alkenes

ALKENES

HydrocarbonsHydrocarbonsHydrocarbonsHydrocarbons

AromaticAromaticAromaticAromaticAliphaticAliphaticAliphaticAliphatic

AlkanesAlkanesAlkanesAlkanes AlkynesAlkynesAlkynesAlkynesAlkenesAlkenesAlkenesAlkenes

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 Ccharacterized by molecular formula CnnHH2n2n

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)


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


4) If a substituent is present, identify its position 4) If a substituent is present, identify its position by number. The double bond takes by number. The double bond takes precedence over alkyl groups and halogens precedence over alkyl groups and halogens when the chain is numbered.when the chain is numbered.

The compound shown above isThe compound shown above is4-bromo-3-methyl-1-butene.4-bromo-3-methyl-1-butene.

HH22CC CHCHCHCHCHCH22BrBr

CHCH33


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 HH22CC CCHCCH33

HH22CC CHCHCHCH22

CHCHHH22CC

HH22CC

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

• Side-by-side overlap of half-filled p orbitals gives a bond

Bonding in EthyleneBonding in EthyleneBonding in EthyleneBonding in Ethylene

IsomersIsomersIsomersIsomers

Isomers are different compounds thatIsomers are different compounds thathave the same molecular formula.have the same molecular formula.

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

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)

Reactions of AlkenesReactions of AlkenesReactions of AlkenesReactions of Alkenes

The characteristic reaction of alkenesThe characteristic reaction of alkenes is addition to the double bond. is addition to the double bond.

+ + AA——BBCC CC AA CC CC BB

Hydrogenation of ethyleneHydrogenation of ethyleneHydrogenation of ethyleneHydrogenation of ethylene

exothermic exothermic HH° = –136 kJ/mol° = –136 kJ/mol

catalyzed by finely divided Pt, Pd, Rh, Nicatalyzed by finely divided Pt, Pd, Rh, Ni

+ H—H+ H—H

CC CC HH CC CC

HH HH

HH HH

HH

HH

HH

HH

HH

H HH

CCCC

AABB

XXYY

H

Mechanism of catalytic hydrogenationMechanism of catalytic hydrogenationMechanism of catalytic hydrogenationMechanism of catalytic hydrogenation

++ EE——YY ––

CC CC EE YYCC CC

General equation for electrophilic additionGeneral equation for electrophilic additionGeneral equation for electrophilic additionGeneral equation for electrophilic addition

++ HH——XX ––

CC CC HH XXCC CC

When EY is a hydrogen halideWhen EY is a hydrogen halideWhen EY is a hydrogen halideWhen EY is a hydrogen halide

When an unsymmetrically substituted alkene reacts with a hydrogen halide, the hydrogen adds to the carbon that has the greater number of hydrogen substituents, and the halogen adds to the carbon that has the fewer hydrogen substituents.

Markovnikov’s RuleMarkovnikov’s RuleMarkovnikov’s RuleMarkovnikov’s Rule

ExampleExampleExampleExample

acetic acidacetic acidBrBr

CHCH33CHCH22CHCHCHCH33CHCH22CHCH33CHCH22CHCHHBrHBr

(80%)(80%)

Markovnikov’s RuleMarkovnikov’s RuleMarkovnikov’s RuleMarkovnikov’s Rule

Acid-Catalyzed Hydration of AlkenesAcid-Catalyzed Hydration of AlkenesAcid-Catalyzed Hydration of AlkenesAcid-Catalyzed Hydration of Alkenes

reaction is acid reaction is acid catalyzed; typical catalyzed; typical

hydration medium is hydration medium is 50% H50% H22SOSO44-50% H-50% H22OO

reaction is acid reaction is acid catalyzed; typical catalyzed; typical

hydration medium is hydration medium is 50% H50% H22SOSO44-50% H-50% H22OO

H—OHH—OHCC CC ++

OHOHCC CCHH

Cyclopentene +Br2

––

Bromonium ion

trans-Stereochemistry in vicinal dibromide

Epoxidation of AlkenesEpoxidation of AlkenesEpoxidation of AlkenesEpoxidation of Alkenes

peroxy acidperoxy acid

CC CC ++

OO

RCORCOOOHH

CCCC

OO

++

OO

RCOHRCOH

Polymerization of alkenesPolymerization of alkenesPolymerization of alkenesPolymerization of alkenes

cationic polymerizationcationic polymerization

free-radical polymerizationfree-radical polymerization

coordination polymerizationcoordination polymerization

Free-Radical Polymerization of EthyleneFree-Radical Polymerization of EthyleneFree-Radical Polymerization of EthyleneFree-Radical Polymerization of Ethylene

