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Chapter 20

Chapter 20. Introduction to General, Organic, and Biochemistry, 10e John Wiley & Sons, Inc Morris Hein, Scott Pattison, and Susan Arena. Unsaturated Hydrocarbons. The aromas of many fragrant plants are mixtures of unsaturated organic molecules. Chapter Outline.

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Chapter 20

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  1. Chapter 20 Introduction to General, Organic, and Biochemistry, 10e John Wiley & Sons, Inc Morris Hein, Scott Pattison, and Susan Arena Unsaturated Hydrocarbons The aromas of many fragrant plants are mixtures of unsaturated organic molecules.

  2. Chapter Outline 20.1Bonding in Unsaturated Hydrocarbons 20.2Nomenclature of Alkenes 20.3Geometric Isomerism in Alkenes 20.4Cycloalkenes 20.5Preparation and Physical Properties of Alkenes 20.6Chemical Properties of Alkenes 20.7Alkynes: Nomenclature and Preparation 2

  3. 20.8Physical and Chemical Properties of Alkynes 20.9Aromatic Hydrocarbons: Structure 20.10Naming Aromatic Hydrocarbons 20.11Polycyclic Aromatic Hydrocarbons 20.12Sources and Physical Properties of Aromatic Hydrocarbons 20.13Chemical Properties of Aromatic Hydrocarbons Chapter Summary Chapter Outline 3

  4. Bonding in Unsaturated Hydrocarbons Unsaturated hydrocarbons enhance our standard of living. They are used to make: • Polyethylene plastic bags and bottles. • Polystyrene Styrofoam cups. • Plastic wraps. • Cosmetics, medicines, flavorings, perfumes. • Detergents, insecticides, and dyes. 4

  5. Bonding in Unsaturated Hydrocarbons Types of Unsaturated Hydrocarbons • Alkenes contain carbon-carbon double bonds. • Alkynes contain carbon-carbon triple bonds. • Aromatic compounds contain benzene rings. 5

  6. Bonding in Unsaturated Hydrocarbons The carbon atoms connected to double bonds in alkenes and aromatic compounds are sp2 hydridized. Figure 20.1Schematic hybridization of 2s2 2p 2p orbitals of carbon to form three sp2 electron orbitals and one p electron orbital. 6

  7. Bonding in Unsaturated Hydrocarbons Figure 20.2 (a) A single sp2 electron orbital and (b) a side view of three sp2 orbitals all lying in the same plane with a p orbital perpendicular to the three sp2 orbitals. 7

  8. Bonding in Unsaturated Hydrocarbons Figure 20.3: Pi (π) and sigma (σ) bonding in ethene. 8

  9. Bonding in Unsaturated Hydrocarbons The carbon atoms connected to triple bonds in alkynes are sp hybridized as shown in Figure 20.3 on the following slide . . . 9

  10. Bonding in Unsaturated Hydrocarbons Figure 20.3: Pi (π) and sigma (σ) bonding in acetylene. 10

  11. Nomenclature in Alkenes IUPAC Rules for Naming Alkenes 1. Identify the longest chain containing the C=C bond. 2. Name the parent alkane and change the –ane ending to –ene. CH3CH2CH3 is propane CH3CH=CH2 is propene 11

  12. Nomenclature in Alkenes 3. Number the carbon chain to give the double bonded carbons the lowest numbers. 4. Number and name branched alkyl groups as shown below. 12

  13. Your Turn! What is the structural formula of 4-methyl-2-pentene?

  14. Your Turn! What is the structural formula of 4-methyl-2-pentene? The name indicates: • Five carbons in the longest chain containing the double bond. • The double bond is between carbons #2 and #3 • A methyl group is on carbon #4.

  15. Your Turn! What is the name of this compound? 15

  16. Your Turn! • Five carbon atoms in the longest chain containing the double bond. • The double bond is between carbons #1 and #2. • The ethyl group is attached to the #2 carbon atom. • The name of this compound is 2-ethyl-1-pentene. 16

  17. Geometric Isomerism in Alkenes Alkenes that have the same molecular formula and the same connectivity between atoms but different spatial orientation of the atoms are called geometric isomers or cis-trans isomers. 17

  18. Geometric Isomerism in Alkenes Alkenes with the a/b pattern shown here will exhibit cis-trans isomerism. 18

  19. Geometric Isomerism in Alkenes If a C=C carbon has two identical groups as shown here, then cis-trans isomerism will not occur in the alkene. 19

  20. Your Turn! Draw the chemical structure of cis-5-chloro-2-hexene. 20

  21. Your Turn! Draw the chemical structure of cis-5-chloro-2-hexene. This molecule contains six carbons with a C=C between carbons #2 and #3, and a Cl atom on carbon #5. 21

  22. Your Turn! Draw the chemical structure of cis-5-chloro-2-hexene. This molecule is also cis because the carbon atoms in the longest chain containing the double bond are on the same side of the double bond. 22

  23. Your Turn! Is this the cis or trans isomer of 3-methyl-2-pentene? 23

  24. Your Turn! Is this the cis or trans isomer of 3-methyl-2-pentene? This istrans-3-methyl-2-pentene because the carbon atoms in the longest chain are on opposite sides of the double bond. 24

