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.