ALKENE AND ALKYNE REACTIONS
Dr. Clower
CHEM 2411
Spring 2014
McMurry (8th ed.) sections 7.7-7.8, 7.10-7.11, 10.3-10.4,
8.2-8.8, 8.10, 8.12, 9.3-9.8, 7.1, 8.1, 9.2, 9.9
Outline
• Reactions of alkenes
• Reactions of alkynes
• Preparation of alkenes and alkynes
• Synthesis
• No reactions of alkyl halides (originally on syllabus)
Reaction Charts
• Help organize reaction details
• Organize charts by reaction
type, starting material, product
• See webpage for template
• Example:
Reaction Type
Oxymercurationdemercuration
Starting
Material
Reagent
Regiochemistry/
stereochemistry
Rearrangement
possible?
Product
Alkene
1. Hg(OAc)2, H2O
2. NaBH4
Markovnikov
Anti addition
no
Alcohol
Reactions of Alkenes
I.
II.
III.
IV.
V.
Allylic halogenation
Electrophilic addition
Reduction
Oxidation
Polymerization
I. Allylic Halogenation
• Similar to radical halogenation of alkanes
• Alkene react with molecular halogen in the presence of
heat or light
• Alkyl halide is produced
• Substitution of –X for –H at the allylic position
• Most stable radical intermediate
• Stabilized by resonance
Allylic Halogenation
• Another set of reagents:
• N-bromosuccinimide (NBS), hn
• Bromination only (no chlorination)
• Product is a racemic mixture (if there is a stereocenter)
Radical Stability
• What is the major product of the reaction of 1-octene with
NBS (in the presence of light)?
• What is the major product of the reaction of 1-octene with
NBS (in the presence of light)?
• Reaction occurs at less sterically hindered carbon and
produces the more stable C=C
• What is the major product of the following reaction?
II. Electrophilic Addition
• Most common reaction of alkenes
• Examples:
• Break p bond of alkene
• Form new s bonds to each C of double bond
• Alkene is nucleophile; reacts with electrophile (HX, H2O, etc.)
• Forms carbocation intermediate
Electrophilic Addition
• General mechanism:
• Step 1:
• Step 2:
• Which step is RDS?
Addition of Hydrogen Halides
• HCl, HBr, HI
• Example: 2-methylpropene + HBr
• What is the major product of the following reaction?
• Stereochemistry of product = racemic mixture
• Carbocation intermediate is planar, sp2 hybridized
• Regiochemistry of reaction
• Which C gets the H? Which C gets the X?
• Reaction is regiospecific for one product
Regiochemistry of Electrophilic Addn.
• Markovnikov’s Rule:
• In the addition of HX (or H2O) to an alkene, the H will add to the
carbon with the greater number of H’s already bonded to it
• The X (or OH) attaches to the carbon with fewer H’s (the more
substituted carbon)
• Product = Markovnikov product
• Opposite product = anti-Markovnikov or non-Markovnikov
• Formed under specific conditions
Markovnikov’s Rule
Markovnikov’s Rule
• Why is the Markovnikov product favored?
• Look at reaction intermediate
• Carbocation
• Markovnikov addition forms the more stable R+
• 3º > 2º > 1º
• More stable carbocation forms faster, will react to give
product
Markovnikov’s Rule
• Draw and name the major product of the following reaction.
• Draw and name the major product of the following reaction.
• Expected product =
• Actual product =
• What happened?
Carbocation Rearrangement
• Carbocation intermediates can rearrange to form a more
stable carbocation structure
• Hydride shift = H:- moves from C adjacent to carbocation
Carbocation Rearrangement
• Alkyl groups can also shift
• Typically methyl or phenyl
(Major product)
Anti-Markovnikov Addition of HBr
• In the presence of peroxides
• H2O2 or R2O2
• Free radical mechanism
• Only HBr, not HCl or HI
Addition of Halogens
• X2 = Br2 or Cl2 (F2 too reactive, I2 unreactive)
• Solvent = inert, nonaqueous
• Stereochemistry = anti addition
• Two X atoms add from opposite sides of the C=C
• Product = a vicinal dihalide
• Two X atoms on adjacent carbons
Mechanism
Addition of Halogens
• Draw the major product of the following reaction.
Addition of Halogens in the
Presence of Water
• Stereochemistry: X and OH add anti
• Regiochemistry: X adds to the less substituted carbon
OH adds to the more substituted carbon
• Mechanism the same as addition of X2, except H2O is the
nucleophile in the second step
Mechanism
Mechanism
• Water attacks the carbon with the largest d+
• Results in OH on more substituted carbon
• Draw the major product of the following reaction.
