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4-2© 2005 John Wiley & Sons, IncAll rights reserved
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)
H
C C
H
H H
H-C C-HEthyne
(an alkyne)
4-3© 2005 John Wiley & Sons, IncAll rights reserved
Unsaturated Hydrocarbons• Arenes: benzene and its derivatives (Ch 9)
CC
CC
C
C
H
H
H
HH
H
Benzene(an arene)
4-4© 2005 John Wiley & Sons, IncAll rights reserved
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-
4-5© 2005 John Wiley & Sons, IncAll rights reserved
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
4-6© 2005 John Wiley & Sons, IncAll rights reserved
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
4-7© 2005 John Wiley & Sons, IncAll rights reserved
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
C
H3C
C
H
CH3
C
HH
C
CH3
HH3C
4-8© 2005 John Wiley & Sons, IncAll rights reserved
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
4-9© 2005 John Wiley & Sons, IncAll rights reserved
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
4-10© 2005 John Wiley & Sons, IncAll rights reserved
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
4-11© 2005 John Wiley & Sons, IncAll rights reserved
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
4-13© 2005 John Wiley & Sons, IncAll rights reserved
IUPAC nomenclature
1 2
34
5
3-Methylcyclo-pentene
CH3
1,6-Dimethylcyclo-hexene
CH3
CH31
65
4
3
2
4-14© 2005 John Wiley & Sons, IncAll rights reserved
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
4-17© 2005 John Wiley & Sons, IncAll rights reserved
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
4-18© 2005 John Wiley & Sons, IncAll rights reserved
IUPAC nomenclature
3-Methyl-1-butyne 6,6-Dimethyl-3-heptyne
1 12 2
3
3 44 5
6 7
4-20© 2005 John Wiley & Sons, IncAll rights reserved
QuestionName this compound:
http://www.chemsynthesis.com/molimg/1/big/16/16207.gif
4-22© 2005 John Wiley & Sons, IncAll rights reserved
Dienes, Trienes, and PolyenesAlkenes that contain more than one double bond are
alkadienes, alkatrienes, and so on.
e.g. 2-methyl-1,4-pentadiene
4-24© 2005 John Wiley & Sons, IncAll rights reserved
Alkylidene GroupsDouble Bonds Fused to Rings
CH2
methylenecyclohexane
CHCH3
ethylidenecyclopentane
4-26© 2005 John Wiley & Sons, IncAll rights reserved
3,3-dimethyl-1,4-cyclohexadiene 2-butyl-1,5-hexadiene
(E) 1-ethylidene-2-methylcyclopentane 3-allylcyclohexene
4-27© 2005 John Wiley & Sons, IncAll rights reserved
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.
4-28© 2005 John Wiley & Sons, IncAll rights reserved
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
4-30© 2005 John Wiley & Sons, IncAll rights reserved
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)
4-32© 2005 John Wiley & Sons, IncAll rights reserved
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
4-33© 2005 John Wiley & Sons, IncAll rights reserved
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
4-35© 2005 John Wiley & Sons, IncAll rights reserved
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
4-36© 2005 John Wiley & Sons, IncAll rights reserved
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