X
C
H
H
C
CH3
CANNOT ROTATE AROUND C=C
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GEOMETRIC ISOMERISM
CH3
H
C
CH3
C
H
CH3
C
CH3
(E) but-2-ene
E entgegen (opposite)
C
H
H
(Z) but-2-ene
Z zusammen (together)
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GEOMETRIC ISOMERISM
PRIORITY: H < CH3 < CH2CH3
Look at atomic numbers of atoms attached to C=C (or
subsequent atoms if the same).
Highest values opposite = E
Highest values together = Z
9
F
CH3
C
6
12
CH3
6
12
1
6, 612
CH2CH3 12,
H
C
C
Br
35
E 2-bromo-3-fluorobut-2-ene
12
6
C
CH3
CH3
12
6
E 3-methylpent-2-ene
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GEOMETRIC ISOMERISM
CH3
H
C
CH3
C
H
CH3
C
CH3
C
H
H
trans but-2-ene
cis but-2-ene
(E) but-2-ene
(Z) but-2-ene
BOTH C ATOMS OF C=C MUST HAVE
TWO DIFFERENT GROUPS
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TASK
CH3
H
C
NO
C
CH3
H
methylpropene
H
H
C
CH3
H
C
C
Z
Cl
Cl
CH3
YES
C
E
H
1-chloropropene
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A2 011
12-Jul-12
TASK
CH2
CH3
C
CH3
NO
C
H
CH3
2-methylpent-2-ene
H
CH2
C
H
CH2
CH3
NO
C
CH2
CH3
2-ethylpent-1-ene
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12-Jul-12
TASK
CH3
CH3
C
C
E
H
CH3
H
CH2
CH3
CH3
YES
C
C
Z
CH2
CH3
3-methylpent-2-ene
H
H
C
H
NO
C
CH2
CH3
but-1-ene
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12-Jul-12
TASK
CH3
CH3
C
CH3
CH2
CH3
C
Z
CH2
C
CH2
CH3
CH3
CH2
CH3
YES
C
E CH3
3,4-dimethylhex-3-ene
CH3
CH3
CH3
Cl
YES
C
Br
C
C
Z
Cl
Br
C
E
CH3
2-bromo-3-chlorobut-2-ene
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A2 011
12-Jul-12