Chemical and Physical Properties: Isomerism

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Lecture materials for the Introductory Chemistry course for Forensic Scientists, University of Lincoln, UK. See http://forensicchemistry.lincoln.ac.uk/ for more details.

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Chemical and Physical Properties: Isomerism

  1. 1. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Isomerism University of Lincoln presentation
  2. 2. Isomerism <ul><li>ISOMERS are compounds with the same molecular formula </li></ul><ul><li>(but different arrangements of atoms) </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  3. 3. Types of Isomerism <ul><li>Structural isomerism </li></ul><ul><li>Stereoisomerism </li></ul><ul><ul><li>Geometric ( cis/trans ) </li></ul></ul><ul><ul><li>Optical </li></ul></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  4. 4. 1. Structural isomerism This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  5. 5. Structural Isomerism This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License C 2 H 6 O These two compounds are structural isomers methoxymethane ethanol
  6. 6. Structural Isomerism This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Structural isomers often have different chemical & physical properties o– m– p– – nitromethylbenzene
  7. 7. Exercise <ul><li>Q What two compounds, with different functional groups, are isomers with the formula: </li></ul><ul><li>C 3 H 6 O 2 </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  8. 8. Answer <ul><li>A carboxylic acid </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License An ester
  9. 9. 2. Stereoisomerism a. Geometric isomerism This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  10. 10. Geometric Isomerism ( cis / trans ) <ul><li>If two compounds have the same formulae and the same structural framework , </li></ul><ul><li>but differ in the spatial arrangement of different atoms or groups about a central atom </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  11. 11. Geometric Isomerism ( cis / trans ) <ul><li>or about a double bond, </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  12. 12. Geometric Isomerism ( cis / trans ) <ul><li>then the compounds are called GEOMETRIC ISOMERS </li></ul><ul><ul><li>Cis– on the same side </li></ul></ul><ul><ul><li>Trans– on the opposite side </li></ul></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  13. 13. Geometric Isomerism ( cis / trans ) <ul><li>[Pt(NH 3 ) 2 Cl 2 ] </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Cis – diamminedichloroplatinum(II) Trans – diamminedichloroplatinum(II)
  14. 14. Geometric Isomerism ( cis / trans ) This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License There is restricted rotation about a C=C double bond and the two isomers are not inter-convertible without first breaking the C=C bond Cis -but-2-ene Trans -but-2-ene
  15. 15. Geometric isomerism helps you see! This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License This reaction is the central feature of human vision 11- cis -retinal 11- trans -retinal 11 12 R R’ R’ 11 12 R Cis Trans
  16. 16. 2. Stereoisomerism b. Optical isomerism This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  17. 17. Optical Isomerism <ul><li>Optical isomerism is caused by a lack of symmetry in a molecule’s structure so that the molecule and its mirror image are actually different </li></ul><ul><li>– like a left hand and a right hand: </li></ul><ul><li>They are non-superimposable </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  18. 18. For example This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Nonsuperimposable mirror images Optical isomers are called ENANTIOMERS
  19. 19. Enantiomers <ul><li>Labels L– and D– are given to two enantiomers to distinguish them from each other </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Only one optical isomer of DOPA called L-DOPA, is effective in the treatment of Parkinson’s disease
  20. 20. Asymmetric Carbon <ul><li>A compound that is NOT SUPERIMPOSABLE on its mirror image is described as being CHIRAL </li></ul><ul><li>Many chiral compounds contain an asymmetric carbon atom – a carbon atom with FOUR different groups attached </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  21. 21. CHIRAL Carbon This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  22. 22. For example – limonene This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Lemons Oranges
  23. 23. For example – limonene This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Lemons Oranges CHIRAL C
  24. 24. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Acknowledgements <ul><li>JISC </li></ul><ul><li>HEA </li></ul><ul><li>Centre for Educational Research and Development </li></ul><ul><li>School of natural and applied sciences </li></ul><ul><li>School of Journalism </li></ul><ul><li>SirenFM </li></ul><ul><li>http:// tango.freedesktop.org </li></ul>

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