ALKENE AND ALKYNE REACTIONS and SYNTHESIS Dr. Sheppard CHEM 2412 Summer 2015 Klein (2 nd ed.)...
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Transcript of ALKENE AND ALKYNE REACTIONS and SYNTHESIS Dr. Sheppard CHEM 2412 Summer 2015 Klein (2 nd ed.)...
ALKENE AND ALKYNE REACTIONSandSYNTHESIS
Dr. Sheppard
CHEM 2412
Summer 2015
Klein (2nd ed.) sections 11.7, 9.1, 9.3, 11.10, 9.4-9.6, 9.8, 9.7, 14.8, 9.9-9.12, 11.11, 10.4, 10.6-10.8, 10.5, 10.9, 10.3, 10.10, 9.13, 10.11, 12.1-12.6
Outline• Nomenclature review• Preparation and reactions of alkenes (Ch. 9 and 11)• Preparation and reactions of alkynes (Ch. 10)• Synthesis (9.13, 10.11, Ch. 12)
Alkene Nomenclature• Similar to alkanes; change infix from “-an-” to “-en-”
• For larger alkenes:
1. Parent is longest C chain containing both carbons of C=C
2. Number chain so C=C has lowest possible number• If the double bond is equidistant from both ends, start numbering at
end nearest the first substituent• Show location of C=C by first number• Alkenes with >1 C=C use “-adiene”, “-atriene”, etc. in place of “-ene”
and show location of all double bonds
3. Name and number substituents and write the full name
4. Determine stereochemistry (cis, trans, E, Z) if applicable
CH2 CH2 CH CH2CH3
ethene (ethylene) propene (propylene)
ExamplesStructure Name
CH3 CH CH CH2
CH2 CH2 CH2
CH3
Alkene substituents
Substituent Name
CH2═ methylene
CH2═CH─ vinyl
CH2═CH─CH2─ allyl
Alkyne Nomenclature• Similar to alkenes; change infix from “-en-” to “-yn-”
• For larger alkynes:
1. Parent is longest C chain containing both carbons of C≡C
2. Number chain so C≡C has lowest possible number• Same rules as alkenes
3. Name and number substituents and write the full name
• Example
C CHCH3
ethyne (acetylene) propyne
CH CH
CH3 C C CH2 CH CH3
CH3
Preparation of Alkenes: Review• Alkyl halide eliminates HX to form an alkene
• Reagent = base
• Alcohol eliminates water to form an alkene• Reagent = acid
Reactions of Alkenes
I. Allylic halogenation
II. Electrophilic addition
III. Reduction
IV. Oxidation
V. Polymerization
Reaction Charts• Help organize reaction details• Organize charts by reaction
type, starting material, product• See webpage for template• Example:
Reaction Type
Starting Material Reagent Regiochemistry/
stereochemistryRearrangement
possible? Product
Oxymercuration-demercuration
Alkene1. Hg(OAc)2, H2O
2. NaBH4 MarkovnikovAnti addition
no Alcohol
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
CH2 CH CH3
Br2
h
Allylic carbon
CH2 CH CH2
Br
+ HBr
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
• Another example:
• Which is the major product?
• What is the major product of the following reaction?
CH2
NBS
h
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
C C + HX C C
H X
C C + H2O C C
H OH
Electrophilic Addition• General mechanism:
• Step 1:
• Step 2:
• Which step is RDS?
C C + E C C
E
C C
E
+ Nu: C C
E Nu
Hydrohalogenation• Addition of hydrogen halides
• HCl, HBr, HI
• Example: cis-3-hexane + HBr
• What is the major product of the following reaction?
• Stereochemistry of product = racemic mixture• Carbocation intermediate is planar, sp2 hybridized
HBr
• Regiochemistry of reaction• Which C gets the H? Which C gets the X?• Reaction is regioselective
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
Mechanism for Hydrohalogenation
Intermediate Structures• Two possible intermediates:
• Draw and name the major product of the following reaction.
HBrCH3 CH2 CH
CH3
CH CH2
• Draw and name the major product of the following reaction.
• Expected product =
• Actual product =
• What happened?
