Post on 07-Feb-2016
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OPTICAL ISOMERISM
R W Grime Ripon Grammar School
Molecular mirror images
Optical isomerism and chirality Enantiomers and racemates
Self-Test: recognising optical isomers from structural formulae
Polarimetry: recognising optical isomers by experiment
Examples of optical isomers and their importance
thalidomide carvone limonene
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• All molecules have a mirror image – but for most molecules it is the same molecule.
fluoromethane
H
CH F
H
H
CHF
H
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• For some molecules the mirror image is a different molecule (the mirror image is non-superimposable).
OH
CH CH3
COOH
OH
CHH3C
HOOC
(-) lactic acid (+) lactic acidin sour milk in muscles
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• Left and right hands are an example of non-superimposable mirror images.
5• 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.
a
Cb c
d
a
Cbc
d
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• 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.
7TASK 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
b) 2-chlorobutane
c) 1-chlorobutane
d) 3-methylhexane
e) butanone
f) 2-methylbutanoic acid
g) butan-2-ol
h) 1-chloro-3-methylpentane
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propan-2-ol
NOT OPTICALLY ACTIVE
CH3 CH CH3
OH
Click here to go back to the optical isomerism task
92-chlorobutane CH3 CH CH2 CH3
Cl
OPTICALLY ACTIVE
CH2CH3
CH Cl
CH3
CH2CH3
CHCl
H3C
Click here to go back to the optical isomerism task
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1-chlorobutane
NOT OPTICALLY ACTIVE
CH2 CH2 CH2 CH3
Cl
Click here to go back to the optical isomerism task
113-methylhexane CH2 CH CH2 CH2
CH3
CH3CH3
OPTICALLY ACTIVE
CH2CH2CH3
CH CH2CH3
CH3
CH2CH2CH3
CHCH3CH2
CH3
Click here to go back to the optical isomerism task
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butanone
NOT OPTICALLY ACTIVE
C CH2 CH3CH3
O
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
142-methylbutanoic acid
CH3 CH2 CH
CH3
C
O
OH
OPTICALLY ACTIVE
CH2CH3
CCH3 COOH
H
CH2CH3
CCH3
HHOOC
Click here to go back to the optical isomerism task
15butan-2-ol
CH3 CH2 CH
OH
CH3
OPTICALLY ACTIVE
CH2CH3
CCH3 OH
H
CH2CH3
CCH3
HHO
Click here to go back to the optical isomerism task
161-chloro-3-methylpentane
CH3 CH2 CH
CH3
CH2 CH2
Cl
OPTICALLY ACTIVE
CH2CH3
CCH3 CH2CH2Cl
H
CH2CH3
CCH3
HCH2ClCH2
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
18• Light is a form of electromagnetic radiation.
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normal light(waves vibrate in all directions)
plane-polarised light(vibrates in only one direction)
plane-polarised light after clockwise rotation
• The wave vibrations are perpendicular to the direction of travel of the wave.
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• Optical isomers rotate the plane of plane polarised light.
(-)-enantiomer(anticlockwise rotation)
(±)-racemate(no overall effect)
(+)-enantiomer(clockwise rotation)
21POLARIMETERS can be used to analyse the effect optical isomers have on plane polarised light:
Heriot Watt University has a web page with an interactive tutorial and self-test questions about this topic: http://scholar.hw.ac.uk/site/chemistry/activity5.asp?outline
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• 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.
25• 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
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H2CCH2
C
NHO O
H
N
O
O
H2CCH2
C
NH OO
H
N
O
O
S thalidomide (effective drug)
The body racemises each enantiomer, so even pure S is
dangerous as it converts to R in the body.
R thalidomide (dangerous drug)
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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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S carvone (caraway seed) R carvone (spearmint)
O
CH3
H C CH2
H3C
O
CH3
HCH2C
CH3
Caraway Seed has a warm, pungent, slightly bitter flavour with aniseed overtones.
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S limonene (lemons) R limonene (oranges)
CH3
HCCH2
CH3
CH3
H C CH2
H3C