200 °C200 °C2000 atm2000 atm

OO22

peroxidesperoxides

HH22CC CHCH22

polyethylenepolyethylene

CHCH22 CHCH22 CHCH22 CHCH22 CHCH22 CHCH22 CHCH22

Free-Radical Polymerization of PropeneFree-Radical Polymerization of PropeneFree-Radical Polymerization of PropeneFree-Radical Polymerization of Propene

HH22CC CHCHCHCH33

polypropylenepolypropylene

CHCH CHCH CHCHCHCHCHCHCHCH CHCH

HH CHCH33 HH CHCH33 HH CHCH33 HH

HH22C=CHCl C=CHCl polyvinyl chloridepolyvinyl chlorideHH22C=CHCC=CHC66HH5 5 polystyrenepolystyreneFF22C=CFC=CF22 TeflonTeflon

HH22C=CHCl C=CHCl polyvinyl chloridepolyvinyl chlorideHH22C=CHCC=CHC66HH5 5 polystyrenepolystyreneFF22C=CFC=CF22 TeflonTeflon

Classes of DienesClasses of Dienes

isolated dieneisolated diene

conjugated dieneconjugated diene

cumulated dienecumulated diene

CC Classification of DienesClassification of DienesClassification of DienesClassification of Dienes

(2(2EE,5,5EE)-2,5-heptadiene)-2,5-heptadiene

(2(2EE,4,4EE)-2,4-heptadiene)-2,4-heptadiene

3,4-heptadiene3,4-heptadiene

CC NomenclatureNomenclatureNomenclatureNomenclature

Isolated dieneIsolated diene

Conjugated dieneConjugated diene

bonds are bonds are independent of independent of

each othereach other

pp orbitals overlap orbitals overlap to give extended to give extended

bond bond encompassing encompassing four carbonsfour carbons

cumulated dienes are less stable thancumulated dienes are less stable thanisolated and conjugated dienesisolated and conjugated dienes

Cumulated DienesCumulated Dienes Cumulated DienesCumulated Dienes

CCCC CC

Structure of AlleneStructure of Allene Structure of AlleneStructure of Allene

131 pm131 pm

118.4°118.4°

linear arrangement of carbonslinear arrangement of carbons

nonplanar geometrynonplanar geometry

1,2-Addition versus 1,4-Addition1,2-Addition versus 1,4-Addition 1,2-Addition versus 1,4-Addition1,2-Addition versus 1,4-Addition 1,2-addition of XY1,2-addition of XY 1,4-addition of XY1,4-addition of XY

XX

YY XX

YY

AlkynesAlkynes

HydrocarbonsHydrocarbonsHydrocarbonsHydrocarbons

AromaticAromaticAromaticAromaticAliphaticAliphaticAliphaticAliphatic

AlkanesAlkanesAlkanesAlkanes AlkynesAlkynesAlkynesAlkynesAlkenesAlkenesAlkenesAlkenes

HCHC CHCHAcetylene and ethyne are both acceptableAcetylene and ethyne are both acceptableIUPAC names forIUPAC names for

NomenclatureNomenclatureNomenclatureNomenclature

Higher alkynes are named in much the sameHigher alkynes are named in much the sameway as alkenes except using an -way as alkenes except using an -yneyne suffix suffixinstead of -instead of -eneene..

HCHC CCHCCH33

PropynePropyne

HCHC CCHCCH22CHCH33

1-Butyne1-Butyne

(CH(CH33))33CCCC CCHCCH33

4,4-Dimethyl-2-pentyne4,4-Dimethyl-2-pentyne

StructureStructureStructureStructure

linear geometry for acetylenelinear geometry for acetylene

CC CCHH HH

120 pm120 pm

106 pm106 pm 106 pm106 pm

CC CCCHCH33 HH

121 pm121 pm

146 pm146 pm 106 pm106 pm

Bonds in AcetyleneBonds in Acetylene Bonds in AcetyleneBonds in Acetylene

Each carbon isEach carbon is

connected to aconnected to a

hydrogen by ahydrogen by a

bond. The two bond. The two

carbons are connectedcarbons are connected

to each other by ato each other by a

bond and two bond and two bonds. bonds.

Acidity of Acetylene andTerminal AlkynesAcidity of Acetylene andTerminal Alkynes

HH

CC CC

Reactions of AlkynesReactions of AlkynesReactions of AlkynesReactions of Alkynes

Acidity Acidity Hydrogenation Hydrogenation Metal-Ammonia Reduction Metal-Ammonia Reduction Addition of Hydrogen HalidesAddition of Hydrogen HalidesHydration Hydration Addition of Halogens Addition of Halogens OzonolysisOzonolysis

Acidity Acidity Hydrogenation Hydrogenation Metal-Ammonia Reduction Metal-Ammonia Reduction Addition of Hydrogen HalidesAddition of Hydrogen HalidesHydration Hydration Addition of Halogens Addition of Halogens OzonolysisOzonolysis

alkenes

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