Chapter 12: Unsaturated Hydrocarbons UNSATURATED HYDROCARBONS contain carbon-carbon multiple bonds. Alkenes C=C double bonds Alkynes triple bonds Aromatics benzene rings 1 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 lowest-numbered carbon. Examples: 1 2 3 4 CH 3 -CH=CH-CH 3 2-butene Step 4: Locate and name attached groups. Step 5: Combine all the names. 1 2 3 4 5 6 CH 3 -CH 2 -CH 2 -CH=CH-CH 3 2-hexene 3 4 NAMING ALKENES WITH MULTIPLE DOUBLE BONDS Step 1: Follow the same naming instructions for alkenes with one double bond, except use the endings ミdiene, - triene, and the like to denote the number of double bonds. Step 2: Indicate the location of all the multiple bonds, including those with rings. EXAMPLE: THE GEOMETRY OF ALKENES In C=C bonds, sp 2 hybrid orbitals are formed by the carbon atoms, with one electron left in a 2p orbital. A representation of sp 2 hybridization of carbon: 5 6 1
During hybridization, two of the 2p orbitals mix with the single 2s orbital to produce three sp 2 hybrid orbitals. One 2p orbital is not hybridized and remains unchanged. This gives a planar shape for the sp 2 bonding orbitals with the unhybridized p orbital perpendicular to the plane of the three sp 2 hybridized orbitals. 7 8 The planar geometry of the sp 2 hybrid orbitals and the ability of the 2p electron to form a pi bond bridge locks the C=C bond firmly in place. 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. Click here to play Chemistry Interactive 9 10 PHYSICAL PROPERTIES OF ALKENES Similar to alkanes Soluble in nonpolar solvents Less dense than water Unpleasant, gasoline-like odors ALKENE REACTIONS Alkenes are quite chemically reactive Alkene reactions follow the pattern: These reactions are called addition reactions. 11 12 2
HALOGENATION Halogenation (addition) reactions produce haloalkanes or alkylhalides. 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. 13 14 MARKOVNIKOV S RULE Unsymmetrical alkene addition reactions follow Markovnikov s rule: When a molecule of H-X adds to an alkene, the H attaches to the carbon already bonded to the most hydrogens. The rich get richer. ADDITION OF SIMPLE ACIDS Addition of simple acids following Markovnikov s rule: CH 2 = CH CH 3 + HBr CH 2 CH CH 3 H Br 15 16 HYDRATION Hydration (addition of water) reactions follow Markovnikov s rule: ADDITION POLYMERIZATION: Don t memorize An addition polymer is a polymer formed by the linking together of many alkene molecules through addition reactions. H 2SO 4 CH 2 = CH CH 3 + H 2 O CH 2 CH CH 3 H OH This reaction requires an acid catalyst. 17 18 3
POLYMERIZATION: Don t memorize Polymers very large molecules made up of repeating units Monomer the starting material that becomes the repeating units of a polymer COPOLYMER: Don t memorize An addition polymer formed by the reaction of two different monomers is a copolymer. 19 20 ALKYNES Ethyne (commonly called acetylene) is the simplest alkyne and is used as a fuel for torches and in making plastics. Alkynes are named in exactly the same ways as alkenes, except the ending yne is used. Examples: 21 22 THE GEOMETRY OF ALKYNES 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: 23 24 4
PHYSICAL PROPERTIES OF ALKYNES Similar to alkanes and alkenes Soluble in nonpolar solvents Less dense than water Low melting and boiling points BENZENE Aromatic compounds contain the benzene ring or one of its structural relatives. Aliphatic compounds don t contain this structure. CHEMICAL PROPERTIES OF ALKYNES Similar to alkenes React by addition reaction with Br 2, H 2, HCl, H 2 O Require twice as many moles of addition reagent as alkenes in reactions that go on to completion 25 26 In benzene, the six p orbital bonding electrons of the sp 2 hybridized carbon atoms can move freely around the ring. A hybrid orbital view of the benzene structure: When two groups are attached to the benzene ring, their positions can be designated by the prefixes ortho (o), meta (m), and para (p). 27 28 When two or more groups are attached, their positions can be indicated by numbering the ring so as to obtain the lowest possible numbers for the attachment positions. PHYSICAL PROPERTIES OF AROMATIC COMPOUNDS Similar to alkanes and alkenes Hydrophobic CHEMICAL PROPERTIES OF AROMATIC COMPOUNDS Aromatic rings are relatively stable chemically and often remain intact during reactions Benzene does not react like alkenes and alkynes Benzene does undergo substitution reactions, in which a ring hydrogen is replaced by some other group 29 30 5