Chapter 20

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. 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. 2

3. 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. 3

4. 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 4

5. 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. 5

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

7. Your Turn! What is the name of this compound? 7

8. 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. 8

9. Geometric Isomerism in Alkenes Alkenes with the a/b pattern shown here will exhibit cis-trans isomerism. 9

10. 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. 10

11. Your Turn! Draw the chemical structure of cis-5-chloro-2-hexene. 11

12. Your Turn! Is this the cis or trans isomer of 3-methyl-2-pentene? 12

13. 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. 13

14. 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. 14

15. Cycloalkenes Naming cycloalkenes Number the carbon atoms in the ring. The carbon atoms with the double bond are given the lowest numbers. 15

16. 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. 16

17. Your Turn! Name the following compounds. 17

18. 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. 18

19. 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) 19

20. 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. 20

21. Preparation and Physical Properties of Alkenes Alkenes can also be prepared by dehydration of alcohols. The reaction is catalyzed by an acid . 21

22. 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. 22

23. 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. 23

24. 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) 24

25. 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. 25

26. Chemical Properties of Alkenes Addition of halogen to ethene (Br2 in this case) forms 1,2-dibromoethane (an alkyl halide). 26

27. 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). 27

28. Chemical Properties of Alkenes Addition of sulfuric acid (H2SO4) to ethene forms ethyl hydrogen sulfate. 28

29. Chemical Properties of Alkenes Addition of water (H2O) to ethene forms ethanol. 29

30. Chemical Properties of Alkenes Addition of alkyl halide (HCl, HBr, HI) to ethene forms chloroethane, bromoethane and iodoethane. 30

31. 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. 31

32. 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 . . . 32

33. 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. 33

34. 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. 34

35. Your Turn! Predict the major products formed when 2-methyl-1-butene reacts with: • H2, Pt/25°C • Cl2 • HCl • H2O, H+ 35

36. Chemical Properties of Alkenes Another typical reaction of alkenes is oxidation of the double bond. For example when an alkene is shaken with a cold, dilute solution of potassium permanganate, KMnO4, the alkene is converted to a glycol (glycols are dihydroxy alcohols). 36

37. Chemical Properties of Alkenes The reaction below is used in the Baeyer test which is a test for the presence of double or triple bonds in an unknown sample. 37

38. Alkynes: Nomenclature and Preparation IUPAC Rules for Naming Alkynes The rules for naming alkynes are the same as those used to name alkenes except the suffix –yne is used to indicate the presence of the C≡C bond. The carbon atoms with the triple bond get the lowest numbers. 38

39. Alkynes: Nomenclature and Preparation 39

40. Alkynes: Nomenclature and Preparation Although triple bonds are very reactive, it is relatively easy to synthesize alkynes. Acetylene, the simplest alkyne, can be prepared inexpensively from calcium carbide and water or by the cracking of methane. 40

41. Your Turn! Name the following compounds. 41

42. Physical and Chemical Properties of Alkynes Physical properties of acetylene …… • a colorless gas. • little odor when pure. • insoluble in water. • a gas at normal temperature and pressure. • subject to explosive decomposition. 42

43. Physical and Chemical Properties of Alkynes Chemical properties of alkynes Alkynes undergo addition reactions similar to those of alkenes. They react with: • Cl2 and Br2 • HCl and HBr • KMnO4 to give a positive result from Baeyer’s test. 43

44. Physical and Chemical Properties of Alkynes Reaction of acetylene with bromine (bromination). HCCH + Br2  CHBr=CHBr HCCH + 2 Br2  CHBr2-CHBr2 44

45. Physical and Chemical Properties of Alkynes HCl addition to unsymmetrical alkynes follows Markovnikov’s rule like the reactions with alkenes. The hydrogen atom adds to the carbon atom that has the largest number of hydrogens. CH3CCH + HCl  CH3CCl=CH2 CH3CCH + 2 HCl  CH3CCl2-CH3 45

46. Aromatic Hydrocarbons: Structure Benzene is an aromatic compound. Aromatic compounds are those that resemble benzene in structure and reactivity. 46

47. Aromatic Hydrocarbons: Structure Benzene is aromatic because it has an unusually stable electronic structure created by six delocalized pi electrons. 47

48. Naming Aromatic Compounds Substituted benzenes are the most common benzene derivatives because substitution is the most common reaction type for benzene. A substituted benzene is derived by replacing one or more hydrogen atoms of benzene by another atom or group of atoms. 48

49. Naming Aromatic Compounds A monosubstituted benzene has the formula C6H5G, where G is the group replacing a hydrogen atom. Monosubstituted benzenes can be named by adding the substituent prefix in front of the word benzene as shown below. 49

50. Naming Aromatic Compounds There are several monosubstituted benzenes that have special names. 50