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8/2/2019 A2 Optical Isomerism
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STEREOISOMERISMSTEREOISOMERISM
OPTICALOPTICAL
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All molecules have a mirror image All molecules have a mirror image
but for most molecules it is the samebut for most molecules it is the samemolecule.molecule.
fluoromethane
H
C
HF
H
H
C
HF
H
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For some molecules the mirror imageFor some molecules the mirror image
is a different molecule (the mirroris a different molecule (the mirrorimage is non-superimposable).image is non-superimposable).
OH
C
H CH3
COOH
OH
C
HH3C
HOOC
(-) lactic acid (+) lactic acid
in sour milk in muscles
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Left and right hands are an example ofLeft and right hands are an example of
non-superimposable mirror images.non-superimposable mirror images.
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This usually happens when a moleculeThis usually happens when a molecule
contains a C atom with four differentcontains a C atom with four different
groups attached (groups attached (chiralchiral // asymmetricasymmetric C).C).
Such molecules are said to beSuch molecules are said to be chiralchiral oror
optically activeoptically active..
a
C
b cd
a
C
bcd
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Chiral IsomersChiral Isomers Chiral isomers have 4 different groupsChiral isomers have 4 different groups
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Achiral IsomersAchiral Isomers Achiral isomers have 2 or more identicalAchiral isomers have 2 or more identical groupsgroups
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The optical isomers are calledThe optical isomers are called
enantiomersenantiomers..
These are distinguished by +/-, D/L orThese are distinguished by +/-, D/L or
more correctly R/S.more correctly R/S.A 50/50 mixture of the two enantiomersA 50/50 mixture of the two enantiomers
is called ais called a racemic mixtureracemic mixture or aor a
racemateracemate..
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TASKTASK Some of the following moleculesSome of the following moleculesare optically active (are chiral).are optically active (are chiral).
For each one, decide whether it isFor each one, decide whether it isoptically active or not.optically active or not.
a) propan-2-ola) propan-2-ol
b) 2-chlorobutaneb) 2-chlorobutane
c) 1-chlorobutanec) 1-chlorobutane
d) 3-methylhexaned) 3-methylhexane
e) butanonee) butanone
f) 2-methylbutanoic acidf) 2-methylbutanoic acid
g) butan-2-olg) butan-2-ol
h) 1-chloro-3-methylpentaneh) 1-chloro-3-methylpentane
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propan-2-olpropan-2-ol
NOT OPTICALLY ACTIVENOT OPTICALLY ACTIVE
CH3 CH CH3
OH
Click here to go back to the optical isomerism task
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2-chlorobutane2-chlorobutane CH3 CH CH2 CH3
Cl
OPTICALLY ACTIVEOPTICALLY ACTIVE
CH2CH3
C
HCl
CH3
CH2CH3
C
HCl
H3C
Click here to go back to the optical isomerism task
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1-chlorobutane1-chlorobutane
NOT OPTICALLY ACTIVENOT OPTICALLY ACTIVE
CH2 CH2 CH2 CH3
Cl
Click here to go back to the optical isomerism task
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3-methylhexane3-methylhexane CH2 CH CH2 CH2
CH3
CH3CH3
OPTICALLY ACTIVEOPTICALLY ACTIVE
CH2CH2CH3
C
HCH2CH3
CH3
CH2CH2CH3
C
HCH3CH2
CH3
Click here to go back to the optical isomerism task
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butanonebutanone
NOT OPTICALLY ACTIVENOT OPTICALLY ACTIVE
C CH2 CH3CH3
O
Click here to go back to the optical isomerism task
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propan-2-olpropan-2-ol CH3 CH CH3
OH
NOT OPTICALLY ACTIVENOT OPTICALLY ACTIVE
Click here to go back to the optical isomerism task
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2-methylbutanoic acid2-methylbutanoic acid
CH3 CH2 CH
CH3
C
O
OH
OPTICALLY ACTIVEOPTICALLY ACTIVE
CH2CH3
CCH3 COOH
H
CH2CH3
CCH3
HHOOC
Click here to go back to the optical isomerism task
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butan-2-olbutan-2-ol
CH3 CH2 CH
OH
CH3
OPTICALLY ACTIVEOPTICALLY ACTIVE
CH2CH3
CCH3 OH
H
CH2CH3
CCH3
HHO
Click here to go back to the optical isomerism task
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1-chloro-3-methylpentane1-chloro-3-methylpentane
CH3 CH2 CH
CH3
CH2 CH2
Cl
OPTICALLY ACTIVEOPTICALLY ACTIVE
CH2CH3
C
CH3 CH2CH2Cl
H
CH2CH3
C
CH3H
CH2ClCH2
Click here to go back to the optical isomerism task
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Molecules that are optical isomers areMolecules that are optical isomers are
calledcalled enantiomers.enantiomers.
