Alkenes and
alkynes
The chemistry of
unsaturation
Alkene 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 structure
• One or more triple bonds
• Linear geometry
• 180º bond angles
• sp hybridization
• One sigma and two pi bonds
H-C≡C-H
Ethyne (acetylene)
Nomenclature
• 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-trans isomerism
trans-2-butene
E-Z notation
cis-2-butene
E-Z 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
Nomenclature
• Higher priority substituents on
opposite sides – E isomer (entgegen,
against)
E-2-chloro-2-butene
Nomenclature
• Cycloalkenes – double bonded
carbons are numbers 1 & 2
4-methylcyclohexene
(A number for the double bond is not
needed)
Nomenclature
• 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
Nomenclature
2-methyl-1,4-cyclohexadiene
Physical properties
• Physical properties of alkenes are
similar to those of alkanes
• Alkene natural products: Terpenes
• Oligomers of isoprene (2-methyl-1,3butadiene)
Terpenes
• Essential oils – two isoprene units –
monoterpenes
Terpenes
• Sesquiterpenes – three isoprenes
• Tetraterpenes (8 isoprenes)
b-carotene (precursor to vitamin A)
Reactions of alkenes
• Addition reactions
• Addition of hydrogen halides to form
alkyl halides
CH2=CH2 + HBr  CH3CH2Br
• Mechanism
– Hydrogen halides are polar
– The positive end is attracted to the
electrons in the double bond
Mechanism of
hydrohalogenation
• protonation step

nucleophile – double bond
electrophile – H of HCl
1º (primary) carbocation
(unfavorable)
Mechanism of
hydrohalogenation

chloroethane
• Carbocation mechanism!
Details of Carbocation
Mechanisms
motion of 1 e-
motion of 2 e-
• Order of stability of carbocations:
3º>2º>1º
• Primary
Details of Carbocation
Mechanisms
• Secondary
• Tertiary
Details of Carbocation
Mechanisms
• 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
Mechanisms
2-methyl-2-butene:
two possible sites
for carbocation
formation
Tertiary carbocation
is always formed –
never the secondary
Details of Carbocation
Mechanisms
• 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 water
• Acid catalyzed addition of water to
form alcohols
Addition of water
• Proceeds by a carbocation
mechanism
• Follows Markovnikov’s rule (“gives
the Markovnikov product”)
• Mechanism

Addition of water

protonated ethanol
Addition of water
ethanol
Addition of halogens
• Addition of halogens to form vicinal
dihalides
CH2=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 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 reactions
• Polymers are long chains of identical
units called monomers
• nCH2=CH2 + initiator  (─CH2CH2─)n
ethane
polyethylene
Mechanism of free radical
polymerization
• Free radicals are species with one
unpaired electron
• Formed by heterolytic bond cleavage
• Initiation
Mechanism of free radical
polymerization
• Propagation
• Termination
Mechanism of free radical
polymerization
Polyethylenes
• 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