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OPTICAL ISOMERISM. OPTICAL ISOMERISM. Occurrence another form of stereoisomerism occurs when compounds have non-superimposable mirror images Isomers the two different forms are known as optical isomers or enantiomers they occur when molecules have a chiral centre

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slide2

OPTICAL ISOMERISM

Occurrenceanother form of stereoisomerism

occurs when compounds have non-superimposable mirror images

Isomers the two different forms are known as optical isomers or enantiomers

they occur when molecules have a chiral centre

a chiral centre contains an asymmetric carbon atom

an asymmetric carbon has four different atoms (or groups)

arranged tetrahedrally around it.

slide3

OPTICAL ISOMERISM

Occurrenceanother form of stereoisomerism

occurs when compounds have non-superimposable mirror images

Isomers the two different forms are known as optical isomers or enantiomers

they occur when molecules have a chiral centre

a chiral centre contains an asymmetric carbon atom

an asymmetric carbon has four different atoms (or groups)

arranged tetrahedrally around it.

CHIRAL CENTRES

There are four different colours arranged tetrahedrally about the carbon atom

2-chlorobutane exhibits optical isomerism because the second carbon atom has four different atoms/groups attached

slide4

OPTICAL ISOMERISM

SPOTTING CHIRAL CENTRES

Look at each carbon atom in the chain and see what is attached to it. For a chiral centre

you need an asymmetric carbon with four different atoms/groups) arranged tetrahedrally around it.

IF A CARBON HAS MORE THAN ONE OF ANY ATOM/GROUP ATTACHED, IT CAN’T BE CHIRAL

C 3 H’s around it NOT chiral

C 2 H’s around it NOT chiral

C 2 H’s around it NOT chiral

C 2 H’s around it NOT chiral

CH3CH2CH2CH2Cl

1-chlorobutane

slide5

OPTICAL ISOMERISM

SPOTTING CHIRAL CENTRES

Look at each carbon atom in the chain and see what is attached to it. For a chiral centre

you need an asymmetric carbon with four different atoms/groups) arranged tetrahedrally around it.

IF A CARBON HAS MORE THAN ONE OF ANY ATOM/GROUP ATTACHED, IT CAN’T BE CHIRAL

C 3 H’s around it NOT chiral

C 2 H’s around it NOT chiral

C 2 H’s around it NOT chiral

C 2 H’s around it NOT chiral

CH3CH2CH2CH2Cl

1-chlorobutane

CH3CH2CHClCH3

C 3 H’s around it NOT chiral

C 2 H’s around it NOT chiral

C H, CH3, Cl,C2H5 around it CHIRAL

C 3 H’s around it NOT chiral

2-chlorobutane

slide6

OPTICAL ISOMERISM

SPOTTING CHIRAL CENTRES

Look at each carbon atom in the chain and see what is attached to it. For a chiral centre

you need an asymmetric carbon with four different atoms/groups) arranged tetrahedrally around it.

IF A CARBON HAS MORE THAN ONE OF ANY ATOM/GROUP ATTACHED, IT CAN’T BE CHIRAL

C 3 H’s around it NOT chiral

C 2 H’s around it NOT chiral

C 2 H’s around it NOT chiral

C 2 H’s around it NOT chiral

CH3CH2CH2CH2Cl

1-chlorobutane

CH3CH2CHClCH3

C 3 H’s around it NOT chiral

C 2 H’s around it NOT chiral

C H, CH3, Cl,C2H5 around it CHIRAL

C 3 H’s around it NOT chiral

2-chlorobutane

(CH3)2CHCH2Cl

C 3 H’s around it NOT chiral

C 2 CH3’s around it NOT chiral

C 2 H’s around it NOT chiral

1-chloro-2-methylpropanane

(CH3)3CCl

C 3 H’s around it NOT chiral

C 3 CH3’s around it NOT chiral

2-chloro-2-methylpropanane

slide7

OPTICAL ISOMERISM

Spatial differences between isomers

  • two forms exist which are NON-SUPERIMPOSABLE MIRROR IMAGES of each other
  • non-superimposable means you you can’t stack one form exactly on top of the other
slide8

OPTICAL ISOMERISM

Spatial differences between isomers

  • two forms exist which are NON-SUPERIMPOSABLE MIRROR IMAGES of each other
  • non-superimposable means you you can’t stack one form exactly on top of the other
  • Some common objects aremirror images and superimposable spoons
  • superimposable but not mirror images books
  • non-superimposable mirror images hands
slide9

OPTICAL ISOMERISM

Spatial differences between isomers

  • two forms exist which are NON-SUPERIMPOSABLE MIRROR IMAGES of each other
  • non-superimposable means you you can’t stack one form exactly on top of the other
  • Some common objects aremirror images and superimposable spoons
  • superimposable but not mirror images books
  • non-superimposable mirror images hands
  • NB For optical isomerism in molecules, both conditions must apply...
  • they must be mirror images AND be non-superimposable
slide10

OPTICAL ISOMERISM

What is a non-superimposable mirror image?