Enantiomers have identical chemicalEnantiomers have identical chemical
and physical properties, except:and physical properties, except: Their effect on plane polarised light;Their effect on plane polarised light;
Their reaction with other chiral moleculesTheir reaction with other chiral molecules
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Light is a form of electromagneticLight is a form of electromagnetic
radiation.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 perpendicularThe wave vibrations are perpendicular
to the direction of travel of the wave.to the direction of travel of the wave.
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Optical isomers rotate the plane ofOptical isomers rotate the plane of
plane polarised light.plane polarised light.
(-)-enantiomer(anticlockwise rotation)
()-racemate(no overall effect)
(+)-enantiomer(clockwise rotation)
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POLARIMETERSPOLARIMETERS
can be used to analyse the effect opticalcan be used to analyse the effect optical
isomers have on plane polarised light:isomers have on plane polarised light:
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Chirality in the Biological WorldChirality in the Biological World
When chiral organic compounds interact withWhen chiral organic compounds interact withliving systems, the two enantiomers oftenliving systems, the two enantiomers ofteninteract very differently.interact very differently.
This is because living systems are chiral atThis is because living systems are chiral atmany different levels in many ways:many different levels in many ways:
- our hands and feet are chiral (of course)- our hands and feet are chiral (of course)
- our taste buds are chiral- our taste buds are chiral- our olfactory sensors (for smell) are chiral- our olfactory sensors (for smell) are chiral
- our enzymes are chiral- our enzymes are chiral
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This is like the idea that a right handThis is like the idea that a right hand
does not fit a left handed glove thedoes not fit a left handed glove the
molecule must be the correct shape tomolecule must be the correct shape to
fit the molecule it is reacting with.fit the molecule it is reacting with.
Many natural molecules are chiral andMany natural molecules are chiral andmost natural reactions are affected bymost natural reactions are affected by
optical isomerism.optical isomerism.
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For example, most amino acids (and soFor example, most amino acids (and so
proteins) are chiral, along with manyproteins) are chiral, along with many
other molecules.other molecules.
In nature, only one optical isomerIn nature, only one optical isomer
occurs (e.g. all natural amino acidsoccurs (e.g. all natural amino acidsrotate the plane of plane polarised lightrotate the plane of plane polarised light
to the left).to the left).
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Many drugs are optically active, withMany drugs are optically active, with
one enantiomer only having theone enantiomer only having thebeneficial effect.beneficial effect.
In the case of some drugs, the otherIn the case of some drugs, the other
enantiomer can even be harmful, e.g.enantiomer can even be harmful, e.g.
thalidomide.thalidomide.
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In the 1960s thalidomide was given toIn the 1960s thalidomide was given to
pregnant women to reduce the effectspregnant women to reduce the effects
of morning sickness.of morning sickness.This led to many disabilities in babiesThis led to many disabilities in babies
and early deaths in many cases.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
http://www.chm.bris.ac.uk/motm/thalidomide/start.htmlhttp://www.chm.bris.ac.uk/motm/thalidomide/start.html8/2/2019 A2 Optical Isomerism
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H2C
CH2
C
NHO O
H
N
O
O
H2C
CH2
C
NH OO
H
N
O
O
S thalidomide (effective
drug)
The body racemises eachenantiomer, 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 worldwideThalidomide was banned worldwide
when the effects were discovered.when the effects were discovered.However, it is starting to be usedHowever, it is starting to be used
again to treat leprosy and HIV.again to treat leprosy and HIV.
Its use is restricted though andIts use is restricted though and
patients have to have a pregnancypatients have to have a pregnancy
test first (women!) and use two formstest first (women!) and use two formsof contraception (if sexually active).of contraception (if sexually active).
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AspartameAspartame
H2N
N
O
OH
O
H
H
O
OH
H
Left-handed Aspartame
"Nutrasweet"
160 times sweeter than sugar
H2N
N
O
OH
O
H
H H
O
OH
Right-handed AspartameNot at all sweet
slightly bitter
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IbuprofenIbuprofen
H3C
O
OH
CH3
O
HO
Left-handed Ibuprofen
Powerful Pain Killer and
Anti-inflamatory Drug
HH
Right-handed Ibuprofen
No Drug Activity
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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
http://geekphilosopher.com/bkg/fruitLemon1.htm8/2/2019 A2 Optical Isomerism
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SummarySummary
Molecules which lack symmetry are ChiralMolecules which lack symmetry are Chiral
Chiral molecules have right-handed and left-Chiral molecules have right-handed and left-
handed formshanded forms
Right-handed and left-handed molecules interactRight-handed and left-handed molecules interactwith living systems in very different ways.with living systems in very different ways. Different smellDifferent smell Different tasteDifferent taste Different drug effectsDifferent drug effects