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Alkenes and Alkenes and alkynesalkynes
The chemistry of The chemistry of unsaturationunsaturation
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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
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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)
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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
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Cis-transCis-trans isomerism isomerism trans-2-butene cis-2-butene
E-Z notation
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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
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NomenclatureNomenclature• Higher priority substituents on
opposite sides – E isomer (entgegen, against)
E-2-chloro-2-butene
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NomenclatureNomenclature• Cycloalkenes – double bonded
carbons are numbers 1 & 2
4-methylcyclohexene(A number for the double bond is not
needed)
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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
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NomenclatureNomenclature
2-methyl-1,4-cyclohexadiene
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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)
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TerpenesTerpenes• Essential oils – two isoprene units –
monoterpenes
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TerpenesTerpenes• Sesquiterpenes – three isoprenes
• Tetraterpenes (8 isoprenes)
b-carotene (precursor to vitamin A)
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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
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Mechanism of Mechanism of hydrohalogenationhydrohalogenation
• protonation step
nucleophile – double bond 1º (primary) carbocation electrophile – H of HCl (unfavorable)
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Mechanism of Mechanism of hydrohalogenationhydrohalogenation
chloroethane• Carbocation mechanism!
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Details of Carbocation Details of Carbocation MechanismsMechanisms
motion of 1 e- motion of 2 e-
• Order of stability of carbocations: 3º>2º>1º
• Primary
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Details of Carbocation Details of Carbocation MechanismsMechanisms
• Secondary
• Tertiary
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Details of Carbocation Details of Carbocation MechanismsMechanisms
• 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
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Details of Carbocation Details of Carbocation MechanismsMechanisms
Tertiary carbocation is always formed – never the secondary
2-methyl-2-butene: two possible sites for carbocation formation
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Details of Carbocation Details of Carbocation MechanismsMechanisms
• 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)
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Addition of waterAddition of water• Acid catalyzed addition of water to
form alcohols
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Addition of waterAddition of water• Proceeds by a carbocation
mechanism• Follows Markovnikov’s rule (“gives
the Markovnikov product”)• Mechanism
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Addition of waterAddition of water
protonated ethanol
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Addition of waterAddition of water
ethanol
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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
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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
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Polymerization reactionsPolymerization reactions• Polymers are long chains of identical
units called monomers
• nCH2=CH2 + initiator (─CH2CH2─)n
ethane polyethylene
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Mechanism of free radical Mechanism of free radical polymerization polymerization
• Free radicals are species with one unpaired electron
• Formed by heterolytic bond cleavage
• Initiation
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Mechanism of free radical Mechanism of free radical polymerizationpolymerization
• Propagation
• Termination
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Mechanism of free radical Mechanism of free radical polymerizationpolymerization
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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