Animation doesn’t work in old versions of Powerpoint

slide11

OPTICAL ISOMERS - DIFFERENCE

  • isomers differ in their reaction to plane-polarised light
  • plane polarised light vibrates in one direction only
  • one isomer rotates light to the right, the other to the left
  • rotation of light is measured using a polarimeter
  • rotation is measured by observing the polarised light coming out towards the observer
slide12

OPTICAL ISOMERS - DIFFERENCE

  • isomers differ in their reaction to plane-polarised light
  • plane polarised light vibrates in one direction only
  • one isomer rotates light to the right, the other to the left
  • rotation of light is measured using a polarimeter
  • rotation is measured by observing the polarised light coming out towards the observer
  • If the light appears to haveturned to the rightturned to the left
  • DEXTROROTATORY LAEVOROTATORY
  • d or + form l or - form
slide13

OPTICAL ISOMERS - DIFFERENCE

  • isomers differ in their reaction to plane-polarised light
  • plane polarised light vibrates in one direction only
  • one isomer rotates light to the right, the other to the left
  • rotation of light is measured using a polarimeter
  • rotation is measured by observing the polarised light coming out towards the observer
  • If the light appears to haveturned to the right turned to the left
  • DEXTROROTATORY LAEVOROTATORY
  • d or + form l or - form
  • Racematea 50-50 mixture of the two enantiomers (dl) or (±) is a racemic mixture.
  • The opposite optical effects of each isomer cancel each other out
  • ExamplesOptical activity is common in biochemistry and pharmaceuticals
  • • Most amino acids exhibit optical activity
  • • many drugs must be made of one optical isomer to be effective
  • - need smaller doses (safer and cost effective)
  • - get reduced side effects
  • - improved pharmacological activity
slide14

A

B

D

C

E

F

OPTICAL ISOMERISM

The polarimeter

A Light source produces light vibrating in all directions

B Polarising filter only allows through light vibrating in one direction

C Plane polarised light passes through sample

D If substance is optically active it rotates the plane polarised light

E Analysing filter is turned so that light reaches a maximum

F Direction of rotation is measured coming towards the observer

If the light appears to have turned to the rightturned to the left

DEXTROROTATORYLAEVOROTATORY

slide15

OPTICAL ISOMERISM

How optical isomers can be formed

Carbonyl compounds undergo nucleophilic addition. If there are two different groups attached to the C=O bond, the possibility of forming optical isomers arises.

THE NUCLEOPHILIC ADDITION OF HCN TO ETHANAL

If the nucleophilic cyanide ion attacks from above one optical isomer is formed

slide16

OPTICAL ISOMERISM

How optical isomers can be formed

Carbonyl compounds undergo nucleophilic addition. If there are two different groups attached to the C=O bond, the possibility of forming optical isomers arises.

THE NUCLEOPHILIC ADDITION OF HCN TO ETHANAL

If the nucleophilic cyanide ion attacks from above one optical isomer is formed

However, attack from below, gives the non-superimposable mirror image of the first

slide17

OPTICAL ISOMERISM

How optical isomers can be formed

Carbonyl compounds undergo nucleophilic addition. If there are two different groups attached to the C=O bond, the possibility of forming optical isomers arises.

THE NUCLEOPHILIC ADDITION OF HCN TO ETHANAL

If the nucleophilic cyanide ion attacks from above one optical isomer is formed

However, attack from below, gives the non-superimposable mirror image of the first

The reactionproduces a mixture of the two optical isomers because both modes of attack are possible

slide18

OPTICAL ISOMERISM

How optical isomers can be formed

Carbonyl compounds undergo nucleophilic addition. If there are two different groups attached to the C=O bond, the possibility of forming optical isomers arises.

THE NUCLEOPHILIC ADDITION OF HCN TO ETHANAL

ANIMATION

The reactionproduces a mixture of the two optical isomers because both modes of attack are possible

slide19

OPTICAL ISOMERISM

Synthesis of 2-hydroxypropanoic acid (lactic acid)

LACTIC ACID can be formed from ethanal in a two stage process.

1. Nucleophilic addition of hydrogen cyanide to ethanal

2 Hydrolysis of the nitrile group

H+ / H2O

HCN

slide20

OPTICAL ISOMERISM

Synthesis of 2-hydroxypropanoic acid (lactic acid)

LACTIC ACID can be formed from ethanal in a two stage process.

1. Nucleophilic addition of hydrogen cyanide to ethanal

2 Hydrolysis of the nitrile group

H+ / H2O

HCN

During the first stage, the nucleophilic CN- ion can attack from below, or above, the aldehyde.

A mixture of the two enantiomers is formed.

slide21

OPTICAL ISOMERISM

Synthesis of 2-hydroxypropanoic acid (lactic acid)

LACTIC ACID can be formed from ethanal in a two stage process.

1. Nucleophilic addition of hydrogen cyanide to ethanal

2 Hydrolysis of the nitrile group

H+ / H2O

HCN

During the first stage, the nucleophilic CN- ion can attack from below, or above, the aldehyde.

A mixture of the two enantiomers is formed.

Acid hydrolysis of the mixture provides a mixture of the two lactic acid forms.

slide22

OPTICAL ISOMERISM - THALIDOMIDE

The one obvious difference between optical isomers is their response to plane polarised light. However, some naturally occurring molecules or specifically synthesised pharmaceuticals show different chemical reactivity.

The drug, THALIDOMIDE is a chiral molecule and can exist as two enantiomers. In the 1960’s it was used to treat anxiety and morning sickness in pregnant women. Tragically, many gave birth to children with deformities and missing limbs.

It turned out that only one of the enantiomers (the structure on the right) was effective and safe; its optically active counterpart was not. The major problem was that during manufacture a mixture of the isomers was produced. The drug was banned world-wide, but not after tens of thousands of babies had been affected.

slide23

OPTICAL ISOMERISM – Other points

The following points are useful when discussing reactions producing optical isomers.

The formation of racemic mixtures is more likely in a laboratory reaction than in a chemical process occurring naturally in the body.

If a compound can exist in more than one form, only one of the optical isomers is usually effective.

The separation of isomers will make manufacture more expensive.

A drug made up of both isomers will require a larger dose and may cause problems if the other isomer is ‘poisonous’ like thalidomide.