Unsaturated Hydrocarbons

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Your Turn! Name the following compounds. 29

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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 secondary carbocation (2o) is produced when the pi electrons of the C=C are attacked by HCl. 48

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

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

Your Turn! Predict the major products formed when 2-methyl-1-butene reacts with: H2, Pt/25°C Cl2 HCl H2O, H+ 51

Your Turn! 2-methyl-1-butene + H2, Pt/25 °C Hydrogen adds to the double bond to form an alkane (2-methylbutane). 52

Your Turn! 2-methyl-1-butene + Cl2 Chlorine adds to the double bond to form an alkyl halide (1,2-dichoro-2-methylbutane). 53

Your Turn! 2-methyl-1-butene + HCl The hydrogen atom adds to the carbon atom with the larger number of hydrogen atoms and the chlorine adds to the other carbon atom (Markovnikov’s rule) to form an alkyl halide (2-chloro-2-methylbutane). 54

Your Turn! 2-methyl-1-butene + H2O The hydrogen atom adds to the carbon atom with the larger number of hydrogen atoms and the –OH group adds to the other carbon atom (Markovnikov’s rule) to form an alcohol. 55

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). 56

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. 57

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. 58

59

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. 60

Your Turn! Name the following compounds. 3-methyl-1-butyne
1-chloro-2-butyne 62

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. 63

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. 64

Reaction of acetylene with bromine (bromination). HCCH + Br2  CHBr=CHBr HCCH + 2 Br2  CHBr2-CHBr2 65

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 66

Aromatic Hydrocarbons: Structure
Benzene is an aromatic compound. Aromatic compounds are those that resemble benzene in structure and reactivity. 67

Benzene is aromatic because it has an unusually stable electronic structure created by six delocalized pi electrons. 68

Figure 20.5 (a) sp2-sp2 orbital overlap to form the carbon ring structure. 69

Figure 20.5 (c) pi electron clouds above and below the plane of the carbon ring. 71

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. 72

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. 73

There are several monosubstituted benzenes that have special names. 74

The word phenyl represents the C6H5- group
The word phenyl represents the C6H5- group. It is used to name benzene derivatives that would otherwise be difficult to name. 75

Disubstituted benzenes The prefixes ortho-, meta-, and para- (abbreviated o-, m-, and p-) are used to name disubstituted benzenes where G is given here as a reference point on the ring. 76

For example, this would be ortho-dichlorobenzene because the chlorine atoms are on adjacent carbon atoms on the ring. 77

The three isomers of dichlorobenzene have different physical properties. Notice how their melting and boiling points are different. 78

When naming disubstituted benzenes the substituents are given in alphabetical order followed by the word benzene. 79

The dimethylbenzenes have the special name xylene. 80

A disubstituted benzene is named as a derivative if one of the substituents corresponds to a benzene derivative with a special name. 81

Naming Polysubstituted Benzenes If there are three groups on the ring, start with number one and number in the direction that gives the lowest number to each group as shown on the next slide . . . 82

Naming Polysubstituted Benzenes 83

Polycyclic Aromatic Compounds
There are many other aromatic ring systems besides benzene. Their structures consist of two or more rings in which two carbon atoms are common to two rings. These compounds are known as polycyclic or fused aromatic ring systems. 84

Polycyclic Aromatic Compounds
Three of the most common hydrocarbons in this category are naphthalene, anthracene, and phenanthrene. 85

Your Turn! Name the following compounds.
1-chloro-2-bromo-4-nitrobenzene 1-ethyl-2-fluorobenzene 3-nitrotoluene 87

Sources and Physical Properties of Aromatic Hydrocarbons
Aromatic hydrocarbons (to include benzene, toluene, napthalene etc.) are produced from petroleum. A limited amount can be prepared from coal tar. Coal tar is only a by-product of coke. Consequently this method is no longer the principal source of aromatic hydrocarbons. 88

Sources and Physical Properties of Aromatic Hydrocarbons
Aromatic hydrocarbons have properties common to all hydrocarbons. They are nonpolar substances, they are insoluble in water but soluble in most organic solvents, and have densities less than the density of water They also burn readily, usually with smoky yellow flames. 89

Aromatic hydrocarbons undergo the following substitution reactions.
Chemical Properties of Aromatic Hydrocarbons Aromatic hydrocarbons undergo the following substitution reactions. Halogenation net addition of –Br or –Cl Nitration net addition of –NO2 Alkylation net addition of –R (alkyl group) 90

Chemical Properties of Aromatic Hydrocarbons
Halogenation: Benzene is halogenated in the presence of a mixture of X2 and an iron(III) halide catalyst. 91

Nitration: Benzene is nitrated in the presence of a mixture of concentrated nitric acid/sulfuric acid at ~50C. 92

Alkylation: The reaction below is an example of the Friedel-Crafts reaction. An alkyl group from an alkyl halide (RX) is substituted on the ring for a hydrogen atom. The reaction is catalyzed by AlCl3. 93

Oxidation of alkyl groups: Alkyl groups on a benzene ring are easily oxidized to benzoic acid. 94

Chapter 20 Summary Unsaturated hydrocarbons consist of alkenes, alkynes, cycloalkenes, and aromatic compounds. Each molecule in this class contains one or more pi (π) bond. Pi (π) bonds result from sideways overlap of p-orbitals on two sp- or sp2-hybridized atoms. Alkenes undergo addition reactions. For unsymmetrical alkenes the product obtained is predicted by Markovnikov’s rule. 95

Chapter 20 Summary Markovnikov’s rule is explained by the variation in stability among carbocations. Aromatic compounds are relatives of benzene and undergo substitution reactions because of the special electronic stability these molecules derive from electron delocalization. Aromatic compounds undergo substitution reaction due the stability of the benzene ring. 96

Unsaturated Hydrocarbons

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