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Do Now. What is the definition of structural isomerism? What are the three different structural isomers? Give an example of each What is the definition of stereoisomerism? What is an example of a stereoisomer? Why does it show stereoisomerism?. Stereoisomerism.

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  1. Do Now • What is the definition of structural isomerism? • What are the three different structural isomers? • Give an example of each • What is the definition of stereoisomerism? • What is an example of a stereoisomer? • Why does it show stereoisomerism?

  2. Stereoisomerism • Which of these molecules can show E-Z isomerism? • CH2=CH2 • CH3-CH3 • RCH=CH2 • RCH=CHR 2. What is the name of this molecule?

  3. OPTICAL ISOMERISM

  4. What does it mean?

  5. What does it mean? Plane with paper Facing the viewer Extended away

  6. Molymods • Using the molymods, try and make different compounds with fourdifferentsubstituent groups attached to a carbon e.g bromochlorofluoromethane. • Once you have made one, make the mirror image of that compound. • Can it be superimposed?

  7. All molecules have a mirror image – but for most molecules it is the same molecule.

  8. For some molecules the mirror image is a different molecule (the mirror image is non-superimposable).

  9. Cannot be superimposed!

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

  11. 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.

  12. Stereoisomerism • Have the same structural formula but different because their atoms or groups are arranged differently in three dimensions. • There are two types: E-Z isomerism and Optical isomerism

  13. Optical Isomerism • When there are four different substituents attached to one carbon atom. • This results in two isomers that are non-superimposable mirror images of one another, but are not identical.

  14. TASKSome of the following molecules are optically active. Use the molymods to help you work out which of these molecules are optically active. a) propan-2-ol e) butanone b) 2-chlorobutane f) 2-methylbutanoic acid c) 1-chlorobutane g) butan-2-ol d) 3-methylhexane h) 1-chloro-3-methylpentane

  15. propan-2-ol NOT OPTICALLY ACTIVE

  16. OPTICALLY ACTIVE 2-chlorobutane

  17. 1-chlorobutane NOT OPTICALLY ACTIVE

  18. OPTICALLY ACTIVE 3-methylhexane

  19. butanone NOT OPTICALLY ACTIVE

  20. propan-2-ol NOT OPTICALLY ACTIVE

  21. OPTICALLY ACTIVE 2-methylbutanoic acid

  22. OPTICALLY ACTIVE butan-2-ol

  23. OPTICALLY ACTIVE 1-chloro-3-methylpentane

  24. Key Points • Optical isomers are chiral • Two isomers are called a pair of enantiomers • Carbon bonded to the four different groups is called a chiral centre • It is often indicated by a *

  25. α – aminopropanoic acid Lactic acid (or 2-hydroxypropanoic acid)

  26. Spot the chiral carbons

  27. Exam Questions

  28. How do they work?

  29. Polarised Light • Optical Isomers • TED-Ed • Plane Polarised Light

  30. 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

  31. The wave vibrations are perpendicular to the direction of travel of the wave.

  32. POLARIMETERS can be used to analyse the effect optical isomers have on plane polarised light:

  33. Optical isomers rotate the plane of plane polarised light. Laevorotatory Dextrorotary

  34. Racemic A racemic mixture (racemate) consists of an equimolar mixture of both enantiomers and is therefore optically inactive, i.e. one isomer cancels out the rotation of plane polarised light caused by the other isomer. (Do NOT say it does not contain optically active molecules etc.)

  35. Notes • Optical rotation can be measured using a polarimeter • One solution will rotate the plane of polarisation clockwise (+ isomer) • One solution will rotate the plane of polarisation anticlockwise (- isomer) • Racemate means there is a 50:50 ratio of each isomer. This means it cancels out the rotation, there is no overall effect and optically inactive.

  36. Question • A dextrorotatory enantiomer has a rotation of +43 degrees. A mixture of this enantiomer and its laevorotary enantiomer has a rotation of -10 degrees. What does this information indicate about the composition of this mixture?

  37. Exam Questions

  38. Exam Questions

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