Unsaturated Hydrocarbons

• Introduction to Unsaturated HC– One or more C-to-C multiple bond present• Double bond is rigid – doesn’t rotate

– Similar physical properties to saturated HC– Very different chemical properties than sat’d HC• Unsat’d HC are MUCH more reactive than saturated HC• Reactions take place at the multiple bond

• When a specific portion of the molecule determines its reactivity that part of the molecule is called a functional group– C-to-C multiple bonds are an example of a

functional group

Classes of Unsaturated HC

• Alkenes – C-to-C double bond(s) present • may be acyclic or cyclic

• Alkynes – C-to-C triple bond(s) present• Aromatic – unsaturated cyclic HC that does

not undergo addition reactions readily– Often a benzene ring present

Alkenes and Cycloalkenes• Functional group: C=C– ALKENES: acyclic alkene

• General formula if only 1 double bond is present: CnH2n

• ??same as…….

Example: • Cycloalkenes: cyclic alkene

• general formula if only one double bond is present: CnH2n - 2

Example:

Naming Alkenes• Name longest continuous chain that contains

the double bond, suffix = “ene”– If C=C is same distance from both ends, start

numbering from end closest to any substituent• Locate C=C by giving # of the lowered

numbered C in the double bond• Use suffixes of “diene”, “triene”, tetrene” if

more than one C=C– 1,3-hexadiene

Naming Cycloalkenes

• Name ring as you would a cycloalkane, but using suffix of “ene”– Cyclopentene….. no # needed to locate C=C

• If there are substituents on the ring– Cs of the double bond are C 1 and 2 – no #needed

to locate C=C in name– Number so substituent(s) have lowest possible #– List in alphabetical order if more than 1

substituent

More naming

• Practice: Page 73, 16 and 20• Common Names to Know– Ethylene

– Propylene

Line Angle Formulas

• Pretty straightforward• Try a few on page 73: 26 & 28

Isomerism in Alkenes

1. Constitutional– Positional isomers…location of double different – Skeletal isomers …longest chain differs

2. Stereoisomers– Cis-trans isomers possible when:• Each C in double bond has 2 different groups attached

Naturally Occurring Alkenes

• Pages 50-52• Good paper potential– Pheremones– Retenoids– Terpenes and isoprenes

• No coverage in lecture

Physical Properties

1. Nonpolar– Solubility

2. Density3. MP and BP - trends4. State at room temperature– For unbranched alkenes

Chemical Properties Alkenes

1. Combustion reaction2. Addition reactions– Symmetrical: H2 and X2

– Unsymmetrical: HX and H-OH• Markovnikov’s Rule applies

3. Polymerization reaction – Not covered, good paper topic

Alkynes

• Limited coverage in class• See pages 62-64

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Bond Angles

• Alkanes

• Alkenes

• Alkynes

Aromatic Hydrocarbons

• Cyclic unsaturated HC that does not undergo addition reactions– delocalized electrons electrons present

• Our focus will be on benzene based aromatic HC– Structure– Naming conventions– Properties: physical and chemical (selected)

Benzene and Benzene Derivatives

• Benzene• Toluene– NOT methylbenzene

• Single substituent – put name of substituent in front of benzene– Chlorobenzene– tertbutylbenzene

2 Substituents - 2 Naming Options

1. Number system– Locate substituents with #s– Goal is lowest possible set of #s– When substituents differ, that with alphabetical

priority is assigned #1…see top of page 68 – When a methyl group is one of the

substituents….

2. Ortho, meta, para system– 2 methyl groups a xylene

Benzene as a Substituent!

• Benzene “hanging off” a chain = a phenyl group– See example 2.6 c on page 68

Properties of Aromatic HC (benzene)

1. Physical properties– You know these….polarity, solubility, density,

state of matter of benzene

2. Chemical properties – many, text covers 2– Alkylation: + R-Cl • AlCl3 catalyst required

– Halogenation: + X2

• FeX3 catalyst required