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alkanes, alkynes, Bettelheim, chem104

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  1. 1. Chapter 12: Alkenes and Alkynes Chem 104 K. Dunlap
  2. 2. Alkenes and Alkynes Alkene: a hydrocarbon that contains one or more carbon-carbon double bonds. – ethylene is the simplest alkene. Alkyne: a hydrocarbon that contains one or more carbon-carbon triple bonds. – acetylene is the simplest alkyne.
  3. 3. Trigonal Planar Structure: – The VSEPR model predicts bond angles of 120° about each carbon of a double bond. – In ethylene, the actual angles are close to 120°. – In substituted alkenes, angles about each carbon of the double bond may be greater than 120° because of repulsion between groups bonded to the double bond.
  4. 4. Cis-Trans Isomerism – because of restricted rotation about a carboncarbon double bond, an alkene with two different groups on each carbon of the double bond shows cis-trans isomerism.
  5. 5. is = c ide es sa m tran s = op pos it e si de
  6. 6. Naming Alkenes – The parent name is that of the longest chain that contains the C=C. – Number the chain from the end that gives the lower numbers to the carbons of the C=C. – Locate the C=C by the number of its first carbon. – Use the ending -ene to show the presence of the C=C – Branched-chain alkenes are named in a manner similar to alkanes; substituted groups are located and named.
  7. 7. Examples:
  8. 8. Naming Alkynes – follow the same rules as for alkenes, but use the ending -yne to show the presence of the triple bond.
  9. 9. Linear compound
  10. 10. Names this compounds:
  11. 11. • Common names are still used for some alkenes and alkynes, particularly those of low molecular weight.
  12. 12. Examples of ALKENES
  13. 13. Physical Properties of Alkenes and Alkynes – Alkenes and alkynes are nonpolar compounds. – The only attractive forces between their molecules are London dispersion forces. – Their physical properties are similar to those of alkanes with the same carbon skeletons. – Alkenes and alkynes are insoluble in water but soluble in one another and in nonpolar organic liquids. – Alkenes and alkynes that are liquid or solid at room temperature have densities less than 1.0 g/mL; they float on water.
  14. 14. The most common reaction is addition:
  15. 15. • Most alkene addition reactions are exothermic. – The products are more stable (lower in energy) than the reactants. – Just because they are exothermic doesn’t mean that alkene addition reactions occur rapidly. – reaction rate depends on the activation energy – Many alkene addition reactions require a catalyst.
  16. 16. Polymers • From the perspective of the organic chemical industry, the single most important reaction of alkenes is polymerization: – polymer: Greek: poly, many and meros, part – monomer: Greek: mono, single and meros, part
  17. 17. Polymers Cont… – Show the structure of a polymer by placing parentheses around the repeating monomer unit. – Place a subscript, n, outside the parentheses to indicate that this unit repeats n times. – The structure of a polymer chain can be reproduced by repeating the enclosed structure in both directions.
  18. 18. following a section of polypropene (polypropylene)
  19. 19. • Low-density polyethylene (LDPE): – a highly branched polymer; polymer chains do not pack well and London dispersion forces between them are weak. – softens and melts above 115°C. – approximately 65% used for the production of films for packaging and for trash bags. • High-density polyethylene (HDPE): – only minimal chain branching; chains pack well and London dispersion forces between them are strong. – has higher melting point than LDPE and is stronger – can be blow molded to squeezable jugs and bottles.
  20. 20. Problems with Polymers 1) Disposal- their stability and resistance to oxidation and attack by chemicals and bacteria *Only 5% of plastics are recycled *Account for 20% of solid waste volume *Incineration releases toxic gases *Plastisizers cause pollution and health problems 2) Depleated petroleum reserves
  21. 21. Solutions 1) Degradable plastics • Incorporation of light sensitive molecules that can be degraded by UV light • Incorporation of biodegradable polymers that bacteria can decompose 1) Reducing plastic usage 2) Recycling • Different types of plastics need to be sorted • Lesser quality plastic • Economical
  22. 22. Polymer Codes