Optical isomers

28
OPTICAL ISOMERISM

description

 

Transcript of Optical isomers

Page 1: Optical isomers

OPTICAL ISOMERISM

Page 2: Optical isomers

• All molecules have a mirror image – but in most of the cases image and object are the same molecule.

fluoromethane

H

CH F

H

H

CHF

H

Page 3: Optical isomers

• 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

Page 4: Optical isomers

• Left and right hands are an example of non-superimposable mirror images.

Page 5: Optical isomers
Page 6: Optical isomers

• 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

Page 7: Optical isomers

• The optical isomers are called enantiomers.

• These are distinguished by +/-, D/L or more correctly Rectus-right/Sinister-left.

• A 50/50 mixture of the two enantiomers is called a racemic mixture or a racemate.

Page 8: Optical isomers

TASK Which of the following molecules are optically active?

1) propan-2-ol

2) 2-chlorobutane

3) 1-chlorobutane

4) 3-methylhexane

5) butanone

6) 2-methylbutanoic acid

7) butan-2-ol

8) 1-chloro-3-methylpentane

Page 9: Optical isomers

propan-2-ol

NOT OPTICALLY ACTIVE

CH3 CH CH3

OH

Page 10: Optical isomers

2-chlorobutane CH3 CH CH2 CH3

Cl

OPTICALLY ACTIVE

CH2CH3

CH Cl

CH3

CH2CH3

CHCl

H3C

Page 11: Optical isomers

1-chlorobutane

NOT OPTICALLY ACTIVE

CH2 CH2 CH2 CH3

Cl

Page 12: Optical isomers

3-methylhexane

OPTICALLY ACTIVE

CH2 CH CH2 CH2

CH3

CH3CH3

CH2CH2CH3

CH CH2CH3

CH3

CH2CH2CH3

CHCH3CH2

CH3

Page 13: Optical isomers

butanone

NOT OPTICALLY ACTIVE

C CH2 CH3CH3

O

Page 14: Optical isomers

propan-2-ol CH3 CH CH3

OH

NOT OPTICALLY ACTIVE

Page 15: Optical isomers

2-methylbutanoic acid

OPTICALLY ACTIVE

CH3 CH2 CH

CH3

C

O

OH

CH2CH3

CCH3 COOH

H

CH2CH3

CCH3

HHOOC

Page 16: Optical isomers

butan-2-ol

OPTICALLY ACTIVE

CH3 CH2 CH

OH

CH3

CH2CH3

CCH3 OH

H

CH2CH3

CCH3

HHO

Page 17: Optical isomers

1-chloro-3-methylpentane

OPTICALLY ACTIVE

CH3 CH2 CH

CH3

CH2 CH2

Cl

CH2CH3

CCH3 CH2CH2Cl

H

CH2CH3

CCH3

HCH2ClCH2

Page 18: Optical isomers

• 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

Page 19: Optical isomers

normal light(waves vibrate in all directions)

plane-polarised light(vibrates in only one direction)

plane-polarised light after clockwise rotation

1-The wave vibrations are perpendicular to the direction of travel of the wave.

Page 20: Optical isomers

• Optical isomers rotate the plane of plane polarised light.

(-)-enantiomer(anticlockwise rotation)

(±)-racemate(no overall effect)

(+)-enantiomer(clockwise rotation)

Page 21: Optical isomers
Page 22: Optical isomers

2- 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.

Page 23: Optical isomers

• For example, most amino acids (and so proteins) are chiral, along with many other molecules.

• The human body can consume amino acids that rotate polarised light to the left.

Page 24: Optical isomers

• 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.

Page 25: Optical isomers

• 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.

Page 26: Optical isomers

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)

Page 27: Optical isomers

S carvone (caraway seed) R carvone (spearmint)

O

CH3

H C CH2

H3C

O

CH3

HCH2C

CH3

Page 28: Optical isomers

S limonene (lemons) R limonene (oranges)

CH3

HCCH2

CH3

CH3

H C CH2

H3C