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11 unsaturated-hydrocarbons(1)


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Chapter 11 of General, Organic, and Biochemistry, Denniston, 7th edition

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11 unsaturated-hydrocarbons(1)

  1. 1. Chapter 11The Unsaturated Hydrocarbons:Alkenes, Alkynes, and Aromatics
  2. 2. 1. StructureAlkenes are hydrocarbons with a double bond. CnH2n
  3. 3. 1. StructureAlkynes are hydrocarbons with a triple bond. CnH2n-2
  4. 4. 1. StructureAlkenes and alkynes are unsaturated (don’t have themaximum number of hydrogens bonded to each carbon). Tegrity lecture video
  5. 5. 1. Comparison
  6. 6. 1. Geometry
  7. 7. 1. Geometry [3.4 Lewis structures] Four groups of electrons  Ethane  tetrahedral  extend toward the corners of a regular tetrahedron  bond angle = 109.5o
  8. 8. 1. Geometry [3.4 Lewis structures] 8
  9. 9. 1. Geometry [3.4 Lewis structures] Three groups of electrons  Ethene  All in the same plane  Trigonal planar  Bond angle = 120o
  10. 10. 1. Geometry [3.4 Lewis structures] 10
  11. 11. 1. Geometry [3.4 Lewis structures] Two groups of electrons  Ethyne  Linear  Bond angle = 180o
  12. 12. 1. Geometry [3.4 Lewis structures] 12
  13. 13. 1. Physical properties Name Melting point Boiling point ethene -160.1oC -103.7oC propene -185.0oC -47.6oC 1-butene -185.0oC -6.1oCmethylpropene -140.0oC -6.6oC ethyne -81.8oC -84.0oC propyne -101.5oC -23.2oC 1-butyne -125.9oC 8.1oC 2-butyne -32.3oC 27.0oC
  14. 14. 1. Physical properties In each case, the alkyne has a higher boiling point than the alkene.  Its structure is more linear.  The molecules pack together more efficiently.  Intermolecular forces are stronger. Tegrity lecture video
  15. 15. 2. NomenclatureThe root name is based on the longest chain thatincludes both carbons of the multiple bond.The –ane ending is changed to –ene for double bondsand –yne for triple bonds. ethyne ethene propyne propene
  16. 16. 2. NomenclatureThe chain is numbered from the end nearest the multiplebond. 2-pentyne 1-butene [not 3-pentyne] [not 3-butene]The position of the multiple bond is indicated with thelower-numbered carbon in the bond.
  17. 17. 2. NomenclatureDetermine the name and number of each substituentand add in front of the name of the parent compound. 5-chloro-4-methyl-2-hexene 2,6-dimethyl-3-octene 5-bromo-4-ethyl-2-heptene
  18. 18. 2. NomenclatureAlkenes with more than one double bond are called alkadienes (2 double bonds) alkatrienes (3 double bonds) etc…Each double bond is designated by its lower-numberedcarbon. 2,4-hexadiene
  19. 19. 2. NomenclatureCycloalkenes must be numbered so the double bond isbetween carbons one and two. 3-chloro-cyclopentene 4-ethyl-5-methylcyclooctene
  20. 20. 2. NomenclatureName the following compounds. CH3CH=C(CH2CH3)2 H2C=C-CH2-CH=CH2 pencast
  21. 21. 2. NomenclatureName the following compounds. pencast
  22. 22. 2. NomenclatureWrite a structural formula for each of the followingcompounds. 1-hexene 1,3-dicholoro-2-butene 4-methyl-2-hexyne pencast 1,4-cyclohexadiene
  23. 23. 2. NomenclatureDraw a structural formula for each of the followingcompounds: 1-bromo-3-hexyne 2-butyne pencast dichloroethyne 9-iodo-1-nonyne
  24. 24. 3. Geometric isomers Rotation around a double bond is restricted, in much the same was as rotation is restricted for the cycloalkanes. In the alkenes, geometric isomers occur when there are two different groups on each of the double-bonded carbon atoms. 1,2-dichloroethene
