A2 Optical Isomerism

8/2/2019 A2 Optical Isomerism

1/35

11

STEREOISOMERISMSTEREOISOMERISM

OPTICALOPTICAL


2/35

22

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


3/35

33

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


4/35

44

Left and right hands are an example ofLeft and right hands are an example of

non-superimposable mirror images.non-superimposable mirror images.


5/35

55

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


6/35

66

Chiral IsomersChiral Isomers Chiral isomers have 4 different groupsChiral isomers have 4 different groups


7/35

77

Achiral IsomersAchiral Isomers Achiral isomers have 2 or more identicalAchiral isomers have 2 or more identical groupsgroups


8/35

88

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..


9/35

99

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


10/35

1010

propan-2-olpropan-2-ol

NOT OPTICALLY ACTIVENOT OPTICALLY ACTIVE

CH3 CH CH3

OH

Click here to go back to the optical isomerism task


11/35

1111

2-chlorobutane2-chlorobutane CH3 CH CH2 CH3

Cl

OPTICALLY ACTIVEOPTICALLY ACTIVE

CH2CH3

C

HCl

CH3

CH2CH3

C

HCl

H3C



12/35

1212

1-chlorobutane1-chlorobutane


CH2 CH2 CH2 CH3

Cl



13/35

1313

3-methylhexane3-methylhexane CH2 CH CH2 CH2

CH3

CH3CH3


CH2CH2CH3

C

HCH2CH3

CH3

CH2CH2CH3

C

HCH3CH2

CH3



14/35

1414

butanonebutanone


C CH2 CH3CH3

O



15/35

1515

propan-2-olpropan-2-ol CH3 CH CH3

OH




16/35

1616

2-methylbutanoic acid2-methylbutanoic acid

CH3 CH2 CH

CH3

C

O

OH


CH2CH3

CCH3 COOH

H

CH2CH3

CCH3

HHOOC



17/35

1717

butan-2-olbutan-2-ol

CH3 CH2 CH

OH

CH3


CH2CH3

CCH3 OH

H

CH2CH3

CCH3

HHO



18/35

1818

1-chloro-3-methylpentane1-chloro-3-methylpentane

CH3 CH2 CH

CH3

CH2 CH2

Cl


CH2CH3

C

CH3 CH2CH2Cl

H

CH2CH3

C

CH3H

CH2ClCH2



19/35

1919

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


20/35

2020

Light is a form of electromagneticLight is a form of electromagnetic

radiation.radiation.


21/35

2121

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.


22/35

2222

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)


23/35

2323

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:


24/35

2424

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


25/35

2525

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.


26/35

2626

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).


27/35

2727

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.


28/35

2828

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.html


29/35

2929

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)


30/35

3030

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).


31/35

3131

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


32/35

3232

IbuprofenIbuprofen

H3C

O

OH

CH3

O

HO

Left-handed Ibuprofen

Powerful Pain Killer and

Anti-inflamatory Drug

HH

Right-handed Ibuprofen

No Drug Activity


33/35

3333

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.


34/35

3434

S limonene (lemons) R limonene (oranges)

CH3

HCCH2

CH3

CH3

H C CH2

H3C
http://geekphilosopher.com/bkg/fruitLemon1.htm


35/35

3535

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

Documents

A2 Optical Isomerism