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General
Organic ChemistryTwo credits
Second Semester 2009
King Saud bin Abdulaziz University for Health Science
Reference Book: Organic Chemistry: A Brief Course, by Robert C. Atkins and Francis A. CareyThird Edition
Instructor: Rabih O. Al-Kaysi, PhD.
Chapter 4Chapter 4
Alkenes & Alkynes
Lecture 5
Alkene NomenclatureAlkene Nomenclature
AlkenesAlkenesAlkenesAlkenes
Alkenes are hydrocarbons that contain a Alkenes are hydrocarbons that contain a carbon-carbon double bondcarbon-carbon double bond
also called "olefins"also called "olefins"
characterized by molecular formula characterized by molecular formula CCnnHH2n2n
said to be "unsaturated"said to be "unsaturated"
Alkene NomenclatureAlkene NomenclatureAlkene NomenclatureAlkene Nomenclature
HH22CC CHCH22 HH22CC CHCHCHCH33
EtheneEtheneoror
EthyleneEthylene(both are acceptable(both are acceptable
IUPAC names)IUPAC names)
PropenePropene
(Propylene is(Propylene issometimes used sometimes used
but is not an acceptablebut is not an acceptableIUPAC name)IUPAC name)
Alkene NomenclatureAlkene NomenclatureAlkene NomenclatureAlkene Nomenclature
1) Find the longest continuous chain that 1) Find the longest continuous chain that includes the double bond.includes the double bond.
2) Replace the -2) Replace the -aneane ending of the unbranched ending of the unbranched alkane having the same number of carbons alkane having the same number of carbons by -by -eneene..
3) Number the chain in the direction that gives 3) Number the chain in the direction that gives the lowest number to the doubly bonded the lowest number to the doubly bonded carbon.carbon.
HH22CC CHCHCHCH22CHCH33 1-Butene1-Butene
Alkene NomenclatureAlkene NomenclatureAlkene NomenclatureAlkene Nomenclature
4) If a substituent is present, identify its position 4) If a substituent is present, identify its position by number. The by number. The double bond is firstdouble bond is first then then alkyl groups and halogens when the chain is alkyl groups and halogens when the chain is numbered.numbered.
The compound shown above isThe compound shown above is4-bromo-3-methyl-1-butene.4-bromo-3-methyl-1-butene.
HH22CC CHCHCHCHCHCH22BrBr
CHCH33
Alkene NomenclatureAlkene NomenclatureAlkene NomenclatureAlkene Nomenclature
4) If a substituent is present, identify its position 4) If a substituent is present, identify its position by number. by number. HydroxylHydroxyl groups take groups take precedence over the double bond when the precedence over the double bond when the chain is numbered.chain is numbered.
The compound shown above isThe compound shown above is2-methyl-3-buten-1-ol.2-methyl-3-buten-1-ol.
HH22CC CHCHCHCHCHCH22OHOH
CHCH33
Alkenyl groupsAlkenyl groupsAlkenyl groupsAlkenyl groups
methylenemethylene
vinylvinyl
allylallyl
isopropenylisopropenyl
CHCHHH22CC
HH22CC CHCHCHCH22
HH22CC CCHCCH33
HH22CC
RememberRemember
Cycloalkene NomenclatureCycloalkene NomenclatureCycloalkene NomenclatureCycloalkene Nomenclature
1) Replace the -1) Replace the -aneane ending of the cycloalkane ending of the cycloalkane having the same number of carbons by -having the same number of carbons by -eneene..
CyclohexeneCyclohexene
Structure and Bonding in Structure and Bonding in AlkenesAlkenes
Structure of EthyleneStructure of EthyleneStructure of EthyleneStructure of Ethylene
bond angles: bond angles: H-C-H = 117°H-C-H = 117°
H-C-C = 121°H-C-C = 121°
bond distances: bond distances: C—H = 110 pmC—H = 110 pm
C=C = 134 pmC=C = 134 pm
planarplanar
• Framework of bonds• Each carbon is sp2 hybridized
Bonding in Ethylene
• Each carbon has a half-filled p orbital
Bonding in Ethylene
• Side-by-side overlap of half-filled p orbitals gives a bond
Bonding in Ethylene
Isomerism in AlkenesIsomerism in Alkenes
IsomersIsomersIsomersIsomers
Isomers are different compounds thatIsomers are different compounds thathave the same molecular formula.have the same molecular formula.
