Chapter 19 Organic Chemistry Saturated Hydrocarbons Organic chemistry
Chapter 19 Organic Chemistry: Saturated Hydrocarbons Organic chemistry is important in nanotechnology. A carbon nanotube forest is shown here. Introduction to General, Organic, and Biochemistry, 10 e John Wiley & Sons, Inc Morris Hein, Scott Pattison, and Susan Arena Chapter Outline
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 2
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 Chapter 19 Summary 3
Chapter Outline Organic Chemistry: History and Scope 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 laboratory synthesis of urea. 4
Chapter Outline What is organic chemistry? 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. Lipstick is made of organic molecules. Cosmetics and perfumes contain organic compounds. 5
Chapter Outline The Carbon Atom: Bonding and Shape Carbon can form saturated compounds (i. e. where 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)). 6
Chapter Outline The Carbon Atom: Bonding and Shape These are the molecular shapes of carbon compounds predicted by the VSEPR Bonding Theory. 7
Chapter Outline The Carbon Atom: Bonding and Shape This figure shows the bonding shape that results when carbon has four single bonds. 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 8
Chapter Outline Your Turn! Estimate the bond angles around 1 C and 2 C in the following structure. 9
Chapter Outline Your Turn! Estimate the bond angles around 1 C and 2 C in the following structure. 10
Chapter Outline Organic Formulas and Molecular Models Structural formulas or models are often used in organic chemistry to illustrate molecules. For example, or in the case of a model. 11
Chapter Outline Figure 19. 3 Types of formulas and models used to represent organic molecules. Each diagram is a representation of a propane molecule. 12
Chapter Outline Organic Formulas and Molecular Models This is an example of how to change a condensed structural formula into a line structure. The table on the next slide summarize formulas and models used in organic chemistry. 13
Formula or model Chapter Outline Definition Lewis structure Shows all bonds, all atoms, and all nonbonding electrons Structural formula Same as Lewis structure except nonbonding electrons are not shown Condensed structural formula Bonds are collapsed so relative positions of atoms are group together Line structure Only bonds and functional groups are shown and each endpoint and bend represents a carbon atom Ball-and-stick model Atoms are shown as balls and bonds as sticks and the shape is shown at the central atom. Space-filling model Bonds are omitted and the size of atoms are emphasized. 14
Chapter Outline Classifying Organic Compounds Functional groups are group of atoms (or one atom) that have specific behavioral characteristics in organic compounds. Organic compounds are classified based on the functional groups that they contain. 15
Chapter Outline Classifying Organic Compounds The list of common functional groups found in organic compounds are shown here on Table 19. 1. 16
Chapter Outline Classifying Organic Compounds The list of important functional groups found in biological compounds is shown here on Table 19. 2. 17
Classifying Organic Compounds Chapter Outline Ethanol contains the alcohol functional group and dimethyl ether contains the ether functional group. They 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 18 molecules.
Chapter Outline 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. 4 on the next slide. Hydrocarbons are also classified as saturated or unsaturated 19 compounds.
