1. 1 Jerry Poteat Science Department Georgia Perimeter College © John Wiley and Sons, Inc. Version 1.0 Chapter 19 Organic Chemistry: Saturated Hydrocarbons Hein * Pattison * Arena * Best *

2. 2 Chapter Outline 19.1 Organic Chemistry: History and Scope 19.2 The Carbon Atom: Bonding and Shape 19.3 Organic Formulas and Molecular Models 19.4 Classifying Organic Compounds 19.5 Hydrocarbons 19.6 Saturated Hydrocarbons: Alkanes 19.7 Carbon Bonding in Alkanes

3. 3 Chapter Outline 19.8 Isomerism 19.9 Naming Organic Compounds 19.10 Introduction to the Reactions of Carbon 19.11 Reactions of Alkanes 19.12 Sources of Alkanes 19.13 Gasoline: A Major Petroleum Product 19.14 Cycloalkanes

4. 4 19.1 Organic Chemistry: History and Scope

5. 5 Organic chemistry is the study of compounds containing carbon. These compounds occur naturally but can be prepared in a laboratory. The early history of organic chemistry included an erroneous theory. ( i.e. that organic compounds exist only in living organisms). The German chemist Wöhler disproved this theory in 1828 with the lab synthesis of urea. What is organic chemistry?

6. 6 Carbon can form a vast array of long chain and ring containing compounds because carbon has the unique ability to bond to itself. Organic compounds include drugs, fuels, toiletries, plastics, and fabrics. You can see why organic chemistry is such an important field of study. What is organic chemistry? Lipstick is made of organic molecules. Cosmetics and perfumes contain organic compounds.

7. 7 19.2 The Carbon Atom: Bonding and Shape

8. 8 Bonding in Carbon Compounds Carbon can form saturated compounds ( i.e. all carbon atoms have four single bonds) or unsaturated compounds (i.e. at least one carbon has a double bond (C=C) or a triple bond (C≡C)).

9. 9 Molecular Shapes of Carbon Compounds Predicted by the VSEPR Bonding Theory

10. 10 Figure 19.1 Tetrahedral structure of carbon: (a) a regular tetrahedron; (b) a carbon atom with tetrahedral bonds; (c) a carbon atom within a regular tetrahedron; (d) a methane molecule, CH 4 Bond Angle and Shape of Methane

11. 11 19.3 Organic Formulas and Molecular Models

12. 12 Formulas and Molecular Models or in the case of a model. Structural formulas or models are often used in organic chemistry to illustrate molecules. For example,

13. 13 Figure 19.3 Types of formulas and models used to represent organic molecules. Each diagram is a representation of a propane molecule.

14. 14 This is an example of how to change a condensed formula into a line structure. The tables in the next two slides summarize formulas and models used in organic chemistry.

15. 15 Lewis structure Structural formula Condensed structural formula Line structure Ball-and-stick model Space-filling model Shows all bonds, all atoms, and all nonbonding e- Same as Lewis structure except nonbonding e- are not shown Bonds are collapsed so relative positions of atoms are group together Only bonds and functional groups are shown and each endpoint and bend represent carbon atoms Atoms are shown as balls and bonds as sticks and the shape is shown at the central atom. Bonds are omitted and the size of atoms are emphasized. Formula or model Definition

16. 16

17. 17 19.4 Classifying Organic Compounds

18. 18 What are functional groups? Functional groups are group of atoms ( or atom) that have specific behavorial characteristics in organic compounds. For example ethanol (i.e. the alcohol functional group) and dimethyl ether (i.e. the ether functional group) have the same molecular formula but the boiling point (b.p.) of ethanol is 78  C while the b.p. of dimethyl ether is -23  C because of the structural differences between the molecules.

19. 19 Organic compounds are organized by functional groups into classes as shown in Table 19.1.

20. 20 19.5 Hydrocarbons

21. 21 Hydrocarbons Hydrocarbons are organic compounds that contain only carbon and hydrogen atoms. (e.g. propane-CH 3 CH 2 CH 3 is a hydrocarbon but ethanol CH 3 CH 2 OH is not ) Hydrocarbons are classified as either aliphatic (i.e those hydrocarbons without benzene rings in the chemical structure) or aromatic.(i.e those hydrocarbons with benzene rings in the chemical structure) as shown in Figure 19.5 in the next slide. Hydrocarbons are also classified as saturated or unsaturated compounds

22. 22 Figure 19.4 Classes of Hydrocarbons

23. 23 Hydrocarbons The principal source of hydrocarbons is fossil fuels which include natural gas, petroleum, and coal. Fossil fuels are the primary source of heat and also a primary source of organic chemicals.

