Chapter 12 Alkenes and Alkynes

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1 BR M 102 lass Notes hapter 12 Page 1 of 8 hapter 12 Alkenes and Alkynes * alkenes = double bonds * alkynes 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: 2 = 2 ethene common name is ethylene 2 = 3 propene common name is propylene In these examples, there is no chance of isomers. What if you could have isomers? 2 = 2 3 ow would you name these? 3 = 3 2. If a chain has 4 or more carbons, number the position of the double bond using the lowest of possible numbers. examples: 2 = butene 3 = 3 2 butene ethyl 3 methyl 1 pentene 2 3

2 BR M 102 lass Notes hapter 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 = 3 l 6 chloro-3,4,6 trimethyl 2 heptene 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 ethyl 1 butene = ethyl 2 hexene 5. yclic compounds containing double bonds are called cycloalkenes. examples: l 3 3 cyclobutene 1 chlorocyclopentene 3,4 dimethylcyclohexene 6. ompounds with 2 or more double bonds are given the suffix diene, triene, tetraene, etc. 2 = = 2 1,3 butadiene 3 2 = = 2 = 3 2 methyl 1,3,6-octatriene

3 BR M 102 lass Notes hapter 12 Page 3 of 8 Geometry and Stereoisomerism of Alkenes * A single bond can rotate. * A double or triple bond cannot rotate. Notice that this structure is flat, that is all the bonds are in one plane. * Because there is no rotation, this allows stereoisomers similar to those we saw with the cyclic alkanes 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 of the double bond must have 2 unlike constituents. l 3 no stereoisomers l 1,1-dichloropropene 3 3 no stereoisomers 3 2-methyl-2-butene

4 BR M 102 lass Notes hapter 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 s gas 5 17 s liquid > 17 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. 3 2 isoprene isoprene unit * 1 is called the head and 4 is called the tail. Myrcene is found in oil of bay and consists of 2 isoprene units connected head to tail myrcene O geraniol Geraniol found in roses is similar to myrcene, but has an alcohol group. * 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.

5 BR M 102 lass Notes hapter 12 Page 5 of 8 hemical Properties of Alkenes * Alkenes like alkanes may undergo combustion, yielding O 2 and water. * Alkenes may undergo other chemical reactions as well. Addition Reactions Something is added across the double bond. + A B A B Addition of 2 reduction reaction + 2 catalyst propene propane Pt * This is also called a catalytic hydrogenation, where an unsaturated hydrocarbon (alkene) is saturated with to become a saturated hydrocarbon (alkane). Addition of 2 & l 2 X2 = 2 or l 2 + X 2 X X butene 2,3 dibromobutane * The same group is added to each of the double bond. * 2 is used as a test for unsaturated hydrocarbons. A drop of 2 is added to the sample. 2 is a red color. If the red disappears, then the sample is unsaturated.

6 BR M 102 lass Notes hapter 12 Page 6 of 8 Addition of X (ydrogen alides) X =, l, F, or I + X X + cyclohexene bromocyclohexane * A different group is added to each. Sometimes 2 different products are possible. Do you make one product or both? Are they made in equal amounts or not? l l chloropropane l chloropropane * Markovnikov s Rule in the addition of X, the goes to the that has more 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 2 O called a hydration reaction. + 2 O acid catalyst O

7 BR M 102 lass Notes hapter 12 Page 7 of 8 3 O O * Addition of 2 O follows Markovnikov s Rule. The goes to the with more s. 3 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. l l l l vinyl chloride vinyl chloride poly vinyl chloride (PV) * 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 2 2 ( 2 2 ) n ethylene polyethylene polymer * The most common way of naming a polymer is to add the prefix poly to the name of the monomer.

8 BR M 102 lass Notes hapter 12 Page 8 of 8 Alkynes Naming Alkynes * Use the same rules as for naming alkenes, except use the suffix yne. examples: IUPA name: ethyne common name: acetylene 3 propyne methyl 2 pentyne * 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 * 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.