Chapter 4: Alkenes and Alkynes

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Chapter 4: Alkenes and Alkynes


Unsaturated Hydrocarbons• Unsaturated hydrocarbon: 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)

HC C

H

H HH-C C-H

Ethyne(an alkyne)


Unsaturated Hydrocarbons• Arenes: 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• but 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°

HC C

H

H H

121.7°

Ethylene

HC 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

• Remember same order of attachment, different arrangement in space

cis-2-Butenemp -139°C, bp 4°C

trans-2-Butenemp -106°C, bp 1°C

CH3C

CH

CH3

CHH

CCH3

HH3C


Cis-Trans Isomerism• trans alkenes are more stable than cis alkenes because

of nonbonded interaction strain between alkyl substituents of the same side of the double bond

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 -en- at the end 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 33

4 4 45 5

56 6


Numbering System (cont)e.g.


IUPAC nomenclature

1 2

34

5

3-Methylcyclo-pentene

CH3

1,6-Dimethylcyclo-hexene

CH3

CH31

654

32


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

ones, are known almost exclusively by their common names

• CH2= methylene• CH2=CH─ vinyl• CH2=CHCH2 ─ allyl

CH2=CH2 CH3CH=CH2 CH3C=CH2

CH3

IUPAC:IsobutylenePropyleneEthyleneCommon:

2-MethylpropenePropeneEthene


QuestionName this compound:


Answer2-methyl-2-butene


Nomenclature of Alkynes• IUPAC nomenclature

• use the -yn- at the end 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


IUPAC nomenclature

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

1 12 2

33 44 5

6 7


Numbering System (cont)e.g. 3-Methyl-1-butyne


QuestionName this compound:

http://www.chemsynthesis.com/molimg/1/big/16/16207.gif


Answer2,2-dimethyl-3-heptyne


Dienes, Trienes, and PolyenesAlkenes that contain more than one double bond are

alkadienes, alkatrienes, and so on.e.g. 2-methyl-1,4-pentadiene


Polyenes


Alkylidene GroupsDouble Bonds Fused to Rings

CH2

methylenecyclohexane

CHCH3

ethylidenecyclopentane


Name These


3,3-dimethyl-1,4-cyclohexadiene 2-butyl-1,5-hexadiene

(E) 1-ethylidene-2-methylcyclopentane 3-allylcyclohexene


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

by the orientation of atoms of the main chain

• Draw cis-2- butene and trans-2-butene

• Cis and trans stereoisomerism in alkenes is not possible when one of the doubly bonded carbons bears two identical substituents.

• Neither 1-butene and 2-methylpropene have no stereoisomers.


Stability of Alkenes

Stability of cis- and trans isomers • Interconversion does not occur spontaneously• Interconversion can be brought about by treating the

alkene with a strong acid• Cis isomers are less stable than trans isomers because

of the steric strain between the two larger substituents on the same side of the double-bond


Relative Stabilities of Alkenes


Sequence Rules: The E,Z Designation

• Z – for German zusammen “together”• High priority groups on each

carbon are on same side of the double-bond

• E – for German entgegen “opposite”• High priority groups on each

carbon are on opposite sides of the double-bond

E,Z system• Sequence rules used to assign priorities to the

substituent groups on the double-bond carbons (alkenes)


E/Z System


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

HC

CO

CC-CH-CH2

is treated as

is treated as


“E” is More Stable than “Z”


Physical Properties• Alkenes and alkynes are nonpolar compounds• Their physical properties 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/m L)• 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)

1 23

4head tail


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

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Chapter 4: Alkenes and Alkynes

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