  25. 25. 3. Cis-trans isomers If both constituents are on the same side of the double bond, the isomer is cis-. cis-1,2-dichloroethene If the constituents are on opposite sides of the double bond, the isomer is trans-. trans-1,2-dichloroethene
  26. 26. 3. Cis-trans isomers Alkenes without substituents also may exhibit cis-trans isomerism. cis-4-octene trans-4-octene
  27. 27. 3. Cis-trans isomers In order for cis and trans isomers to exist, neither double- bonded carbon may have two identical substituents. 2-methyl-2-butene no cis/trans isomerism 1-butene no cis/trans isomerism
  28. 28. 3. Cis-trans isomers 28
  29. 29. 3. Cis-trans isomers Which of the following compounds can exist as geometric isomers?  1-bromo-1-chloro-2,2-dimethylpropene  1,1-dichloroethene  1,2-dibromoethene  3-ethyl-2-methyl-2-hexene
  30. 30. 5. Reactions of alkenes and alkynes The most common reactions of alkenes and alkynes are addition reactions.  Hydrogenation: addition of H2  Halogenation: addition of X2  Hydration: addition of H2O  Hydrohalogenation: addition of HX
  31. 31. 5. General addition reaction A double bond consists of  a sigma bond: two electrons concentrated on a line between the two connected atoms;  a pi bond: two electrons concentrated in planes above and below the sigma bond.
  32. 32. 5. General addition reaction 32
  33. 33. 5. General addition reaction In an addition reaction, the pi bond is lost and its electrons become part of the single bonds to A and B.
  34. 34. 5. General addition reaction For hydrogenation, halogenation, hydration, and hydrohalogenation, identify the A and B portions of what is being added to the double bond.  hydrogenation, H2  halogenation, X2 (where X = F, Cl, Br, or I)  hydration, H2O  hydrohalogenation, HX (where X = F, Cl, Br, or I)
  35. 35. 5. Hydrogenation In hydrogenation of an alkene, one molecule of hydrogen (H2) adds to one mole of double bonds. Reaction conditions:  platinum, palladium, or nickel catalyst  [sometimes] heat and/or pressure
  36. 36. 5. Hydrogenation In hydrogenation of an alkyne, two molecules of hydrogen (H2) add to one mole of triple bonds. Reaction conditions: same as for alkenes.
  37. 37. 5. Hydrogenation Compare the products resulting from the hydrogenation of trans-2-pentene and cis-2-pentene. pencast
  38. 38. 5. Hydrogenation Compare the products resulting from the hydrogenation of 1-butene and cis-2-butene. pencast
  39. 39. 5. Vegetable oil and margarine Why does hydrogenation make oils more solid? MP = 13-14oC MP = 69.6oC MP = 62.9oC
  40. 40. 5. Halogenation In halogenation of an alkene, one mole of a halogen (Cl2, Br2, I2) adds to one mole of double bonds. Since halogens are more reactive than hydrogen, no catalyst is needed.
  41. 41. 5. Halogenation In halogenation of an alkyne, two moles of a halogen (Cl2, Br2, I2) add to one mole of double bonds.
  42. 42. 5. Halogenation Draw the structure and write a balanced equation for the halogenation of each of the following compounds.  3-methyl-1,4-hexadiene  4-bromo-1,3-pentadiene  3-chloro-2,4-hexadiene pencast
  43. 43. 5. Halogenation A solution of bromine in water has a reddish-orange color. A simple test for the presence of an alkene or alkane is to add bromine water.  If a double or triple bond is present, the bromine will be used up in a halogenation Test of cyclohexane reaction and the color will and cyclohexene disappear.
  44. 44. 5. Hydration In hydration, one mole of water (H2O) is added to one mole of double bonds. A trace of acid is required as a catalyst.
  45. 45. 5. Hydration Unlike hydrogenation and halogenation, hydration is not a symmetric addition to a double bond. If the double bond is not symmetrically located in the molecule, there are two possible hydration products.
