Unsaturated Hydrocarbons Chapter 20 Hein Best Pattison Arena

Unsaturated Hydrocarbons Chapter 20 Hein * Best * Pattison * Arena Version 1. 0 Colleen Kelley Chemistry Department Pima Community College 1 © John Wiley and Sons, Inc.

Chapter Outline 20. 1 Bonding in Unsaturated Hydrocarbons 20. 8 Physical and Chemical Properties of Alkynes 20. 9 Aromatic Hydrocarbons: Structure 20. 10 Naming Aromatic Hydrocarbon 20. 2 Nomenclature of Alkenes 20. 3 Geometric Isomerism in Alkenes 20. 11 Polycyclic Aromatic 20. 4 Cycloalkenes Hydrocarbons 20. 5 Preparation and Physical 20. 12 Sources and Physical Properties of Alkenes Properties of Aromatic 20. 6 Chemical Properties of Hydrocarbons Alkenes 20. 13 Chemical Properties of 20. 7 Alkynes: Nomenclature and Aromatic Hydrocarbons Preparation 2

• Unsaturated hydrocarbons enhance our lives in many ways: 1. 2. 3. 4. Polyethylene plastic bags and bottles Polystyrene Styrofoam cups Plastic wraps Essential oils in plants contain multiple bonds between carbon atoms. – Cosmetics, medicines, flavorings, perfumes 5. Hydrocarbons also form rings of carbon atoms (aromatics) – Detergents, insecticides, and dyes 3

Bonding in Unsaturated Hydrocarbons 4

• The unsaturated hydrocarbons consist of three families of homologous compounds that contain multiple bonds between carbon atoms. • Alkenes contain carbon-carbon double bonds. • Alkynes contain carbon-carbon triple bonds. • Aromatic compounds contain benzene rings. 5

The four orbitals available for bonding in alkenes are three sp 2 orbitals and one p orbital. Figure 20. 1 Schematic hybridization of 2 s 22 px 12 py 1 orbitals of carbon to form three sp 2 electron orbitals and one p electron orbital 6

Figure 20. 2 (a) A single sp 2 electron orbital and (b) a side view of three sp 2 orbitals all lying in the same plane with a p orbital perpendicular to the three sp 2 7 orbitals.

• The carbon-carbon pi ( ) bond is much weaker and, as a consequence, much more reactive than the carbon-carbon sigma ( ) bond. 8

• The formation of a triple bond between carbon atoms, as in acetylene, CH CH, may be visualized as shown below. • These pi bond electrons are not as tightly held by the carbon nuclei as the sigma bond electrons. Acetylene, consequently, is a very reactive substance. 9

Nomenclature of Alkenes The general formula for alkenes is: Cn. H 2 n 10

IUPAC Rules for Naming Alkenes 1. Select the longest continuous carbon-carbon chain that contains the double bond. 2. Name this parent compound as you would an alkane, but change the –ane ending to –ene. CH 3 CH 2 CH 3 propane CH 3 CH=CH 2 propene 11

IUPAC Rules for Naming Alkenes 3. Number the carbon chain of the parent compound starting with the end nearer to the double bond. Use the smaller of the two numbers on the double-bonded carbon atoms to indicate the position of the double bond. Place this number in front of the alkene name. CH 3 CH=CHCH 3 2 - butene CH 3 CH 2 CH=CH 2 1 -pentene 12

IUPAC Rules for Naming Alkenes 4. Branch chains and other groups are treated as in naming alkanes, by numbering and assigning them to the carbon atom to which they are bonded. 13

How would we write the structural formula for 4 -methyl-2 -pentene? • The name indicates: – Five carbons in the longest chain – A double bond between carbons 2 and 3 – A methyl group on carbon 4 14

Write a structural formula for: 7 -methyl-2 -octene • Octene indicates an 8 -carbon chain • The chain contains a C=C between carbons 2 and 3. • There is a –CH 3 group on carbon 7 15

Name this compound: • Longest chain containing C=C is 5 carbons • 2 -ethyl-1 -pentene 16

Geometric Isomerism in Alkenes 17

Geometric Isomerism in Alkenes • Compounds containing a carbon-carbon double bond (pi bond) have restricted rotation about that double bond. • This restricted rotation in a molecule gives rise to a type of isomerism known as geometric isomerism. • Isomers that differ from each other only in the geometry of their molecules and not in the order of their atoms are known as geometric isomers. – They are also called cis-trans isomers. 18

Geometric Isomers in Alkenes 19

An alkene shows cis-trans isomerism when each carbon atom of the double bond has two different kinds of groups attached to it. 20

An alkene does not show cis-trans isomerism if one carbon of the double bond has two identical groups attached to it. 21

Draw a structure for cis-5 -chloro-2 -hexene 22

Is the compound below the cis or trans isomer? trans-3 -methyl-2 -pentene 23

Dienes • Many compounds have more than one C=C. • Compounds with two C=C are called dienes. • Compounds with three C=C are called trienes. 24

Cycloalkenes 25

Cycloalkenes • As the name implies, cycloalkenes are cyclic compounds that contain a C=C in the ring. • The carbons of the double bond are assigned numbers 1 and 2. 26

Cycloalkenes 27

Preparation and Physical Properties of Alkenes 28

Preparation of Alkenes (ie, an alkene is a product of each) • Cracking • Dehydration of Alcohol • Dehydrohalogenation of Alkyl Halides • Hydrogenation of alkynes • Dehalogenation of vicinal dihalides 29

Cracking • Cracking, or pyrolysis, is the process in which saturated hydrocarbons are heated to very high temperatures in the presence of a catalyst (usually silica-alumina): Heat Alkane (Cn. H 2 n+2) Mixture of alkenes + Alkanes + H 2 (g) catalyst ~500°C 2 CH 3 CH 2 CH 3 CH 3 CH=CH 2 + CH 2=CH 2 + CH 4 + H 2 30

Dehydration of Alcohols Dehydration involves the elimination of a molecule of water from a reactant molecule. Ex: 31

Dehydrohalogenation of alkyl halides involves eliminating a hydrogen and a halogen Ex 32

Hydrogenation of alkynes involves adding hydrogen (H 2) to an alkyne Ex: 33

Dehalogenation of vicinal dihalide Ex: involves removing a halogen molecule (X 2) 34

Physical Properties of Alkenes • Alkenes have physical properties very similar to the corresponding alkanes. 35

Chemical Properties of Alkenes 36

Addition Reactions of Alkenes • Addition at the C=C bond is the most common reaction of alkenes. – H 2 – Br 2 and Cl 2 – HBr, HCl – H 2 SO 4 – H 2 0 37