IsomersIsomersIsomersIsomers
StereoisomersStereoisomersStereoisomersStereoisomersConstitutional isomersConstitutional isomersConstitutional isomersConstitutional isomers
IsomersIsomersIsomersIsomers
StereoisomersStereoisomersStereoisomersStereoisomersConstitutional isomersConstitutional isomersConstitutional isomersConstitutional isomers
different connectivitydifferent connectivitysame connectivity;same connectivity;
different arrangementdifferent arrangementof atoms in spaceof atoms in space
IsomersIsomersIsomersIsomers
StereoisomersStereoisomersStereoisomersStereoisomersConstitutional isomersConstitutional isomersConstitutional isomersConstitutional isomers
consider the isomeric alkenes of consider the isomeric alkenes of
molecular formula Cmolecular formula C44HH88
2-Methylpropene2-Methylpropene1-Butene1-Butene
cis-cis-2-Butene2-Butene trans-trans-2-Butene2-Butene
CC CC
HH
HH HH
CHCH22CHCH33
HH33CC
CC CC
CHCH33
HH
HHHH
CHCH33
CC CC
HH33CC
HH
CC CC
HH
HHHH33CC
HH33CC
2-Methylpropene2-Methylpropene1-Butene1-Butene
cis-cis-2-Butene2-Butene
CC CC
HH
HH HH
CHCH22CHCH33
HH
CHCH33
CC CC
HH33CC
HH
CC CC
HH
HHHH33CC
HH33CC
Constitutional isomersConstitutional isomersConstitutional isomersConstitutional isomers
2-Methylpropene2-Methylpropene1-Butene1-Butene
trans-trans-2-Butene2-Butene
CC CC
HH
HH HH
CHCH22CHCH33
HH33CC
CC CC
CHCH33
HH
HH
CC CC
HH
HHHH33CC
HH33CC
Constitutional isomersConstitutional isomersConstitutional isomersConstitutional isomers
cis-cis-2-Butene2-Butene trans-trans-2-Butene2-Butene
HH33CC
CC CC
CHCH33
HH
HHHH
CHCH33
CC CC
HH33CC
HH
StereoisomersStereoisomersStereoisomersStereoisomers
Stereochemical NotationStereochemical NotationStereochemical NotationStereochemical Notation
cis (identical or cis (identical or analogous substituents analogous substituents on same side)on same side)
trans (identical or trans (identical or analogous substitutents analogous substitutents on opposite sides)on opposite sides)
Naming Stereoisomeric Naming Stereoisomeric Alkenes by the Alkenes by the E-ZE-Z Notational Notational
SystemSystem
Stereochemical NotationStereochemical NotationStereochemical NotationStereochemical Notation
cis and trans are useful when substituents are cis and trans are useful when substituents are identical or analogous (oleic acid has a cis identical or analogous (oleic acid has a cis double bond)double bond)
cis and trans are ambiguous when analogies cis and trans are ambiguous when analogies are not obviousare not obvious
CC CC
CHCH33(CH(CH22))66CHCH22 CHCH22(CH(CH22))66COCO22HH
HH HH
Oleic acidOleic acid
ExampleExampleExampleExample
What is needed:What is needed:
1) 1) systematic body of rules for ranking systematic body of rules for ranking substituentssubstituents
2)2) new set of stereochemical symbols othernew set of stereochemical symbols otherthan cis and transthan cis and trans
CC CC
HH FF
ClCl BrBr
CC CC
The E-Z Notational SystemThe E-Z Notational SystemThe E-Z Notational SystemThe E-Z Notational System
EE : : higher ranked substituents on higher ranked substituents on oppositeopposite sides sides
ZZ : : higher ranked substituents on higher ranked substituents on samesame side side
higherhigher
lowerlower
CC CC
The E-Z Notational SystemThe E-Z Notational SystemThe E-Z Notational SystemThe E-Z Notational System
EE : : higher ranked substituents on higher ranked substituents on oppositeopposite sides sides
ZZ : : higher ranked substituents on higher ranked substituents on samesame side side
higherhigher
lowerlower
CC CC
The E-Z Notational SystemThe E-Z Notational SystemThe E-Z Notational SystemThe E-Z Notational System
EE : : higher ranked substituents on higher ranked substituents on oppositeopposite sides sides
ZZ : : higher ranked substituents on higher ranked substituents on samesame side side
EntgegenEntgegen
higherhigher
higherhigherlowerlower
lowerlower
CC CCCC CC
The E-Z Notational SystemThe E-Z Notational SystemThe E-Z Notational SystemThe E-Z Notational System
EE : : higher ranked substituents on higher ranked substituents on oppositeopposite sides sides
ZZ : : higher ranked substituents on higher ranked substituents on samesame side side
EntgegenEntgegen ZusammenZusammen
higherhigher
higherhigherlowerlower
lowerlower
lowerlower
higherhigher
lowerlower
higherhigher
CC CCCC CC
The E-Z Notational SystemThe E-Z Notational SystemThe E-Z Notational SystemThe E-Z Notational System
Answer: Answer: They are ranked in order of They are ranked in order of decreasing atomic number. decreasing atomic number.
