OPTICAL ISOMERISM - B Pharma Notes · 2021. 5. 4. · OPTICAL ISOMERISM SPOTTING CHIRAL CENTRES Look at each carbon atom in the chain and see what is attached to it. For a chiral

© Ramaiah University of Applied Sciences

1Faculty of Pharmacy

OPTICAL ISOMERISM

Occurrence another form of stereoisomerismoccurs when compounds have non-superimposablemirror images

Isomers the two different forms are known as optical isomers orenantiomers they occur when molecules have a chiral centrea chiral centre contains an asymmetric carbon atoman asymmetric carbon has four different atoms (or groups)arranged tetrahedrally around it.

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OPTICAL ISOMERISM

There are fourdifferent colours

arranged tetrahedrallyabout the carbon

atom

2-chlorobutane exhibitsoptical isomerism because

the second carbon atom hasfour different atoms/groups

attached

CHIRAL CENTRES

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OPTICAL ISOMERISM

SPOTTING CHIRAL CENTRES

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

CH3CH2CH2CH2Cl1-chlorobutane

C 3 H’s around it C2 H’s around it C 2H’s around it C 2H’s around it

NOT chiralNOT chiralNOT chiralNOT chiral

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OPTICAL ISOMERISM

SPOTTING CHIRAL CENTRES



CH3CH2CHClCH3

2-chlorobutane




C 3 H’s around it C 2 H’saround itC H, CH3, Cl,C2H5 around itC 3 H’s around it

NOT chiralNOT chiralCHIRALNOT chiral

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OPTICAL ISOMERISMSPOTTING CHIRAL CENTRES



CH3CH2CHClCH3

2-chlorobutane




C 3 H’s around it C 2 H’saround itC H, CH3, Cl,C2H5 around itC 3 H’s around it

NOT chiralNOT chiralCHIRALNOT chiral

(CH3)3CCl2-chloro-2-methylpropanane

C 3 H’s around it C3 CH3’s around it

NOT chiralNOT chiral

(CH3)2CHCH2Cl1-chloro-2-methylpropanane

C 3 H’s around it C2 CH3’s around it C 2 H’s around it

NOT chiralNOT chiralNOT chiralwww.bp

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OPTICAL ISOMERISM

••

Spatial differences between isomers

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

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OPTICAL ISOMERISM

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Some common objects are mirror images and superimposablesuperimposable but not mirror imagesnon-superimposable mirror images

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OPTICAL ISOMERISM

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Some common objects are mirror images and superimposablesuperimposable but not mirror imagesnon-superimposable mirror images

spoonsbookshands

NB For optical isomerism in molecules, both conditions must apply... theymust be mirror images AND be non-superimposable

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OPTICAL ISOMERISM

What is a non-superimposable mirror image?

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OPTICAL ISOMERS - DIFFERENCE•••••

isomers differ in their reaction to plane-polarised lightplane polarised light vibrates in one direction onlyone isomer rotates light to the right, the other to the leftrotation of light is measured using a polarimeterrotation is measured by observing the polarised light coming out towards the observer

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• If the light appears to have turned to the rightDEXTROROTATORYd or + form

turned to the leftLAEVOROTATORYl or - form

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• If the light appears to have turned to the rightDEXTROROTATORYd or + form

turned to the leftLAEVOROTATORYl or - form

Racemate a 50-50 mixture of the two enantiomers (dl) or (±) is a racemic mixture. Theopposite optical effects of each isomer cancel each other out

Examples••---

Optical activity is common in biochemistry and pharmaceuticalsMost amino acids exhibit optical activitymany drugs must be made of one optical isomer to be effectiveneed smaller doses (safer and cost effective)get reduced side effectsimproved pharmacological activity

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OPTICAL ISOMERISM

If the light appears to have turned to the rightDEXTROROTATORY

turned to the leftLAEVOROTATORY

A.B.

Light source produces light vibrating in all directionsPolarising filter only allows through light vibrating in one

directionCPlane polarised light passes through sampleD If substance is optically active it rotates the plane polarisedlightE Analysing filter is turned so that light reaches a maximumF Direction of rotation is measured coming towards the observer

The polarimeter

BA

C DE

F

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OPTICAL ISOMERISM - THALIDOMIDEThe one obvious difference between optical isomers is their response to plane polarised light.However, some naturally occurring molecules or specifically synthesised pharmaceuticalsshow different chemical reactivity. The drug, THALIDOMIDE is a chiral molecule and can exist as two enantiomers. In the1960’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 andsafe; its optically active counterpart was not. The major problem was that duringmanufacture a mixture of the isomers was produced. The drug was banned world- wide, butnot after tens of thousands of babies had been affected.

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OPTICAL ISOMERISM – Other pointsThe 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 chemicalprocess occurring naturally in the body. If a compound can exist in more than one form, only one of the optical isomers is usuallyeffective. 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 theother isomer is ‘poisonous’ like thalidomide.

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Summary

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Disclaimer

All data and content provided in this presentationare taken from the reference books, internet –websites and links, for informational purposesonly.

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OPTICAL ISOMERISM - B Pharma Notes · 2021. 5. 4. · OPTICAL ISOMERISM SPOTTING CHIRAL CENTRES Look at each carbon atom in the chain and see what is attached to it. For a chiral