ALKENES

HydrocarbonsHydrocarbons AromaticAromaticAliphaticAliphatic AlkanesAlkanes AlkynesAlkynes AlkenesAlkenes

Alkene Nomenclature

AlkenesAlkenes Alkenes are hydrocarbons that contain a carbon-carbon double bond also called "olefins" characterized by molecular formula C n H 2n said to be "unsaturated"

Alkene Nomenclature H2CH2CH2CH2C CH 2 H2CH2CH2CH2C CHCH 3 Etheneor Ethylene (both are acceptable IUPAC names) Propene (Propylene is sometimes used but is not an acceptable IUPAC name)

Alkene Nomenclature 1) Find the longest continuous chain that includes the double bond. 2) Replace the -ane ending of the unbranched alkane having the same number of carbons by -ene. 3) Number the chain in the direction that gives the lowest number to the doubly bonded carbon. H2CH2CH2CH2C CHCH 2 CH 3 1-Butene

Alkene Nomenclature 4) If a substituent is present, identify its position by number. The double bond takes precedence over alkyl groups and halogens when the chain is numbered. The compound shown above is 4-bromo-3-methyl-1-butene. H2CH2CH2CH2C CHCHCH 2 Br CH 3

Alkene Nomenclature 4) If a substituent is present, identify its position by number. Hydroxyl groups take precedence over the double bond when the chain is numbered. The compound shown above is 2-methyl-3-buten-1-ol. H2CH2CH2CH2C CHCHCH 2 OH CH 3

Alkenyl groups methylenevinylallylisopropenyl H2CH2CH2CH2C CCH 3 H2CH2CH2CH2C CHCH 2 CH H2CH2CH2CH2C H2CH2CH2CH2C

Structure of Ethylene bond angles: H-C-H = 117° H-C-C = 121° bond distances: C—H = 110 pm C=C = 134 pm Planar

Side-by-side overlap of half-filled p orbitals gives a  bond Bonding in Ethylene

IsomersIsomers Isomers are different compounds that have the same molecular formula.

2-Methylpropene 1-Butene cis-2-Butene trans-2-Butene C CHH H CH 2 CH 3 H3CH3CH3CH3C C C CH 3 HH H C C H3CH3CH3CH3CH C C H H H3CH3CH3CH3C H3CH3CH3CH3C

cis-2-Butene trans-2-Butene H3CH3CH3CH3C C C CH 3 HH H C C H3CH3CH3CH3CH StereoisomersStereoisomers

Stereochemical Notation cis (identical or analogous substituents on same side) trans (identical or analogous substitutents on opposite sides)

Reactions of Alkenes The characteristic reaction of alkenes is addition to the double bond. + A—B C C A C C B

Hydrogenation of ethylene exothermic  H° = –136 kJ/mol catalyzed by finely divided Pt, Pd, Rh, Ni + H—H    C C H C CHHHH H H H H H

H H H C C ABX Y H Mechanism of catalytic hydrogenation

+ E—Y  –––– C C E Y C C General equation for electrophilic addition

+ H—X  –––– C C H X C C When 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 Rule

ExampleExample acetic acid Br CH 3 CH 2 CHCH 3 CH 2 CH 3 CH 2 CH HBr(80%) Markovnikov’s Rule

Acid-Catalyzed Hydration of Alkenes reaction is acid catalyzed; typical hydration medium is 50% H 2 SO 4 -50% H 2 O reaction is acid catalyzed; typical hydration medium is 50% H 2 SO 4 -50% H 2 O H—OH C C + OH C C H

Cyclopentene +Br 2

– Bromonium ion

trans-Stereochemistry in vicinal dibromide

Epoxidation of Alkenes peroxy acid C C + O RCOOH C C O + O RCOH

Polymerization of alkenes cationic polymerization free-radical polymerization coordination polymerization

Free-Radical Polymerization of Ethylene 200 °C 2000 atm O 2 peroxides H2CH2CH2CH2C CH 2 polyethylene

Free-Radical Polymerization of Propene H2CH2CH2CH2C CHCH 3 polypropyleneCHCHCHCHCHCHCHH CH 3 H H H

H 2 C=CHCl  polyvinyl chloride H 2 C=CHC 6 H 5  polystyrene F 2 C=CF 2  Teflon H 2 C=CHCl  polyvinyl chloride H 2 C=CHC 6 H 5  polystyrene F 2 C=CF 2  Teflon

Classes of Dienes

isolated diene conjugated diene cumulated diene C Classification of Dienes

(2E,5E)-2,5-heptadiene (2E,4E)-2,4-heptadiene 3,4-heptadiene C NomenclatureNomenclature

Isolated diene Conjugated diene  bonds are independent of each other p orbitals overlap to give extended  bond encompassing four carbons

cumulated dienes are less stable than isolated and conjugated dienes Cumulated Dienes CC C

Structure of Allene 131 pm 118.4° linear arrangement of carbons nonplanar geometry

1,2-Addition versus 1,4-Addition 1,2-addition of XY 1,4-addition of XY X Y X Y

Alkynes

HydrocarbonsHydrocarbons AromaticAromaticAliphaticAliphatic AlkanesAlkanes AlkynesAlkynes AlkenesAlkenes

HC CH Acetylene and ethyne are both acceptable IUPAC names for NomenclatureNomenclature Higher alkynes are named in much the same way as alkenes except using an -yne suffix instead of -ene. HC CCH 3 Propyne HC CCH 2 CH 3 1-Butyne (CH 3 ) 3 CC CCH 3 4,4-Dimethyl-2-pentyne

StructureStructure linear geometry for acetylene C C H H 120 pm 106 pm C C CH 3 H 121 pm 146 pm 106 pm

 Bonds in Acetylene Each carbon is connected to a hydrogen by a  bond. The two carbons are connected to each other by a  bond and two  bonds.

Acidity of Acetylene andTerminal Alkynes H C C

Reactions of Alkynes AcidityHydrogenation Metal-Ammonia Reduction Addition of Hydrogen Halides Hydration Addition of Halogens OzonolysisAcidityHydrogenation Metal-Ammonia Reduction Addition of Hydrogen Halides Hydration Addition of Halogens Ozonolysis