HBrCH3 CH2 CH
CH3
CH CH2
CH3 CH2 CH
CH3
CH CH3
Br
2-bromo-3-methylpentane
CH3 CH2 C
CH3
CH2 CH3
Br
3-bromo-3-methylpentane
Carbocation Rearrangement• Carbocation intermediates can rearrange to form a more
stable carbocation structure• Hydride shift = H:- moves from C adjacent to carbocation
CH3 CH2 C
CH3
CH CH3
H
CH3 CH2 C
CH3
CH2 CH3
CH3 CH2 C
CH3
CH2 CH3
Br
3-bromo-3-methylpentane
Br
CH3 CH2 CH
CH3
CH CH2
H Br
hydride shif t
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
• If product has a stereocenter, a racemic mixture is produced
HBrCH3 C
CH3
CH CH3 CH3 C
CH3
CH2 CH3
Br
HBrCH3 C
CH3
CH CH3 CH3 CH
CH3
CH CH3peroxides
Br
(Markovnikov)
(anti-Markovnikov)
Mechanism
Stability of Intermediate• In both Markovnikov and anti-Markovnikov reactions, the
most stable intermediate is formed
Hydration• Addition of water• Three methods:
A. Acid-catalyzed hydration
B. Oxymercuration-demercuration
C. Hydroboration-oxidation
A. Acid-catalyzed hydration
• Regiochemistry = Markovnikov• Stereochemistry = racemic mixture• Acid catalyst typically H2SO4 or H3PO4 (or just H3O+)
• Carbocation intermediate, so rearrangement can occur• Example:
CH CH2R CH CH3R
OH
H2O
H+
Mechanism
• Draw the major product of the following reaction.
H2O
H2SO4
B. Oxymercuration-demercuration• Step 1: Alkene reacts with mercuric acetate• Step 2: Reduction with sodium borohydride
• Regiochemistry = Markovnikov• Anti addition of OH and H (add on opposite sides of C=C)• No rearrangements• Milder conditions than H3O+
• Electrophile is +HgOAc• Formed by dissociation of AcO-Hg-OAc
• Intermediate is bridged mercurinium ion
CH CH2R CH CH2R
OH
Hg(OAc)2
H2O
HgOAc
NaBH4CH CH2R
OH
H
Oxymercuration-demercuration• Predict the products:
1. Hg(OAc)2, H2O
2. NaBH4
Oxymercuration-demercuration• Oxymercuration mechanism:
• Demercuration mechanism: radical
Oxymercuration-demercuration
• Draw the major product for each of the following reactions.
1. Hg(OAc)2, H2O
2. NaBH4
CH3
1. Hg(OAc)2, H2O
2. NaBH4
CH3
CH3
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 C with more H’s because less steric hindrance• Leads to anti-Markovnikov product
• Second step: H2O2/NaOH replace –BH2 with –OH • Complicated mechanism• Keep same stereochemistry (syn)
CH CH2R CH CH2R
BH2H BH2 H
CH CH2R
BH2H
H2O2
NaOHCH CH2R
OHH
• Draw the major product of the following reactions.
1. BH3 THF
2. H2O2, NaOH
CH3
1. BH3 THF
2. H2O2, NaOH
• Draw the major product formed when the following alkene undergoes (a) acid-catalyzed hydration, (b) oxymercuration-demercuration, and (c) hydroboration-oxidation.
CH3 CH CH CH2
CH3
Halogenation
• Addition of halogens• X2 = Br2 or Cl2 (F2 too reactive, I2 unreactive)
• Solvent = inert, nonaqueous• Stereochemistry = anti addition• Product = a vicinal dihalide
• Two X atoms on adjacent carbons
• Example:
CH CH2R + X2
r.t.
CH2Cl2(CHCl3, CCl4)
CH CH2R
X
X
Mechanism
C C Br Br+
Addition of Halogens
• Draw the major product of the following reaction.
Br2
CCl4
CH3
Halohydrin formation
• 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
• Example:
C CH2R
R
Br
C CH2R
R
Br
not C CH2R
R
Br
• Draw the major product of the following reaction.
Br2
H2O
CH3
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
No enantiomer in this example because the product is achiral