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UNSATURATED HYDROCARBONS

UNSATURATED HYDROCARBONS. Unsaturated hydrocarbons contain carbon-carbon multiple bonds. Alkenes contain carbon-carbon double bonds (C=C ). Alkynes contain carbon-carbon triple bonds (C ≡ C). Aromatics contain benzene rings. NAMING ALKENES.

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UNSATURATED HYDROCARBONS

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  1. UNSATURATED HYDROCARBONS • Unsaturated hydrocarbonscontain carbon-carbon multiple bonds. • Alkenes contain carbon-carbon double bonds (C=C ). • Alkynescontain carbon-carbon triple bonds (C≡C). • Aromatics contain benzene rings.

  2. NAMING ALKENES • Step 1: Name the longest chain that contains the C=C bond. Use the IUPAC root and the –ene ending. • Step 2: Number the longest chain so the C=C bond gets the lowest number possible. • Step 3: Locate the C=C bond with the lower-numbered carbon. Examples: 1 2 3 4 CH3-CH=CH-CH3 2-butene 6 5 4 3 2 1 CH3-CH2-CH2-CH=CH-CH3 2-hexene

  3. NAMING ALKENES(continued) • Step 4: Locate and name attached groups. • Step 5: Combine all the names.

  4. NAMING ALKENESWITH MULTIPLE DOUBLE BONDS • Step 1:Follow the same naming instructions for alkenes with one double bond, except use the endings –diene, – triene, and the like to denote the number of double bonds. • Step 2:Indicate the location of all the multiple bonds. • EXAMPLES:

  5. THE GEOMETRY OFALKENES • In C=C bonds, sp2 hybrid orbitals are formed by the carbon atoms, with one electron left in a 2p orbital. A representation of sp2 hybridization of carbon:

  6. THE GEOMETRY OFALKENES (continued) • During hybridization, two of the 2p orbitals mix with the single 2s orbital to produce three sp2 hybrid orbitals. One 2p orbital is not hybridized and remains unchanged.

  7. THE GEOMETRY OFALKENES (continued) • This gives a planar shape for the sp2 bonding orbitals with the unhybridized p orbital perpendicular to the plane of the three sp2 hybridized orbitals.

  8. THE GEOMETRY OFALKENES (continued) • The planar geometry of the sp2 hybrid orbitals and the ability of the 2p electron to form a “pi bond” bridge locks the C=C bond firmly in place.

  9. THE GEOMETRY OFALKENES (continued) • Because there is no free rotation about the C=C bond, geometric isomerism is possible. • cis- isomers have two similar or identical groups on the same side of the double bond. • trans- isomers have two similar or identical groups on opposite sides of the double bond.

  10. THE GEOMETRY OFALKENES (continued) • Geometric isomers have different physical properties.

  11. PHYSICAL PROPERTIES OFALKENES • Similar to alkanes • Nonpolar • Insoluble in water • Soluble in nonpolar solvents • Less dense than water • Unpleasant, gasoline-like odors

  12. PHYSICAL PROPERTIES OFALKENES

  13. ALKENEREACTIONS • Alkenes are quite chemically reactive. • Alkene reactions follow the pattern: • These reactions are called addition reactions.

  14. ALKENEREACTIONS (continued)

  15. ALKENEREACTIONS (continued) • HALOGENATION • Halogenation (addition) reactions produce haloalkanes or alkyl halides.

  16. ALKENEREACTIONS (continued) • HYDROGENATION • Hydrogenation (addition) reactions can occur in the presence of a catalyst (Pt, Pd, or Ni). • The hydrogenation of vegetable oils is an important commercial process. • Polyunsaturated molecules contain several double bonds. • Hydrogenation of polyunsaturated molecules raises their melting points.

  17. ALKENEREACTIONS (continued) • MARKOVNIKOV’S RULE • Unsymmetrical alkene addition reactions follow Markovnikov’s rule which states that when a molecule of H-X adds to an alkene, the H predominantly attaches to the carbon already bonded to the most hydrogens. “The rich get richer.”

  18. ALKENEREACTIONS (continued) • ADDITION OF SIMPLE ACIDS • Addition of simple acids when Markovnikov’s rule is not required: • Addition of simple acids following Markovnikov’s rule:

  19. ALKENEREACTIONS (continued) • HYDRATION • Hydration (addition of water) reactions follow Markovnikov’s rule: • This reaction requires an acid catalyst.

  20. ALKENEREACTIONS (continued) • Hydration reactions are believed to occur in three steps. • Step 1: • H+ from acid catalyst is attracted to the electrons in the carbon-carbon double bond. • It becomes bonded to one of the carbon atoms by a sharing of electrons. • The other carbon atom from the double bond becomes an extremely reactive carbocation (positively charged carbon atom with only three bonds). • The carbocationattracts the oxygen atom (with two unshared pairs of electrons) in a water molecule.

  21. ALKENEREACTIONS (continued) • Step 2: • One pair of oxygen electrons form a covalent bond with the carbocation.

  22. ALKENEREACTIONS (continued) • Step 3: • H+ is lost to produce the alcohol. • Note: Catalyst is regenerated in this step.

  23. ALKENEREACTIONS (continued) • ADDITION POLYMERIZATION • An addition polymer is a polymer formed by the linking together of many alkene molecules through addition reactions.

  24. POLYMERIZATION • Polymers are very large molecules made up of repeating units. • A monomers is the starting material that becomes the repeating units of a polymer.

  25. COPOLYMER • An addition polymer formed by the reaction of two different monomers is a copolymer.

  26. COMMON ADDITION POLYMERS

  27. COMMON ADDITION POLYMERS

  28. ALKYNES • Ethyne (commonly called acetylene) is the simplest alkyne and is used as a fuel for torches and in making plastics.

  29. ALKYNE NOMENCLATURE • Alkynes are named in exactly the same ways as alkenes, except the ending –yne is used. • Examples:

  30. THE GEOMETRY OFALKYNES • In C≡C bonds, sp hybrid orbitals are formed by the carbon atoms, with two electrons left in unhybridized 2p orbitals. A representation of sp hybridization of carbon:

  31. THE GEOMETRY OFALKYNES(continued) • During hybridization, one 2p orbital mixes with the single 2s orbital to produce two sp hybrid orbitals. Two 2p orbitals are not hybridized and remain unchanged. • This gives a linear shape for the sp bonding orbitals with the unhybridized p orbitals perpendicular to the line of the two sp hybridized orbitals.

  32. THE GEOMETRY OFALKYNES(continued) • A carbon-carbon sigma bond forms by the overlap of one sp hybrid orbital of each carbon atom. • The other sp hybrid orbital of each carbon atom overlaps with a 1s orbital of a hydrogen atom to form a carbon-hydrogen sigma bond. • The remaining pair of unhybridized p orbitals of each carbon atom overlap sideways to form a pair of pi bonds between the carbon atoms.

  33. ALKYNEPROPERTIES • PHYSICAL PROPERTIES OF ALKYNES • Similar to alkanes and alkenes • Nonpolar • Insoluble in water • Soluble in nonpolar solvents • Less dense than water • Low melting and boiling points • CHEMICAL PROPERTIES OF ALKYNES • Similar to alkenes • React by addition reaction with Br2, H2, HCl, H2O • Require twice as many moles of addition reagent as alkenes in reactions that go on to completion

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