  25. Geometric Isomerism in Alkenes Many compounds have more than one C=C. Compounds with two C=C are called dienes as shown below. Compounds with three C=C are called trienes. 25

  26. Cycloalkenes Cycloalkenes are cyclic compounds with a C=C bond in the ring. The cyclo- in the name indicates that the molecule is cyclic and the –ene ending indicates that there is a double bond in the molecule. 26

  27. Cycloalkenes • Naming cycloalkenes • Number the carbon atoms in the ring. The carbon atoms with the double bond are given the lowest numbers. 27

  28. Cycloalkenes Naming cycloalkenes with two double bonds. The double bonds are given the lowest numbers. Diene in the name indicates that each molecule contains two double bonds. 28

  29. Your Turn! Name the following compounds. 29

  30. Your Turn! Name the following compounds. 2-methyl-1,3-cyclohexadiene 1-bromo-4-methylcyclohexene 30

  31. Preparation and Physical Properties of Alkenes Common preparation methods for alkenes start with saturated organic molecules. Atoms must be removed to form the double bonds. Alkene synthesis commonly means “getting rid” of some atoms in elimination reactions. 31

  32. Preparation and Physical Properties of Alkenes Two examples of alkene preparation are cracking and dehydration of alcohols. • Cracking (splitting of large hydrocarbon molecules to form smaller ones) • Dehydration of alcohols (elimination of H2O from an alcohol molecule) 32

  33. Preparation and Physical Properties of Alkenes • Alkenes can be prepared by cracking petroleum (i.e. crude oil) using a catalyst like silica-alumina as shown in the reaction below. 33

  34. Preparation and Physical Properties of Alkenes Alkenes can also be prepared by dehydration of alcohols. The reaction is catalyzed by an acid . 34

  35. Preparation and Physical Properties of Alkenes The physical properties of alkenes are similar to alkanes. Alkenes are nonpolar and insoluble in water but soluble in organic solvents. 35

  36. Chemical Properties of Alkenes What type of reaction might be expected for an alkene (or an alkyne)? Both alkenes and alkynes have fewer than the maximum of four atoms bonded per carbon. These molecules are more reactive than the corresponding alkanes and readily undergo addition reactions. 36

  37. Chemical Properties of Alkenes Alkenes undergo addition reactions to the C=C bond with these reactants. • Hydrogen (H2) • Halogens (Br2, Cl2) • Hydrogen halides (HBr, HCl, HI) • Sulfuric acid (H2SO4) • Water (H2O) 37

  38. Chemical Properties of Alkenes Consider the reactions of ethene first. Addition of hydrogen (H2) to ethene forms ethane. This is type of addition reaction is called a hydrogenation reaction. 38

  39. Chemical Properties of Alkenes Addition of halogen to ethene (Br2 in this case) forms 1,2-dibromoethane (an alkyl halide). 39

  40. Chemical Properties of Alkenes During the reaction of bromine with an alkene, the red-orange color of bromine (flask on the left) dissipates to form a colorless alkyl halide (flask on the right). 40

  41. Chemical Properties of Alkenes Addition of sulfuric acid (H2SO4) to ethene forms ethyl hydrogen sulfate. 41

  42. Chemical Properties of Alkenes Addition of water (H2O) to ethene forms ethanol. 42

  43. Chemical Properties of Alkenes Addition of alkyl halide (HCl, HBr, HI) to ethene forms chloroethane, bromoethane and iodoethane. 43

  44. Chemical Properties of Alkenes The preceding examples dealt with ethene, but reactions of this kind occur on almost any molecule that contains a carbon–carbon double bond. If a symmetrical molecule such as Cl2 is added to a larger alkene like propene only one product, 1,2-dichloropropane, is formed. 44

  45. Chemical Properties of Alkenes If an unsymmetrical molecule such as HCl is added to propene, two products are theoretically possible, depending on the carbon atom that the hydrogen atom connects to. The two possible products are 1-chloropropane and 2-chloropropane . . . 45

  46. Chemical Properties of Alkenes However only one product of the two possible products is produced. This occurs because addition reactions involving unsymmetrical alkenes follow Markovnikov’s rule. 46

  47. Chemical Properties of Alkenes What is Markovnikov’s rule? It is a rule that states the H in HX adds to the C=C carbon that has the largest number of hydrogen atoms. This rule can be explained by a reaction mechanism(i.e. the specific steps from reactants to products). 47

  48. Chemical Properties of Alkenes • HX addition to alkenes is a two-step reaction mechanism. A carbocation is produced in step one. (A carbocation is an ion where a carbon atom has a positive charge.) • 1. In step one a secondarycarbocation(2o)is produced when the pi electrons of the C=C are attacked by HCl. 48

  49. Chemical Properties of Alkenes • 2. In step two the chloride ion produced in step one adds to the carbon atom with the positive chargeto produce the product. Note: A 2 carbocation is more stable than a 1 carbocation so the 2 carbocationforms preferentially over the 1 carbocation. This difference in carbocation stabilty is the basis for Markovnikov’s rule. 49

  50. Chemical Properties of Alkenes There are four types of carbocations and these can be arranged by their relative stability 50

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