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
Details of Carbocation Details of Carbocation MechanismsMechanisms
• Secondary
• Tertiary
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
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
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)
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