Chapter Outline Figure 19. 4 Classes of Hydrocarbons 20
Chapter Outline 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. 21
Chapter Outline Saturated Hydrocarbons: Alkanes are saturated hydrocarbons that are either straightchain or branched-chain hydrocarbons. 22
Chapter Outline Saturated Hydrocarbons: Alkanes form a homologous series of straight-chain molecules as seen in Table 19. 3 on the next slide. Members of a homologous series have similar structures but different chemical formulas. The general formula for all non-cyclic alkanes is shown here. 23
Chapter Outline Saturated Hydrocarbons: Alkanes 24
Chapter Outline Your Turn! Write the formula for an alkane that has nine carbon atoms. Write the condensed structural formula for the straight -chain alkane that has nine carbon atoms. 25
Chapter Outline Your Turn! Write the formula for an alkane that has nine carbon atoms. Cn. H 2 n+2 = C 9 H 20 Write the condensed structural formula for the straight -chain alkane that has nine carbon atoms. CH 3 CH 2 CH 2 CH 3 26
Chapter Outline Carbon Bonding in 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. 27
Chapter Outline Carbon Bonding in Alkanes Carbon has the ability to form four sigma bonds because of its ability to hybridize its valence shell electron orbitals (i. e the s and the 2 p orbitals) as shown in Figure 19. 5 on the next slide. 28
Chapter Outline Carbon Bonding in Alkanes Figure 19. 5 Rearrangement of carbon 2 s and 2 p orbitals to form four equivalent sp 3 hybrid orbitals. 29
Chapter Outline Carbon Bonding in Alkanes Figure 19. 6 shows the tetrahedral nature of the sp 3 hybridized orbitals. 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. 30
Chapter Outline Isomerism The properties of an organic substance are dependent on its molecular structure. The majority of organic compounds are made up of relatively few elements: carbon, hydrogen, oxygen, nitrogen, and the halogens. The valence bonds or points of attachment may be represented in structural formulas by a corresponding number of dashes attached to each atom. 31
Chapter Outline Isomerism Carbon can form four single bonds each, nitrogen can form three single bonds, oxygen can only form two single bonds, and hydrogen and the halogens can form one single bond each. 32
Chapter Outline Isomerism Because carbon can form four single bonds, alkanes with four or more carbon atoms can form isomers. Isomers are molecules that have the same molecular formula but different structural formula. 33
Chapter Outline Isomerism For example, it is possible to write two structural formulas corresponding to the molecular formula C 4 H 10 (i. e. , butane and isobutane are isomers of each other). Notice the balland-stick models of these molecules on the next slide. 34
Chapter Outline Figure 19. 7 Ball-and-stick models illustrating the structural formulas of several alkanes. 35
Chapter Outline Your Turn! There are several isomers of heptane, C 7 H 16. Draw structural formulas and condensed structural formulas for the two isomers that only have one branching methyl group. 36
Chapter Outline Your Turn! There are several isomers of heptane, C 7 H 16. Draw structural formulas and condensed structural formulas for the two isomers that only have one branching methyl group. 37
Chapter Outline Naming Organic Compounds How do chemists 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 38 compounds.
Chapter Outline 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. 4. 39
Chapter Outline Naming Alkyl Groups 40
Chapter Outline Naming Organic Compounds Carbon atoms are often classified based on the number of carbon atoms directly attached to them. Primary (1 ) A carbon with one carbon attached Secondary (2 ) A carbon with two carbons attached Tertiary (3 ) A carbon with three carbons attached 41
Chapter Outline Naming Organic Compounds Here is an example of a molecule with all three types of carbon atoms. 42
Chapter Outline Your Turn! Identify all the carbon atoms in the structure below as 1 o, 2 o or 3 o. 43
Chapter Outline Your Turn! Identify all the carbon atoms in the structure below as 1 o, 2 o or 3 o. 44
Chapter Outline IUPAC Rules for Naming Alkanes 1. Identify and then name the longest continuous chain of carbon atoms. The longest chain in this molecule contains six carbon atoms. 45
Chapter Outline IUPAC Rules for Naming Alkanes 2. Number the chain so any branch alkyl groups have the lowest possible number. There is only one correct way to number the carbon atoms in the longest chain. 46
Chapter Outline IUPAC Rules for Naming Alkanes 3. The methyl group is attached to the #3 carbon atom not the #4 carbon atom. The IUPAC name for the molecule is 3 -methylhexane. 47