  46. 46. 5. Hydration The predominant product is determined by Markovnikov’s rule: The rich get richer. OR: The carbon that already has more hydrogens will get the hydrogen from the water. Hydration of propene: + H 2O 
  47. 47. 5. Hydration Write a balanced equation for the hydration of each of the following compounds:  2-butene  2-ethyl-3-hexene pencast  2,3-dimethylcyclohexene Alkynes undergo a much more complicated hydration that you don’t need to remember at this time!
  48. 48. 5. Hydrohalogenation Like hydration, hydrohalogenation is an asymmetric addition to a double bond.  Hydrohalogenation also follows Markovnikov’s rule.
  49. 49. 5. Hydrohalogenation 2-butene + HBr  ? 3-methyl-2-hexene + HCl  ? cyclopentene + HI  ?pencast
  50. 50. 6. Aromatic compounds Consider the following molecular formulas for unsaturated hydrocarbons:  Hexane (all single bonds): C6H14  Cyclohexane (one ring): C6H12  Hexene (one double bond): C6H12  Hexadiene (two double bonds): C6H10  Cyclohexene (one ring, one double bond): C6H10  Hexatriene (three double bonds): C6H8  Cyclohexadiene (one ring, two double bonds): C6H8
  51. 51. 6. Aromatic compounds The molecular formula for benzene is C6H6.  The structure must be highly unsaturated.  One ring, three double bonds? Reactions of benzene:  Benzene does not decolorize bromine solutions.  Benzene does not undergo typical addition reactions.  Benzene reacts mainly by substitution. The first three items are opposite from what is expected from unsaturated compounds. The last item is identical to what is expected for alkanes.
  52. 52. 6. Benzene structure The benzene ring consists of:  six carbon atoms  joined in a planar hexagonal arrangement  with each carbon bonded to one hydrogen atom. Two equivalent structures proposed by Kekulé are recognized today as resonance structures. The real benzene molecule is a hybrid with each resonance structure contributing equally to the true structure.
  53. 53. 6. Benzene structure Sigma and pi bonding in benzene: The sharing of six electrons over the entire ring gives the benzene structure extra stability. Removing any one of the six electrons would destroy that stability.
  54. 54. 6. Nomenclature Most single-substituent compounds are named as derivatives of benzene.  Bromobenzene  Ethylbenzene
  55. 55. 6. Nomenclature A few “common” names have been adopted as IUPAC nomenclature.  toluene  phenol  aniline  xylene (any benzene ring with two methyl groups)
  56. 56. 6. Nomenclature There are three ways for the methyl groups on xylene to be arranged.  1,2 [ortho-xylene]  1,3 [meta-xylene]  1,4 [para-xylene]
  57. 57. 6. Nomenclature The substituent created by removing one hydrogen from the benzene ring is called phenyl-.  2-phenylhexane  3-phenylcyclopentene
  58. 58. 6. Nomenclature The substituent consisting of a –CH2 attached to a benzene ring is called benzyl-.  Benzyl chloride
  59. 59. 6. Polynuclear aromatic hydrocarbons These consist of rings joined along one side. Good news! You don’t have to memorize these names!
  60. 60. 6. Reactions of benzene Because of the stability of benzene’s ring structure, only substitution reactions are characteristic.  Halogenation: substitution of one or more halogen atoms for hydrogen atoms.  Cl2 requires FeCl3 catalyst.  Br2 requires FeBr3 catalyst.  Nitration: substitution of one or more nitro- (-NO2) groups for hydrogen atoms.  Requires nitric acid and concentration sulfuric acid.  Sulfonation: substitution of one sulfonic acid (-SO3H) group for a hydrogen atom.  SO3 reactant and concentration sulfuric acid.
  61. 61. 7. Heterocyclic aromatic compounds Heterocyclic aromatic compounds have at least one non- carbon atom incorporated in an aromatic ring or polynuclear aromatic compound.  Many of these compounds are biologically important.  Components of DNA and RNA  Components of hemoglobin and chlorophyll  Pharmaceuticals pyridine