Hydration
• Addition of water
• Three methods:
A. Acid-catalyzed hydration
B. Oxymercuration-demercuration
C. Hydroboration-oxidation
A. Acid-catalyzed hydration
• Regiochemistry = Markovnikov
• Acid catalyst typically H2SO4 or H3PO4 (or just H3O+)
• Carbocation intermediate, so rearrangement can occur
Mechanism
• Draw the major product of the following reaction.
B. Oxymercuration-demercuration
• Step 1: Alkene reacts with mercuric acetate
• Step 2: Reduction with sodium borohydride
• Regiochemistry =Markovnikov
• Stereochemistry = anti addition of OH and H
• No rearrangements
• Milder conditions than H3O+
• Electrophile is +HgOAc
• Formed by dissociation of AcO-Hg-Oac
• Intermediate is bridged mercurinium ion (similar to bromonium)
Oxymercuration-demercuration
• Draw the major product for each of the following reactions.
C. Hydroboration-oxidation
• Anti-Markovnikov product
• Syn addition of H and OH (add on same side of C=C)
• No rearrangements
• THF stabilize highly reactive BH3
Hydroboration-oxidation
• Mechanism of first step:
• BH2 on the right because less steric hindrance
• Leads to anti-Markovnikov product
• Second step: H2O2/NaOH replace –BH2 with –OH
• Keep same stereochemistry (syn)
• Draw the major product of the following reaction.
• Draw the major product formed when the following alkene
undergoes (a) acid-catalyzed hydration, (b) oxymercurationdemercuration, and (c) hydroboration-oxidation.
Oxidation and Reduction
• What is oxidation?
• What is reduction?
• Classify these reactions as oxidation or reduction:
• CH3─CH═CH2 → CH3─CH2─CH3
• CH3─CH2─OH → CH3─CO2H
III. Reduction
• Catalytic hydrogenation
• Seen before with heat of hydrogenation (alkene stability)
• Catalyst = metal, usually Pd, Pt, or Ni
• Reaction takes place on metal surface
• Stereochemistry = syn (both H’s add to same side of C=C)
Mechanism
Catalytic Hydrogenation
• This reduction does not work with C=O, C=N, or benzene
except at very high P or T, or with a special catalyst
IV. Oxidation
• Three types
A. Epoxidation
B. Hydroxylation
C. Oxidative cleavage
A. Epoxidation
• Formation of epoxide
• Cyclic ether
• Example:
• Reagent is peroxy acid (RCO3H)
• Stereochemistry = syn
• Another method: treat halohydrin with base:
B. Hydroxylation
• Formation of a 1,2-diol/glycol/vicinal diol
• Methods:
1. Opening of epoxide using aqueous acid
• Product is trans diol
• Mechanism:
Hydroxylation
2. Addition of osmium tetroxide (OsO4) or potassium
permanganate (KMnO4)
• How do you know these are both oxidizing agents?
• Reaction includes some appropriate work-up
• H2O2 or NaHSO3, H2O for OsO4
• HO- (aq) for KMnO4
• Stereochemistry = syn
• Draw the major product of the following reaction.
C. Oxidative Cleavage
• Oxidize and alkene and split the C=C
• Results in formation of 2 carbonyls
• Type of carbonyls depends on alkene structure and the oxidizing
agent used
• Three types of oxidizing agents
1. Ozone
2. Potassium permanganate
3. Periodic acid
Oxidative Cleavage
1. Ozone
• Ozonolysis
• Reagents: 1. O3
2. (CH3)2S or Zn, H3O+
• Products = 2 carbonyls (ketones or aldehydes)
• Terminal alkenes give CO2
Oxidative Cleavage
2. KMnO4
• Reagents: KMnO4 (excess or concentrated) and heat or acid
• Use heat and excess KMnO4 to split intermediate glycol
• Products = 2 carbonyls (ketones or carboxylic acids)
• Aldehydes oxidize to carboxylic acids in KMnO4
• Terminal alkenes still give CO2
Oxidative Cleavage
2. HIO4
• Specifically used to split glycol
• Draw the major product for each of the following reactions.
V. Polymerization
• Polymer = large molecule synthesized by covalently
linking single parts (monomers)
• Biological polymers: proteins, cellulose, nucleic acids
• Organic polymers: plastics
• Chain-growth polymers: made from alkene monomers
• Radical reaction
Chain-growth Polymerization
• Initiation by peroxides:
• Propagation:
• Termination:
R─CH2CH2• + •CH2CH2─R →
R─CH2CH2CH2CH2─R
Chain-growth Polymers
• Draw the structure of poly(vinyl chloride).