Chapter Outline IUPAC Rules for Naming Alkanes 4. Use prefixes (i. e. di, tri, tetra etc. ) if two or more of the same alkyl group branches appear on the longest chain. One methyl group is on the #2 carbon atom and another methyl group is on the #3 carbon atom. 48
Chapter Outline IUPAC Rules for Naming Alkanes 5. When several different groups are attached to the same parent compound, list the groups in alphabetical order (e. g. chloro is listed before isopropyl and isopropyl is listed before methyl). 49
Chapter Outline Your Turn! Give the IUPAC name for the this alkane. 50
Chapter Outline Your Turn! Give the IUPAC name for the this alkane. 2 -methylhexane 51
Chapter Outline Your Turn! Write the structural formula for 5 -ethyl-3 methyloctane. 52
Chapter Outline Your Turn! Write the structural formula for 5 -ethyl-3 methyloctane. 53
Chapter Outline Introduction to the Reactions of Carbon The importance of organic chemistry is illustrated by the variety of reactions that occur at the carbon atom which include four major types. • • Oxidation-reduction Substitution Addition Elimination 54
Chapter Outline Introduction to the Reactions of Carbon Oxidation involves a higher number of oxygen atoms bonding to carbon. Carbon dioxide is more oxidized than methane so this is an oxidation reaction. 55
Chapter Outline Introduction to the Reactions of Carbon Reduction is the reverse of oxidation and involves a higher number of hydrogen atoms bonding to carbon. Methanol is more reduced than carbon dioxide so this is a reduction. 56
Chapter Outline Introduction to the Reactions of Carbon Substitution is the exchange of one atom for another at the carbon atom. In this reaction a bromine atom changes position with a hydrogen atom in methane. 57
Chapter Outline Introduction to the Reactions of Carbon Addition is a reaction where two reactants join to form a single product. 58
Chapter Outline Introduction to the Reactions of Carbon Elimination is the splitting of a single reactant into two products. 59
Chapter Outline Your Turn! Classify the following reaction as substitution, elimination or addition. 60
Chapter Outline Your Turn! Classify the following reaction as substitution, elimination or addition. Elimination 61
Chapter Outline Reactions of Alkanes Reaction Class Name of Reaction Oxidation Combustion Reactions of alkanes can be classified as shown in this table. Substitution Halogenation Elimination Dehydrogenation Decomposition Cracking Rearrangement Isomerization 62
Chapter Outline Reactions of Alkanes Combustion (oxidation) 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. 63
Chapter Outline Reactions of Alkanes Halogenation (substitution): Here a chlorine atom is replacing a hydrogen atom on an ethane molecule. 64
Chapter Outline Reactions of Alkanes Dehydrogenation (elimination): Here two hydrogen atoms are removed from a propane molecule to form propene. 65
Chapter Outline Reactions of Alkanes Cracking (decomposition): Here an alkane undergoes decomposition to form a smaller alkane and an alkene. 66
Chapter Outline Reactions of Alkanes Isomerization (rearrangement): Here an unbranched alkane (pentane) undergoes isomerization to form a branched compound that is an isomer of pentane (2 methylbutane). 67
The chlorination of methane with excess chlorine can produce each of the compounds shown in Table 19. 5. 68 Chapter Outline
Chapter Outline Reactions of Alkanes 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. 69
Chapter Outline Reactions of Alkanes Examples of Alkyl Halides 70
Chapter Outline Reactions of Alkanes The mechanism of halogenation consists of three steps: initiation, propagation, and termination as shown on the next few slides. A mechanism is the individual steps that lead to the product from the reactant. Overall Reaction 71
Chapter Outline Reactions of Alkanes Step I Initiation: Formation of chlorine free radicals. 72
Chapter Outline Reactions of Alkanes Step II Propagation: A chlorine free radical reacts with the alkane to form hydrogen chloride and an alkyl free radical. The alkyl free radical reacts with a molecule of chlorine to form another chlorine free radical and an alkyl halide. 73
Chapter Outline Reactions of Alkanes Step III Termination: 1) The reaction of two chlorine free radicals; 2) the reaction of a chlorine free radical and an alkyl free radical to form an alky halide; 3) the reaction of two alkyl free radicals to form a larger alkane. 74
Chapter Outline Free Radical Stability and Reactivity Experimental results show that tertiary (3 o) free radicals are the most stable and the most easily formed radicals in chemical reactions. 75
Chapter Outline Free Radical Stability and Reactivity Secondary (2 o) free-radicals are next in stability and primary (1 o) free radicals are the least stable. 76
