Alkenes and alkynes The chemistry of unsaturation.

Alkenes and Alkenes and alkynesalkynes

The chemistry of The chemistry of unsaturationunsaturation

Alkene structureAlkene structure• Unsaturated hydrocarbons – those with

one or more double or triple bonds. General formula is CnH2n (for one double bond)

• Trigonal planar geometry, sp2 hybridization

• Bond angles close to 120º • No rotation about C=C

Alkyne structureAlkyne structure• One or more triple bonds • Linear geometry• 180º bond angles• sp hybridization• One sigma and two pi bonds

H-C≡C-HEthyne (acetylene)

NomenclatureNomenclature• Position of double bond is given by

number of first doubly bonded carbon

• 1-butene CH2=CHCH2CH3

• 2-butene CH3CH=CHCH3

• Longest chain always includes double/triple bonds

Cis-transCis-trans isomerism isomerism trans-2-butene cis-2-butene

E-Z notation

E-ZE-Z Notation Notation• Cahn-Ingold sequence priority –

priority is given to higher MW substituents

• Higher priority substituents on same side – Z isomer (zusammen, or together)

Z-2-chloro-2-butene

NomenclatureNomenclature• Higher priority substituents on

opposite sides – E isomer (entgegen, against)

E-2-chloro-2-butene

NomenclatureNomenclature• Cycloalkenes – double bonded

carbons are numbers 1 & 2

4-methylcyclohexene(A number for the double bond is not

needed)

NomenclatureNomenclature• Dienes, trienes and polyenes• Prefixes di=2, tri=3, etc. are used to

indicate the number of double bonds• Each double bond gets its own

location number

3-chloro-1,3-pentadiene

NomenclatureNomenclature

2-methyl-1,4-cyclohexadiene

Physical propertiesPhysical properties• Physical properties of alkenes are

similar to those of alkanes• Alkene natural products: Terpenes• Oligomers of isoprene (2-methyl-1,3-

butadiene)

TerpenesTerpenes• Essential oils – two isoprene units –

monoterpenes

TerpenesTerpenes• Sesquiterpenes – three isoprenes

• Tetraterpenes (8 isoprenes)

b-carotene (precursor to vitamin A)

Reactions of alkenes Reactions of alkenes • Addition reactions• Addition of hydrogen halides to form

alkyl halidesCH2=CH2 + HBr CH3CH2Br

• Mechanism– Hydrogen halides are polar– The positive end is attracted to the

electrons in the double bond

Mechanism of Mechanism of hydrohalogenationhydrohalogenation

• protonation step

nucleophile – double bond 1º (primary) carbocation electrophile – H of HCl (unfavorable)

Mechanism of Mechanism of hydrohalogenationhydrohalogenation

chloroethane• Carbocation mechanism!

Details of Carbocation Details of Carbocation MechanismsMechanisms

motion of 1 e- motion of 2 e-

• Order of stability of carbocations: 3º>2º>1º

• Primary


• Secondary

• Tertiary


• More substituted carbocations are more stable because alkyl groups are slightly electron donating, and they stabilize the carbocation by diluting the positive charge.

• Most stable carbocation possible will be formed


Tertiary carbocation is always formed – never the secondary

2-methyl-2-butene: two possible sites for carbocation formation


• Markovnikov’s rule – in hydrohalide addition, the hydrogen adds to the carbon that already has the most hydrogens bonded to it.

• The halogen adds to the carbon that has the most carbons attached (the location of the positive charge)

Addition of waterAddition of water• Acid catalyzed addition of water to

form alcohols

Addition of waterAddition of water• Proceeds by a carbocation

mechanism• Follows Markovnikov’s rule (“gives

the Markovnikov product”)• Mechanism

Addition of waterAddition of water

protonated ethanol

Addition of waterAddition of water

ethanol

Addition of halogensAddition of halogens• Addition of halogens to form vicinal

dihalidesCH2=CH2 + Br2 BrCH2CH2Br

• Carried out in pure reagent, CCl4 or other inert solvent

• Additions to cyclic alkenes always give the trans product (“anti” addition)

• Discoloration of bromine solutions is a test for alkenes

Addition of hydrogenAddition of hydrogen• Addition of hydrogen (reduction) to

form alkanes – catalytic hydrogenation

• Powdered metal catalyst is used – usually Pd, Pt, Ni, Ru

• H2 is used under high pressure

Polymerization reactionsPolymerization reactions• Polymers are long chains of identical

units called monomers

• nCH2=CH2 + initiator (─CH2CH2─)n

ethane polyethylene

Mechanism of free radical Mechanism of free radical polymerization polymerization

• Free radicals are species with one unpaired electron

• Formed by heterolytic bond cleavage

• Initiation

Mechanism of free radical Mechanism of free radical polymerizationpolymerization

• Propagation

• Termination

Mechanism of free radical Mechanism of free radical polymerizationpolymerization

PolyethylenesPolyethylenes

• Low density polyethylene (LDPE) • Highly branched, clear, low melting

– Made via radical mechanism– Used for packaging, trash bags

• High density polyethylene (HDPE)– Linear, opaque, high melting– Made via an ionic mechanism– Used for milk and water jugs, grocery bags,

squeezable bottles

Alkenes and alkynes The chemistry of unsaturation.

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