William Brown

Thomas Poon

www.wiley.com/college/brown

Chapter Four

Alkenes and Alkynes

Unsaturated Hydrocarbons• Contains one or more carbon-carbon double or

triple bonds.– Alkene: contains a carbon-carbon double bond and

has the general formula CnH2n

– Alkyne: contains a carbon-carbon triple bond and has the general formula CnH2n-2

Ethene(an alkene)

H

C C

H

H H

H-C C-HEthyne

(an alkyne)

Unsaturated Hydrocarbons• Arene: benzene and its derivatives (Ch 9)

CC

CC

C

C

H

H

H

HH

H

Benzene(an arene)

Benzene and Phenyl Groups• We do not study benzene and its derivatives until

Chapter 9.– However, we show structural formulas of compounds

containing the phenyl group before that time.– The phenyl group is not reactive under any of the

conditions we describe in Ch 3-8.

Benzene Alternative representations for the phenyl group

C6H5- Ph-

Structure of Alkenes• The two carbon atoms of a double bond and the

four atoms bonded to them lie in a plane, with bond angles of approximately 120°.

H

C C

H

H H

121.7°

Ethylene

H

C C

CH3

H H

124.7°

Propene

Structure of Alkenes• According to the orbital overlap model, a double

bond consists of • a s bond formed by overlap of sp2 hybrid orbitals.• a p bond formed by overlap of parallel 2p orbital.

• Rotating by 90°breaks the pi bond.

Cis-Trans Isomerism• Because of restricted rotation about a C-C double

bond, groups on adjacent carbons are either cis or trans to each other.

cis-2-Butenemp -139°C, bp 4°C

trans-2-Butenemp -106°C, bp 1°C

C

H3C

C

H

CH3

C

HH

C

CH3

HH3C

Cis-Trans Isomerism– Because of nonbonded interaction strain between

alkyl substituents of the same side of the double bond, trans alkenes are more stable than cis alkenes.

trans-2-Butene cis-2-Butene

Structure of Alkynes• The functional group of an alkyne is a carbon-

carbon triple bond.• A triple bond consists of

• one s bond formed by the overlap of sp hybrid orbitals.

• two p bonds formed by the overlap of sets of parallel 2p orbitals.

Nomenclature of Alkenes• IUPAC Nomenclature

• Use the infix -en- to show the presence of a carbon-carbon double bond.

• Number the parent chain to give the 1st carbon of the double bond the lower number.

• Follow IUPAC rules for numbering and naming substituents.

• For a cycloalkene, the double bond must be numbered 1,2.

Nomenclature of Alkenes• IUPAC nomenclature

1-Hexene 4-Methyl-1-hexene2-Ethyl-3-methyl-

1-pentene

1 1

12 2 2

3 3

34 4 4

5 5

56 6

1 2

34

5

3-Methylcyclo-pentene

CH3

1,6-Dimethylcyclo-hexene

CH3

CH31

65

4

3

2

Nomenclature of Alkenes• Some alkenes, particularly low-molecular-weight

ones, are known almost exclusively by their common names.

CH2=CH2 CH3CH=CH2 CH3C=CH2

CH3

IUPAC:IsobutylenePropyleneEthyleneCommon:

2-MethylpropenePropeneEthene

Nomenclature of Alkynes• IUPAC nomenclature

• Use the infix -yne to show the presence of a carbon-carbon triple bond.

• Number the parent chain to give the 1st carbon of the triple bond the lower number.

• Follow IUPAC rules for numbering and naming substituents.

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

1 12 2

3

3 44 5

6 7

Configuration: Cis-Trans• The cis-trans system: configuration is determined

by the orientation of atoms of the main chain

cis-3,4-Dimethyl-2-pentene

1

2 3

4C

H

C

CH3

CH(CH3)2H3C

trans-3-Hexene

C

H

C

CH2CH3

HCH3 CH2

Configuration: E,Z• Assign a priority to the substituents on each

carbon of the double bond.– If the groups of higher priority are on the same side of

the double bond, the configuration is Z (German: zusammen, together).

– If the groups of higher priority are on opposite sides of the double bond, the configuration is E (German: entgegen, opposite).

Z (zusammen) E (entgegen)

C

higher

C

higher

lowerlower

C

lower higher

C

lowerhigher

Configuration: E,Z• Priority rules

1. Priority is based on atomic number; the higher the atomic number, the higher the priority.

2. If priority cannot be assigned on the basis of the atoms bonded directly to the double bond, look to the next set of atoms; priority is assigned at the first point of difference.

(53)(35)(17)(16)(8)(7)(6)(1)

Increasing priority

-H -CH3 -NH2 -OH -SH -Cl -Br -I

Increasing priority

(8)(7)(6)(1)-CH2-OH-CH2-NH2-CH2-CH3-CH2-H

Configuration - E,Z3. Atoms participating in a double or triple bond are

considered to be bonded to an equivalent number of similar atoms by single bonds.

-CH=CH2

O

-CH

O

H

C

C

O

CC

-CH-CH2is treated as

is treated as

Configuration - E,Z• Example: name each alkene and specify its configuration by the E,Z

system.

(a) (b)

(d)

(c)Cl

Cl

Br

Cl(e)

Cis-Trans Isomerism– The configuration of the double bond in cyclopropene

through cycloheptene must be cis; these rings are not large enough to accommodate a trans double bond.

– Cyclooctene is the smallest cycloalkene that can accommodate a trans double bond.

H

H

CH3

C H3

trans-Cyclooctene cis-Cyclooctene

Cis-Trans Isomerism• Dienes, trienes, and polyenes– For an alkene with n carbon-carbon double bonds,

each of which can show cis-trans isomerism, 2n cis-trans isomers are possible.

– Consider 2,4-heptadiene; it has four cis-trans isomers, two of which are drawn here.

Cis-Trans Isomerism– Vitamin A has five double bonds. – Four of the five can show cis-trans isomerism.– Vitamin A is the all-trans isomer.

Vitamin A aldehyde (retinal)

enzyme-catalyzedoxidation

H

OVitamin A (retinol)

OH

Physical Properties• Alkenes and alkynes are nonpolar compounds.– The only attractive forces between their molecules are

dispersion forces.• The physical properties of alkenes and alkynes are

similar to those of alkanes of similar carbon skeletons.– Those that are liquid at room temperature are less

dense than water (1.0 g/mL).– They dissolve in each other and in nonpolar organic

solvents.– They are insoluble in water.

Terpenes• Terpene: a compound whose carbon skeleton can

be divided into two or more units identical with the carbon skeleton of isoprene.

2-Methyl-1,3-butadiene (Isoprene)

12

34head tail

Terpenes– Myrcene, C10H16, a

component of bayberry wax and oils of bay and verbena.

– Menthol, from peppermint.

OH

Terpenes• Vitamin A (retinol)– The four isoprene units in vitamin A are shown in red.– They are linked head to tail, and cross linked at one

point (the blue bond) to give the six-membered ring.

OH

Alkenes and Alkynes

End Chapter 4