Chapter Outline Free Radical Stability and Reactivity Therefore the major product formed in the chlorination of propane corresponds to the secondary free radical, i. e. 2 -chloropropane. The minor product corresponds to the primary free radical. 77
Chapter Outline Your Turn! Write an equation for each step in the free-radical mechanism forming 2 -chlorobutane. 78
Chapter Outline Your Turn! Write an equation for each step in the free-radical mechanism forming 2 -chlorobutane. Initiation 79
Chapter Outline Your Turn! Write an equation for each step in the free-radical mechanism forming 2 -chlorobutane. Propagation 80
Chapter Outline Your Turn! Write an equation for each step in the free-radical mechanism forming 2 -chlorobutane. Termination 81
Chapter Outline 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. 82
Chapter Outline Gasoline: A Major Petroleum Product Gasoline is a mixture of hydrocarbons. Without additives it causes knocking which is a detonation of the air-fuel mixture in an engine. Figure 19. 8 Uses of petroleum The octane rating of gasoline is the knocking performance based on the percent isooctane in an isooctane/heptane mixture. (higher octane ratings = less knocking) 83
Chapter Outline Gasoline: A Major Petroleum Product Manufacturers of gasoline have switched from tetraethyl lead to more environmentally friendly additives like MTBE (methyl-tert-butyl ether) and ethanol. 84
Chapter Outline Gasoline: A Major Petroleum Product These additives boost octane ratings like lead additives but are better for the environment. However studies have shown that MTBE can pollute ground water and is also being replaced. 85
Chapter Outline Cycloalkanes are cyclic alkanes with the general formula Cn. H 2 n. 86
Chapter Outline Cycloalkanes The physical properties and the chemical reactivity of cycloalkanes are similar to their open-chain counterparts (the alkanes) with two exceptions. Cyclopropane and cyclobutane are more reactive. 87
Chapter Outline Cycloalkanes 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. 88
Chapter Outline Cycloalkanes These molecules cannot change shape to significantly change their bond angles like cycloalkanes that have larger ring sizes. 89
Chapter Outline Cycloalkanes The result of the bond strain in cyclopropane is that it reacts readily with molecules like bromine as shown here. 90
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° instead of 109. 5° ). 91 Chapter Outline
The cyclohexane molecule is puckered (as shown in the chair conformation) with bond angles about 109. 5° as found in hexane. 92 Chapter Outline
Chapter Outline Cycloalkanes Cyclohexane can exist in either the chair or boat conformation. A conformation is the three-dimensional spatial arrangement of the atoms in a molecule. 93
Chapter Outline Cycloalkanes Figure 19. 11 Conformations of cyclohexane: (a) chair conformation; (b) boat conformation. Axial hydrogens are shown in blue in the chair conformation. 94
Chapter Outline Cycloalkanes When naming monosubstituted cycloalkanes: 1. Name the substituent 2. Name the parent cyclohexane Both of these structures are the same molecule and have the same name (methylcyclohexane). methyl + cyclohexane = methylcyclohexane 95
Chapter Outline Cycloalkanes When naming polysubstituted cycloalkanes. 1. 2. 3. Number the ring (clockwise or counterclockwise) to give the substituted groups the lowest possible numbers. Name the groups and their locations. Name the parent cyclohexane. 96
Chapter Outline Cycloalkanes 97
Chapter Outline Your Turn! Write structural formulas for 1 -bromo-2 methylcyclopentane and 1, 1 -dichloro-3 ethylcyclohexane. 98
Chapter Outline Your Turn! Write structural formulas for 1 -bromo-2 methylcyclopentane and 1, 1 -dichloro-3 ethylcyclohexane. 1 -bromo-2 -methylcyclopentane 99
Chapter Outline Your Turn! Write structural formulas for 1 -bromo-2 methylcyclopentane and 1, 1 -dichloro-3 ethylcyclohexane. 1, 1 -dichloro-3 -ethylcyclohexane 100
Chapter Outline 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. 101
Chapter Outline Chapter 19 Summary § 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. 102
Chapter Outline Chapter 19 Summary § Reactions of organic molecules are classified as oxidation-reduction, substitution, elimination, and addition. § Alkanes undergo combustion but need special reaction conditions to undergo halogenation, dehydrogenation, cracking, and isomerization. 103
Chapter Outline Chapter 19 Summary § Cycloalkanes (Cn. H 2 n ) exist in conformations with hydrogen on each carbon in axial or equatorial position § Cyclopropane and cyclobutane are unusually reactive because of strained bonds. § The most stable form of cyclohexane is the chair conformation. 104
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