24. 24 19.6 Saturated Hydrocarbons: Alkanes

25. 25 Alkanes are saturated hydrocarbons that are either straight- chain or branched-chain hydrocarbons.

26. 26 Homologous Series Alkanes form a homologous series as seen in Table 19.2. Members of a homologous series have similar structures but different chemical formulas. The general formula for the homologous series of open-chain alkanes is shown here.

27. 27 C n H 2n + 2 general formula for open-chain alkanes

28. 28 19.7 Carbon Bonding in Alkanes

29. 29 Alkanes contain saturated carbon atoms that have four sigma bonds Sigma bonds are linear bonds formed by a pair of electrons in overlapping atomic orbitals. Carbon has the ability to form two, three, or four sigma bonds because of its ability to hybridize its valence shell electron orbitals ( i.e the s and the 2p orbitals) as shown in Figure 19.5..

30. 30 Figure 19.5 Rearrangement of carbon 2s and 2p orbitals to form four equivalent sp 3 hybrid orbitals.

31. 31 Figure 19.6 (a) A single sp 3 -hybridized orbital; (b) four sp 3 -hybridized orbitals in a tetrahedral arrangement; (c) sp 3 and s orbitals overlapping to form four C-H sigma bonds in methane.

32. 32 19.8 Isomerism

33. 33 Open-chain alkanes with four or more carbon atoms can form isomers. Isomers are molecules that have the same number and same type of atoms but the atoms are connected differently. For example, it is possible to write two structural formulas corresponding to the molecular formula C 4 H 10 i.e., butane and isobutane which are isomers of each other.

34. 34 Figure 19.7 Ball-and-stick models illustrating the structural formulas of several alkanes.

35. 35 Organic chemists often write structural formulas as condensed structural formulas to save time.

36. 36 Alkanes are nonpolar compounds with low boiling points. Many organic compounds are derivatives of alkanes. Many of these compounds have one or more of the following elements in them. The dashed lines indicate the number of bonds each element can form. For example nitrogen can form three bonds while oxygen can only form two bonds.

37. 37 19.9 Naming Organic Compounds

38. 38 How do chemist name organic compounds ? Organic compounds are named using the IUPAC System (International Union of Pure and Applied Chemistry). The IUPAC System was developed at an international science meeting in Geneva in 1892. The IUPAC System is essentially a set of rules that allow for a systematic and uniform method of naming compounds.

39. 39 Naming Alkyl Groups Alkyl groups have one hydrogen less than the respective alkane. The general formula for alkyl groups is shown here and a list of alkyl groups are shown in Table 19.3.

40. 40

41. 41 Carbon atoms are often classified in this way: Primary (1  ) Secondary (2  ) Tertiary (3  ) A carbon with one carbon attached A carbon with two carbons attached A carbon with three carbons attached Here is an example of a molecule with all three types of carbons.

42. IUPAC Rules for Naming Alkanes 1. Identify and then name the longest continuous chain.

43. IUPAC Rules for Naming Alkanes 2. Number the chain so any branch alkyl groups have the lowest possible number.

44. IUPAC Rules for Naming Alkenes 3. Use prefixes if two or more ( i.e. di, tri, tetra etc. ) of the same alkyl group branches appear on the chain.

45. Example I. Give the IUPAC name for the this alkane. 2-methylhexane Example II. Write the formula for 3-methyl-5- ethyloctane.

46. 46 19.10 Introduction to the Reactions of Carbon

47. 47 Reaction Types The importance of organic chemistry is illustrated by the variety of reactions that occur at the carbon atom which include five major types: oxidation, reduction, substitution, elimination, and addition. Oxidation involves a higher number of oxygen atoms bonded to carbon.

48. 48 Reaction Types (Reduction) Reduction is the reverse of oxidation and involves a higher number of hydrogen atoms bonded to carbon.

49. 49 Reaction Types ( Substitution ) Substitution is the exchange of one atom for another at the carbon atom. In the reaction here a bromine atom changes position with a hydrogen atom in methane.

50. 50 Reaction Types (Elimination and Addition) Elimination is the splitting of a single reactant into two products. Addition is the opposite of elimination where two reactants form a single product.

51. 51 19.11 Reactions of Alkanes

52. 52 Oxidation Substitution Elimination Decomposition Rearrangement Combustion Halogenation Dehydrogenation Cracking Isomerization Reaction Class Name of Reaction Reactions of alkanes can be classified as shown in the table below.

53. 53 Combustion Alkanes are a valuable energy source due to their ability to react with oxygen. Alkanes release a large amount of energy as shown here. Alkanes generally are considered to be unreactive and for reactions other than combustion a catalyst or other special reaction conditions are necessary.

54. 54 Halogenation

55. 55 Dehydrogenation Cracking

56. 56 Isomerization

57. 57 The chlorination of methane can produce each of the compounds shown in Table 19.4.

