Chapter 3Alkenes and Alkynes

Unsaturated Hydrocarbons• Contain carbon-carbon multiple bonds.

Alkenes C=C double bonds

Alkynes C≡C triple bonds

Aromatics benzene rings

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Alkenes• Structure:

• The VSEPR model predicts bond angles of 120° about each carbon of a double bond.

H

C C

H

H H

121.7°H3C

C C

H

H H

124.7°

Ethylene Propene

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Alkenes• Cis-trans isomerism

• because of restricted rotation about a carbon-carbon double bond, an alkene with two different groups on each carbon of the double bond shows cis-trans isomerism.

CIS TRANS

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Problem 43, p. 355

(c)

(d)

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Naming Alkenes

• Step 1: Name the longest chain that contains the C=C bond. Use the IUPAC root and the –ene ending.

• Step 2: Number the longest chain so the C=C bond gets the lowest number possible.

• Step 3: Designate the C=C bond in the name with the lowest-numbered carbon.

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Examples:1 2 3 4

CH3-CH=CH-CH3

2-butene

6 5 4 3 2 1

CH3-CH2-CH2-CH=CH-CH3

2-hexene

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Naming Alkenes, cont.• Step 4: Locate and name attached groups.• Step 5: Combine all the names as you did with

alkanes.

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Alkynes - IUPAC Names• follow the same rules as for alkenes, but use the

ending -yne to show the presence of the triple bond.

CH3CHC CHCH3

CH3CH2C CCH2CCH3

CH3

CH3

3-Methyl-1-butyne 6,6-Dimethyl-3-heptyne

1 1

2 23

3 44 5

6 7

Example 12-1, p. 333

(d)

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Cycloalkenes• To name a cycloalkene:

• number the carbon atoms of the ring double bond 1 and 2 in the direction that gives the lower number to the substituent encountered first.

• number and list substituents in alphabetical order.

1 2

3

4

5

1

2

3

4

5

6

3-Methylcyclopentene(not 5-methylcyclopentene)

4-Ethyl-1-methylcyclohexene(not 5-ethyl-2-methylcyclohexene)

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Example 12-3, p. 336

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Physical Properties• Alkenes and alkynes are nonpolar compounds.

• The only attractive forces between their molecules are London dispersion forces.

• Their physical properties are similar to those of alkanes with the same carbon skeletons.

• Alkenes and alkynes are insoluble in water but soluble in one another and in nonpolar organic liquids.

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Alkene Reactions

• Alkenes are quite chemically reactive• Many reactions are addition reactions:

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Reactions of Alkenes• The most common reaction is addition

CC

C C

C C Br2

HCl

H2O

CC H2

C CBr Br

C CH Cl

C CH OH

C CHH

Descriptive Name(s )Reaction

+

+

+

+

bromination

hydrochlorination

hydration

hydrogenation(reduction)

halogenation

hydrohalogenation

Alkene Reactions, cont.• Hydrogenation (addition) reactions can occur in the presence of

a catalyst (Pt, Pd, or Ni).

The hydrogenation of vegetable oils is an important commercial process.

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Addition of H2• Virtually all alkenes add H2 in the presence of a

transition metal catalyst, commonly Pd, Pt, or Ni.

HH3C

C C

H CH3

PdCH3CH2CH2CH3

trans-2-Butene

+ H225°C, 3 atm

Butane

Pd+ H2

Cyclohexene Cyclohexane

25°C, 3 atm

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Addition of a Halogen (Halogenation)• Addition takes place readily at room temp.

• reaction is generally carried out using pure reagents, or mixing them in a nonreactive organic solvent

• addition of Br2 is a useful qualitative test for the presence of a carbon-carbon double bond

Br2CH2Cl2

Br

Br+

1,2-DibromocyclohexaneCyclohexene

CH3CH=CHCH3 Br2 CH2Cl2CH3CH-CHCH3

Br Br

2,3-Dibromobutane2-Butene

+

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Addition of HX (Hydrohalogenation)• Addition of HX (HCl, HBr, or HI) to an alkene

• H adds to one carbon of the C=C and X to the other.

• Markovnikov’s rule: H adds to the less substituted carbon and X to the more substituted carbon.

CH2=CH2 HCl CH2-CH2

ClH

Chloroethane(Ethyl chloride)

Ethylene

+

CH3CH=CH2 HCl CH3CH-CH2

HClCH3CH-CH2

ClH

1-Chloropropane(not formed)

2-ChloropropanePropene

+

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•Hydration-addition of water

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Addition of H2O

• Hydration follows Markovnikov’s rule; H adds to the less substituted carbon and OH adds to the more substituted carbon.

CH3CH=CH2 H2OH2SO4

CH3CH-CH2

HOH

Propene 2-Propanol+

1.

2.

3.

4.

5.

Polymerization• Polymers – long chain products made up of repeating units.• Monomer – the starting material that becomes the repeating

units of a polymer.

Table 12-2, p. 349

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Polymerization• Show the structure of a polymer by placing

parentheses around the repeating monomer unit.• Place a subscript, n, outside the parentheses to

indicate that this unit repeats n times.• The structure of a polymer chain can be reproduced

by repeating the enclosed structure in both directions.• following a section of polypropene (polypropylene)