20.6 Stereoisomerism

1. 20.6 Stereoisomerism 20.6.1 Describe stereoisomers as compounds with the same structural formula but with different arrangements of atoms in space. 20.6.2 Describe and explain geometrical isomerism in non-cyclic alkenes. 20.6.3 Describe and explain geometrical isomerism in C3 and C4 cycloalkanes. 20.6.4 Explain the difference in the physical and chemical properties of geometrical isomers. 20.6.5 Describe and explain optical isomerism in simple organic molecules. 20.6.6 Outline the use of a polarimeter in distinguishing between optical isomers. 20.6.7 Compare the physical and chemical properties of enantiomers.

2. Structural Isomer • Discussed in SL material. • Have same chemical formula (same amount of atoms), but totally different structures (and therefore names) • Ex: C4H10 • Has 2 different structural isomers • Butane CH3CH2CH2CH3 • Methyl propane CH3CH(CH3)CH3 • Can have different branches, position of functional groups, or completely different functional groups.

3. Stereoisomer • Molecules have atoms attached together in same order • Different spatial arrangement (3-D) • Two types required with IB • Geometric • Optical • On exam draw full structural diagrams to show stereoisomers properly.

4. Geometric Isomer • Restricted rotation about the bond • Cyclic structure (ie. Benzene) • Double bond (ie. Alkenes) • The atoms become fixed in place. • Using models can be helpful in understanding geometric isomers.

5. Not an isomer...

6. This is an isomer

7. Geometric isomer: Cis- same groups on same side of the double bond or ring.

8. Geometric isomers: Trans- same groups on opposite sides of double bond or ring.

9. This is not an isomer

10. Physical properties Cis Trans Polar molecules Dipole-dipole interactions + van der Waals’ forces between its molecules Higher B.P Lower M. P due to lack of symmetry. Not packed so closely together. Non polar molecules Only van der Waals’ forces between its molecules Lower B. P Pack together more closely because of their symmetry Higher M. P.

11. Chemical Properties Cis- Trans- Usually very similiar. One exception you need to know. Cis and trans-butenedioic acid Cis-butenedioic acid heated reacts to form butenedioic anhydride + H2O Trans- butenedioic acid heated sublimes, but does not chemically react.

12. Cyclic molecules Cycloalkanes contain a ring of carbons, single bonded. Bond angles are different than regular single bonds Cyclopropane (60 o triangle) Cyclobutane (90 o square) Can have cis- and trans- isomers because of lack of movement about the bond.

13. Draw on board • Cis-1,2-dimethylcyclopropane • Trans-1,2-dimethylcyclopropane • Cis-1,3-dichlorocyclobutane • Trans-1,3-dichlorocyclobutane

14. Optical Isomers It occurs when substances have the same molecular and structural formulae, but one cannot be superimposed on the other. They are mirror images of each other

15. The molecule on the left will not turn into the molecule on the right – unless you break and make some bonds! Molecules like this are said to be chiral and the different forms are called enantiomers.

16. Optical isomers • occur when there is an asymmetric carbon atom. • An asymmetric carbon atom is one which is bonded to four different groups (may be complex (ex: C2H5) or simple (ex: Cl)) • http://cwx.prenhall.com/petrucci/medialib/media_portfolio/text_images/083_Chirality.MOV

17. Optical isomers can rotate the plane of polarisation of plane-polarised light: one enantiomer rotates the polarised light clockwise (to the right) and is the (+) enantiomer the other rotates the polarised light anticlockwise (to the left) and is called the (–) enantiomer. A mixture containing equal amounts of both enantiomers is not optically active (it will not rotate the plane of polarisation). It is called a racemic mixture or racemate.

19. Properties of optical isomers • Enantiomers have identical physical and chemical properties with two exceptions • Optical activity • Reactivity with other chiral molecules

20. Optical activity • http://cwx.prenhall.com/petrucci/medialib/media_portfolio/text_images/084_OpticalActivity.MOV • Plane polarized light: If light is passed through a polarizer, only a single wavelength of light may pass through. (all other waves are blocked out) • A solution of enantiomers changes the rotation of light being passed through a polarizer. • They are optically active molecules

21. Rotation of light The extent and direction of rotation can be deduced. Concentration of solutions, wavelength of light used and sample path length is kept constant. Different enantiomers (at same conc.) rotate plane-polarized light in equal but opposite directions. Remember racemic mixtures don’t rotate any light.

22. Reactivity • When a racemic mixture is reacted with a single enantiomer of another chiral compound, the two components (+) and (-) enantiomers react to produce different products. • The products have different chemical and physical properties. Ex: 1960’s drug thalidomide: one enantiomer reaction was used to relieve symptoms of morning sickness, the other caused severe fetus malformations

23. Sources • http://www.chemguide.co.uk/basicorg/isomerism/geometric.html • http://www.chemguide.co.uk/organicprops/alkanes/background.html • http://www.creative-chemistry.org.uk/molecules/optical.htm • Play with this: http://www.wellesley.edu/Chemistry/Flick/chem341/opticalisomers.html