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Chapter 12 Alkenes and Alkynes BRCC CHM 102 Class Notes Chapter 12 Page 1 of 8 Chapter 12 Alkenes and Alkynes * alkenes – C=C double bonds * alkynes – CC triple bonds * aromatics or arenes – alternating double and single bonds such as in benzene * saturated compounds – contain only single bonds (cannot add more hydrogens) * unsaturated compounds – contain one or more multiple bonds – more hydrogen may be added to these compounds Rules for Naming Alkenes 1. Use the rules for naming alkanes, but assign a name by adding a prefix from the table to the root –ene. examples: CH2 = CH2 ethene common name is ethylene CH2 = CH – CH3 propene common name is propylene In these examples, there is no chance of isomers. What if you could have isomers? CH2 = CH – CH2 – CH3 How would you name these? CH3 – CH = CH – CH3 2. If a chain has 4 or more carbons, number the position of the double bond using the lowest of possible numbers. examples: CH = CH – CH – CH 1 – butene 2 2 3 CH3 – CH = CH – CH3 2 – butene CH3 H3CH2CCC CH2 2 – ethyl – 3 – methyl – 1 – pentene H CH2CH3 BRCC CHM 102 Class Notes Chapter 12 Page 2 of 8 3. Side groups including halogens are named the same way as for alkanes. The double bond takes precedence over alkyl and halogen groups to start the numbering. example: 7 1 CH3 CH3 CH3 | | | CH3 – C – CH2 – CH – C = CH – CH3 6–chloro-3,4,6–trimethyl–2–heptene | Cl 4. The parent chain is the longest continuous chain which contains both carbons of the double bond. The double bond must be in the parent chain. example: CH3 – CH2 – CH – CH2 – CH3 2–ethyl–1–butene | | CH2 CH3 – CH2 – CH2 – C = CH – CH3 | 3–ethyl–2–hexene CH2 – CH3 5. Cyclic compounds containing double bonds are called cycloalkenes. CH examples: Cl 3 H C 3 cyclobutene 1–chlorocyclopentene 3,4–dimethylcyclohexene 6. Compounds with 2 or more double bonds are given the suffix –diene, –triene, –tetraene, etc. example: CH2 = CH – CH = CH2 1,3–butadiene CH3 | CH2 = C – CH = CH – CH2 – CH = CH – CH3 2–methyl–1,3,6-octatriene BRCC CHM 102 Class Notes Chapter 12 Page 3 of 8 Geometry and Stereoisomerism of Alkenes * A single bond can rotate. H H H C C H H H A double or triple bond cannot rotate. H Notice that this * H structure is flat, that CC is all the bonds are in H H one plane. * Because there is no rotation, this allows stereoisomers similar to those we saw with the cyclic alkanes. H3C H3C H CH3 CC CC H CH3 H H trans–2–butene cis–2–butene * The physical and chemical properties of these two compounds are different. * For the trans isomer, the parent chain crosses the double bond. * For the cis isomer, the parent chain is entirely on the same side of the double bond. * In order for cis and trans isomers to exist, each C of the double bond must have 2 unlike constituents. Cl CH3 CC no stereoisomers Cl H 1,1-dichloropropene H3C CH3 CC no stereoisomers H3C H 2-methyl-2-butene BRCC CHM 102 Class Notes Chapter 12 Page 4 of 8 Physical Properties of Alkenes * Alkanes and alkenes have similar physical properties. (see chapter 11 notes) At room temperature, they exist as: 2 – 5 C’s gas 5 – 17 C’s liquid > 17 C’s solid * They are insoluble in water with densities less than water. Terpenes (naturally occurring alkenes) * terpenes are made up of 2 or more isoprene units. CH3 C H2C C C CH2 CCCC H 1234 isoprene isoprene unit * C1 is called the head and C4 is called the tail. Myrcene is found in oil of bay and consists of 2 isoprene units connected head to tail. CH2 C H C CH HO 2 Geraniol found in roses is similar to H2C CH2 myrcene CH myrcene, but has an alcohol group. geraniol C H3CCH3 * Terpenes are widely