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Geometrical and Optical Isomerism in Organic Chemistry Known as STEREOISOMERS.

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Presentation on theme: "Geometrical and Optical Isomerism in Organic Chemistry Known as STEREOISOMERS."— Presentation transcript:

1 Geometrical and Optical Isomerism in Organic Chemistry Known as STEREOISOMERS

2 TYPES OF ISOMERISM Occurs due to the restricted rotation of C=C double bonds... two forms - CIS and TRANS STRUCTURAL ISOMERISM STEREOISOMERISM GEOMETRICAL ISOMERISM OPTICAL ISOMERISM CHAIN ISOMERISM Same molecular formula but different structural formulae Occurs when molecules have a chiral centre. Get two non- superimposable mirror images. Same molecular formula but atoms occupy different positions in space. POSITION ISOMERISM FUNCTIONAL GROUP ISOMERISM

3 STEREOISOMERISM Molecules have the SAME MOLECULAR FORMULA but the atoms are joined to each other in a DIFFERENT SPACIAL ARRANGEMENT - they occupy a different position in 3-dimensional space. There are two types... GEOMETRICAL ISOMERISM OPTICAL ISOMERISM

4 GEOMETRICAL ISOMERISM IN ALKENES Introduction an example of stereoisomerism found in some, but not all, alkenes occurs due to the RESTRICTED ROTATION OF C=C bonds get two forms.... CIS Groups/atoms are on the SAME SIDE of the double bond TRANS Groups/atoms are on OPPOSITE SIDES across the double bond Isomers- have different physical properties - e.g. boiling points, density - have similar chemical properties - in most cases

5 GEOMETRICAL ISOMERISM RESTRICTED ROTATION OF C=C BONDS Single covalent bonds can easily rotate. What appears to be a different structure in an alkane is not. Due to the way structures are written out, they are the same. ALL THESE STRUCTURES ARE THE SAME BECAUSE C-C BONDS HAVE FREE ROTATION Animation doesnt work in old versions of Powerpoint

6 GEOMETRICAL ISOMERISM RESTRICTED ROTATION OF C=C BONDS C=C bonds have restricted rotation so the groups on either end of the bond are frozen in one position; it isnt easy to flip between the two. This produces two possibilities. The two structures cannot interchange easily so the atoms in the two molecules occupy different positions in space. Animation doesnt work in old versions of Powerpoint

7 GEOMETRICAL ISOMERISM Isomerism in butene There are 3 structural isomers of C 4 H 8 that are alkenes *. Of these ONLY ONE exhibits geometrical isomerism. BUT-1-ENE2-METHYLPROPENE trans BUT-2-ENEcis BUT-2-ENE * YOU CAN GET ALKANES WITH FORMULA C 4 H 8 IF THE CARBON ATOMS ARE IN A RING

8 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 a chiral centre contains an asymmetric carbon atom an asymmetric carbon has four different atoms (or groups) arranged tetrahedrally around it.

9 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 a chiral centre contains an asymmetric carbon atom an asymmetric carbon has four different atoms (or groups) arranged tetrahedrally around it. 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 CHIRAL CENTRES

10 OPTICAL ISOMERISM Spatial differences between isomers two forms exist which are NON-SUPERIMPOSABLE MIRROR IMAGES of each other non-superimposable means you you cant stack one form exactly on top of the other Some common objects aremirror images and superimposablespoons superimposable but not mirror imagesbooks non-superimposable mirror imageshands NBFor optical isomerism in molecules, both conditions must apply... they must be mirror images AND be non-superimposable

11 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 CANT BE CHIRAL CH 3 CH 2 CHClCH 3 CH 3 CH 2 CH 2 CH 2 Cl C 3 Hs around itNOT chiral C 2 Hs around itNOT chiral C 3 Hs around itNOT chiral C 2 Hs around itNOT chiral C H, CH 3, Cl,C 2 H 5 around itCHIRAL C 3 Hs around itNOT chiral (CH 3 ) 3 CCl C 3 Hs around itNOT chiral C 3 CH 3 s around itNOT chiral 1-chlorobutane 2-chlorobutane 2-chloro-2-methylpropanane (CH 3 ) 2 CHCH 2 Cl C 3 Hs around itNOT chiral C 2 CH 3 s around itNOT chiral C 2 Hs around itNOT chiral 1-chloro-2-methylpropanane

12 OPTICAL ISOMERISM What is a non-superimposable mirror image? Animation doesnt work in old versions of Powerpoint

13 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 have turned to the right turned to the left DEXTROROTATORY LAEVOROTATORY d or + forml or - form

14 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 have turned to the right turned to the left DEXTROROTATORY LAEVOROTATORY d or + forml or - form Racematea 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

15 Optical isomer disaster…

16 OPTICAL ISOMERISM The polarimeter If the light appears to have turned to the right turned to the left DEXTROROTATORY LAEVOROTATORY 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 AB C D E F


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