58. 58 Alkyl Halides Alkyl halides are compounds that have an X ( X = -F (fluoro), -Cl (chloro), -Br (bromo), or - I (iodo) attached to a carbon atom. RX is the general formula for alkyl halides. Examples of Alkyl Halides

59. 59 Mechanism of Halogenation The mechanism of halogenation consists of three steps: initiation, propagation, and termination as shown here. A mechanism is the individual steps that lead to the product from the reactant. Overall Reaction Step I Initiation

60. 60 Mechanism of Halogenation Step III Termination Step II Propagation

61. 61 Free Radical Stability and Reactivity Experimental results show tertiary free- radicals are the most stable and most easily formed radicals in chemical reactions and secondary free-radicals are next. Therefore the major product formed in the chlorination of propane corresponds to the secondary free-radical, i.e. 2-chloropropane.

62. 62 19.12 Sources of Alkanes 19.13 Gasoline: A Major Petroleum Product

63. 63 Sources of Alkanes Alkanes are produced from natural gas and petroleum. Natural gas and petroleum are formed by the decay of plants and animals Natural gas and petroleum are non-renewable sources of energy

64. 64 Gasoline is a major petroleum product The octane rating of gasoline is the knocking performance based on the percent isooctane in an isooctane/heptane mixture. (higher octane ratings = less knocking) Gasoline is mostly hydrocarbons and without additives it causes knocking which is a detonation of the air-fuel mixture in an engine. Figure 19.8 Uses of petroleum

65. 65 Reformulated Gasoline and the Environment Manufacturers of gasoline have switched from tetraethyl lead to more environmentally friendly additives like MTBE (methyl-tert-butyl ether) and ethanol. These additives boost octane ratings just like lead additives but were determined to be better for the environment. However studies have shown that MTBE can pollute ground water and it is also being replaced.

66. 66 19.14 Cycloalkanes

67. 67 Cycloalkanes Cycloalkanes are cyclic alkanes with the general formula C n H 2n.

68. 68 Properties of Cycloalkanes The physical properties and the chemical reactivity of cycloalkanes are similar to their open-chain counterparts with two exceptions: cyclopropane and cyclobutane.

69. 69 Reactivity of Cyclopropane Cyclopropane and cyclobutane have higher reactivity because their bonds are strained. The basis for the strain is the deviation of their bond angles from the normal tetrahedral carbon bond angle of 109.5 . These molecules cannot change shape to significantly change their bond angles like cycloalkanes that have larger ring sizes.

70. 70 Reactivity of Cyclopropane The result of the bond strain in cyclopropane is that it reacts readily with molecules like bromine as shown here.

71. 71 Figure 19.10 Ball-and-stick models of cyclopropane, hexane, and cyclohexane. In cyclopropane, all the carbon atoms are in one plane (bond angles = 60°). The cyclohexane molecule is puckered (as shown in the chair conformation) with bond angles about 109.5° as found in hexane.

72. 72 Conformations of Cyclohexane Cyclohexane can exist in either the chair or boat conformation. A conformation is the three-dimensional spatial arrangement of the atoms in a molecule. Figure 19.11 Conformations of cyclohexane: (a) chair conformation; (b) boat conformation. Axial hydrogens are shown in blue in the chair conformation.

73. 73 Naming Cycloalkanes When naming monosubstituted cycloalkanes: 1. Name the substituent 2. Name the parent cyclohexane Both of these structures are the same molecule and are named this way: methyl + cyclohexane = methylcyclohexane

74. 74 Naming Polysubstituted Cycloalkanes When naming polysubstituted cycloalkanes: 1. Number the ring in a way that gives the smallest total number. 2. Name the substituent and its location. 3. Name the parent cyclohexane.

75. 75 Chapter 19 Summary  Organic chemistry is the chemistry of carbon compounds.  Alkanes are hydrocarbons that are either saturated (no double or triple bonds) or unsaturated ( one or more multiple bonds ).  Organic compounds are classified by functional groups which is the smaller part of the molecule that determines the chemical profile of the molecule.  Organic molecules can be represented by ball-and stick models, space-filling models, Lewis structures, line structures, and condensed structural formulas.  Saturated carbon atoms are sp 3 hybridized which form can four sigma bonds.

76. 76 Chapter 19 Summary  Reactions of organic molecules are classified as substitution, elimination, and addition.  Alkanes undergo combustion but need special reaction conditions to undergo halogenation, dehydrogenation, cracking, and isomerization.  Cycloalkanes (C n H 2n ) exist in conformations with hydrogen on each carbon in axial or equatorial position  Cyclopropane is unusually reactive because of strained bonds.  The most stable form of cyclohexane is the chair conformation.