Chapter 13 Alkenes and Alkynes & Aromatic Compounds Chapter Outline 13.1 Alkenes and Alkynes 13.2 Nomenclature of Alkenes and Alkynes 13.3 Cis Trans Isomers 13.4 Alkenes in Food and Medicine 13.6 Reactions of Alkenes 13.8 Polymers 13.9 Aromatic Compounds 13.10 Nomenclature of Benzene Derivatives 13.11 Focus on Health & Medicine 13.13 Reactions of Aromatic Compounds 1
Unsaturated Hydrocarbons Unsaturated hydrocarbons are compounds that contain carbon carbon multiple bonds. Alkenes contain a double bond, alkynes contain a triple bond and Aromatic hydrocarbons contain a benzene ring, (as six-membered ring with three double bonds) These compounds have a functional group, making them much more reactive than alkanes. Thousands of biologically active molecules contain these functional groups, and many useful synthetic products result from their reactions. 2
13.1 Alkenes and Alkynes Alkenes are compounds that contain a carbon carbon double bond. The general molecular formula of an alkene is C n H 2n Ethylene or ethene (C 2 H 4 ) is the simplest alkene. Each carbon of ethylene is surrounded by three atoms forming a trigonal planar. All atoms of ethylene lie in the same plane, and all bond angles are 120. Ethylene is a hormone that regulates plant growth and fruit ripening. 3
Alkynes are compounds that contain a carbon carbon triple bond. The general molecular formula for an alkyne is C n H 2n 2 Acetylene (C 2 H 2 ) or ethyne is the simplest alkyne. Each carbon of acetylene is surrounded by two atoms, making each carbon linear with bond angles of 180. 4
13.2 Nomenclature of Alkenes And Alkynes In the IUPAC system: An alkene is identified by the suffix -ene.the simplest alkene, is CH 2 =CH 2 called ethene in the IUPAC system, but it is commonly called ethylene. An alkyne is identified by the suffix -yne. The simplest alkyne, C 2 H 2, is called ethyne in the IUPAC system, but it is commonly named acetylene. Step [1] Step [2] Number the carbon chain from the end that gives the multiple bond the lower number. Step [3] Number and name the substituents and write the name. 5
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Dienes Compounds with two double bonds are called dienes. They are named by changing the ane ending of the parent alkane to the suffix - diene. Each double bond gets its own number. 7
Problem13.1 Give the IUPAC name for the following compound. [1] Find the longest chain containing both carbon atoms of the multiple bond. [2] Number the chain to give the double bond the lower number. Numbering from right to left is preferred since the double bond begins at C2 (not C3). The molecule is named as a 2-pentene. [3] Name and number the substituents and write the complete name. 8
The alkene has two methyl groups located at C2 and C3. Use the prefix di- before methyl 2,3-dimethyl. Cycloalkenes In naming cycloalkenes, the double bond is located between C1 and C2, (and the 1 is usually omitted in the name). The ring is numbered to give the first substituent the lower number. 1-methylcyclopentene 3-methylcyclohepten 1,6-dimethylcyclohexene 9
Problem 13.2 1. Give the IUPAC name for each compound. 2. Problem 13.3: Draw the structure corresponding to each IUPAC name: a) 5,5-dimethyl-3-heptyne; b) 1,3-dimethylcyclohexene. 10
Problem 13.4 Give the structure corresponding to each of the following names. a) 4-methyl-1-hexene b) 2,5-dimethyl-3-hexyne c) 5-ethyl-2-methyl-2-heptene d) 1-propylcyclobutene e) 2-methyl-2-octene f) 1,3-cyclohexadiene g) 5-ethyl-2-methyl-1-nonene h) 4-ethyl-1-decyne 11
13.3 Cis trans Isomers With 2-butene, there are two ways to arrange the atoms on the double bond. The two CH 3 groups can be on the same side of the double bond or they can be on opposite sides of the double bond. These molecules are different compounds with the same molecular formula; that is, they are isomers. Problem 13.5 : Draw cis- and trans-3-hexene. 12
Problem 13.6: Draw a condensed structure for each compound: (a) cis-2-octene; (b) trans-3-heptene; (c) trans-4-methyl-2-pentene. Problem 13.7: For which compounds are cis and trans isomers possible? 13.6 Reactions of Alkenes And Alkynes Alkenes and alkynes undergo addition reactions. In an addition reaction, new groups X and Y are added to a starting material. One bond of the double bond is broken and two new single bonds are formed. 13