distributed biological compounds. They illustrate how small molecules are used in building large molecules in nature. Vitamin A is made up of 4 isoprene units. BRCC CHM 102 Class Notes Chapter 12 Page 5 of 8 Chemical Properties of Alkenes * Alkenes like alkanes may undergo combustion, yielding CO2 and water. * Alkenes may undergo other chemical reactions as well. Addition Reactions Something is added across the double bond. A B CC + A – B C C Addition of H reduction reaction 2 H H CC catalyst + H 2 C C example: Pt H3CHC CH2 + H2 CH3CH2CH3 propene propane * This is also called a catalytic hydrogenation, where an unsaturated hydrocarbon (alkene) is saturated with H to become a saturated hydrocarbon (alkane). Addition of Br & Cl 2 2 X = Br or Cl 2 2 2 X X CC + X2 C C Br Br H3CHC CHCH3 + Br2 H3CHC CHCH3 2 – butene 2,3 – dibromobutane * The same group is added to each C of the double bond. * Br2 is used as a test for unsaturated hydrocarbons. A drop of Br2 is added to the sample. Br2 is a red color. If the red disappears, then the sample is unsaturated. BRCC CHM 102 Class Notes Chapter 12 Page 6 of 8 Addition of HX (Hydrogen Halides) X = Br, Cl, F, or I H X CC + HX C C example: H + HBr Br cyclohexene bromocyclohexane * A different group is added to each C. Sometimes 2 different products are possible. Do you make one product or both? Are they made in equal amounts or not? example: Cl H H3CHC CH2 H CHC CH 2 – chloropropane 3 2 + HCl H Cl H3CHC CH2 1 – chloropropane * Markovnikov’s Rule – in the addition of HX, the H goes to the C that has more H’s. The rich get richer. * Therefore you would make more 2 – chloropropane. * This addition reaction is said to be regioselective, or rather there is a preference in which bonds are broken and formed. Addition of H2O – called a hydration reaction. H OH CC acid catalyst + H O 2 C C BRCC CHM 102 Class Notes Chapter 12 Page 7 of 8 example: H+ OH H + H O H3C CH2 2 H CCCH 3 2 CH3 CH 3 * Addition of H2O follows Markovnikov’s Rule. The H goes to the C with more H’s. Addition Polymers * polymer – a long chain made up of repeating units. * polymerization – process of making polymers from simple units called monomers. * addition polymers – made by double-bonded molecules adding to each other. These are sometimes called chain-growth polymers because the stepwise addition of monomers is like adding links to a chain. Cl Cl Cl Cl + C C C C H2C CH H2C CH H2 H H2 H vinyl chloride vinyl chloride poly vinyl chloride (PVC) * repeating unit – the smallest molecular fragment which contains the nonredundant features of an entire chain. It is usually drawn inside parentheses followed by a subscript n to indicate how many times it is repeated. * ethylene (ethene) is a common monomer used to produce polymers n H2CCH2 (CH2CH2 )n ethylene polyethylene polymer * The most common way of naming a polymer is to add the prefix “poly” to the name of the monomer. BRCC CHM 102 Class Notes Chapter 12 Page 8 of 8 Alkynes Naming Alkynes * Use the same rules as for naming alkenes, except use the suffix “yne.” examples: IUPAC name: ethyne HC CH common name: acetylene H C CCH 3 propyne CH3 4 – methyl – 2 – pentyne H3C C CCCH3 H * Alkynes undergo addition reactions and follow Markovnikov’s Rule. * Since addition to the triple bond yields a product with a double bond, a second addition reaction is possible. example: Br Br HC CH + Br2 + Br2 HC CH Br Br HC CH Br Br * Alkynes have similar physical properties to alkenes and alkanes. They are nonpolar and undergo combustion reactions. With a triple bond, these combustion reactions are high energy. .
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