EntgegenEntgegen ZusammenZusammen
higherhigher
higherhigherlowerlower
lowerlower
lowerlower
higherhigher
lowerlower
higherhigher
Question: How are substituents ranked?Question: How are substituents ranked?
The Cahn-Ingold-Prelog (CIP) SystemThe Cahn-Ingold-Prelog (CIP) SystemThe Cahn-Ingold-Prelog (CIP) SystemThe Cahn-Ingold-Prelog (CIP) System
The system that we use was devised byThe system that we use was devised byR. S. CahnR. S. CahnSir Christopher IngoldSir Christopher IngoldVladimir PrelogVladimir Prelog
(1)(1) Higher atomic number outranks lower Higher atomic number outranks lower atomic numberatomic number
Br > FBr > F Cl > HCl > H
higherhigher
lowerlower
BrBr
FF
ClCl
HH
higherhigher
lowerlower
CC CC
CIP RulesCIP RulesCIP RulesCIP Rules
(1)(1) Higher atomic number outranks lower Higher atomic number outranks lower atomic numberatomic number
Br > FBr > F Cl > HCl > H
((Z Z )-1-Bromo-2-chloro-1-fluoroethene)-1-Bromo-2-chloro-1-fluoroethene
higherhigher
lowerlower
BrBr
FF
ClCl
HH
higherhigher
lowerlower
CC CC
CIP RulesCIP RulesCIP RulesCIP Rules
* Do not remember the name just if it is E or Z* Do not remember the name just if it is E or Z
Physical Properties of AlkenesPhysical Properties of Alkenes
= 0 D= 0 D
CC CC
HH HH
HHHH
= 0.3 D= 0.3 D
HH
HH HH
CC CC
HH33CC
Dipole momentsDipole momentsDipole momentsDipole moments
What is direction of What is direction of dipole moment?dipole moment?
Does a methyl group Does a methyl group donate electrons to the donate electrons to the double bond, or does it double bond, or does it withdraw them?withdraw them?
= 0 D= 0 D
CC CC
HH HH
HHHH
= 1.4 D= 1.4 D
CC CC
HH HH
ClClHH
= 0.3 D= 0.3 D
HH
HH HH
CC CC
HH33CC
Dipole momentsDipole momentsDipole momentsDipole moments
Chlorine is Chlorine is electronegative and electronegative and attracts electrons.attracts electrons.
= 1.4 D= 1.4 D
CC CC
HH HH
ClClHH
= 0.3 D= 0.3 D
HH
HH HH
CC CC
HH33CC = 1.7 D= 1.7 D
HH
HH ClCl
CC CC
HH33CC
Dipole momentsDipole momentsDipole momentsDipole moments
Dipole moment Dipole moment of 1-of 1-chloropropene chloropropene is equal to the is equal to the sum of the sum of the dipole dipole moments of moments of vinyl chloride vinyl chloride and propene.and propene.
= 1.7 D= 1.7 D
= 1.4 D= 1.4 D
CC CC
HH HH
ClClHH
= 0.3 D= 0.3 D
HH
HH HH
CC CC
HH33CC
HH
HH ClCl
CC CC
HH33CC
Dipole momentsDipole momentsDipole momentsDipole moments
Therefore, a Therefore, a methyl group methyl group donates donates electrons to electrons to the double the double bond.bond.