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13.6A Addition Of Hydrogen (Hydrogenation) Hydrogenation (addition of hydrogen (H 2 ) to an alkene). The addition of H 2 occurs only in the presence of a metal catalyst such as palladium (Pd). Problem 13.8 Draw the product of the following reaction. Pd 15
13.6B Addition Of Halogen (Halogenation) Problem 13.10 Draw the product of the reaction. CH 3 CH 2 CH=CH 2 + Cl 2 13.6C Addition of Hydrogen Halides (Hydrohalogenation) Hydrohalogenation is the addition of HX (X =Cl, Br or I) to an alkene. 16
Hydrohalogenation readily occurs with HCl, HBr or HI. The product of hydrohalogenation is an alkyl halide RX. HX can be added to the double bond to give two positional isomers when an unsymmetrical alkene is used as starting material. 17
Markovnikov s Rule In the addition of HX to an unsymmetrical alkene, the H atom bonds to the less substituted carbon atom that is, the carbon that has more H s to begin with. Problem 13.11 positional isomers What product is formed when 2-methylpropene [(CH 3 ) 2 C= CH 2 ] is treated with HBr? 18
13.6D Addition of Water (Hydration) Hydration is the addition of water to an alkene The hydration of unsymmetrical alkenes follows Markovnikov s Rule Problem 13.9 What product is formed when each alkene is treated with H 2 and a Pd catalyst? Problem 13.12 19
Addition Reactions of 1-butyne 20
13.8 POLYMERS Polymers are large molecules made up of repeating units of smaller molecules called monomers, covalently bonded together. Problem 13.13 What polymer is formed when CH 2 =CHCO 2 CH 3 (methyl acrylate) is polymerized? 21
Problem sets on chapter 13: 13.33-13.46 22
Benzene (C 6 H 6 ) is the simplest aromatic hydrocarbon August Kekulé proposed that benzene was a rapidly equilibrating mixture of two compounds, each containing a six-membered ring with three alternating bonds (sometimes a single bond and sometimes a double bond). These structures are known as Kekulé structures. Each of these representations has the same arrangement of atoms. Only the location of the electrons is different. These structures are called resonance structures. 23
13.10 Nomenclature of Benzene Derivatives Mono Substituted Benzene To name a benzene ring with one substituent, name the substituent and add the word benzene. Common names used for monosubstituted benzenes: O O CH 3 benzaldehyde H OH benzoic acid O acetophenone 24
Di-Substituted Benzene If the two groups on the benzene ring are different, alphabetize the names of the substituents preceding the word benzene. If one substituent is part of a common root, name the molecule as a derivative of that mono-substituted benzene. There are three different ways that two groups can be attached to a benzene ring, so the prefixes: ortho, meta, or para are used to designate the relative position of the two substituents. 25 25
Nomenclature of Benzene Derivatives: (Polysubstituted Benzene) Number to give the lowest possible around the ring. Alphabetize the substituent names. If the two groups on the benzene ring are different, alphabetize the names of the substituents preceding the word benzene. If one substituent is part of a common root, name the molecule as a derivative of that mono-substitutedbenzene 26
Compounds containing two or more benzene rings that share C- C bonds are called Polycyclic Aromatic Hydrocarbons (PAHs). There are two different ways to join three rings together, forming anthracene and phenanthrene. 1. Name each of the following aromatic compounds a. b. 2. Draw the structure corresponding to each name a)pentylbenzene c) m-bromoaniline b) 4-chloro-1,2-diethylbenzene d) o-dichlorobenzene 27
Stability of Benzene: Aromatic compounds are benzene and compounds resemble benzene. They are unsaturated compounds that do not undergo the addition reactions as alkenes. But Benzene undergoes electrophilic aromatic substitution in which a hydrogen atom is replaced by an electrophile E + such as X +, NO 2 + 28
Three specific reactions of benzene. In each reaction, only one hydrogen atom on benzene is replaced by another atom or group: -Cl, -NO 2, or SO 3 H, respectively. Problems 1. What product is formed when p-dichlorobenzene is treated with each reagent: (a) Cl 2, FeCl 3 : (b) HNO 3, H 2 SO 4 : (c) SO 3, H 2 SO 4 2. When toluene (methylbenzene) is treated with Cl 2 and FeCl 3, a total products are possible. Draw the structure of the three products and give their IUPAC name Textbook Problems 13.80, 13.81, 13.83, 13.85, 13.88, 13.90,13.91, 13.92, 13.101, 13.107, 13.109 29