Chapter 6: Reactions of Alkenes: Addition Reactions

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Chapter 6 Reactions of Alkenes Addition
Reactions
Disparlure sex attractant of the female gypsy
moth. (A type of pheromone.)
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Addition Reactions
I. Hydrogenation of Alkenes
oleic acid (unsaturated)
stearic acid (saturated)
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I. Hydrogenation of Alkenes
A. Heats of hydrogenation
DHº -30 kcal/mol
DHº -30.1 kcal -28.1 -27.2 -27.8 -26.7
More substituted ? more stable.
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I. Hydrogenation of Alkenes
A. Heats of hydrogenation
less stable
more stable
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I. Hydrogenation of Alkenes
B. Stereochemistry of hydrogenation
anti addition
syn addition
Catalytic hydrogenation proceeds by syn addition
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I. Hydrogenation of Alkenes
Question 6-1. Give the product of the following
reaction. Click on the arrow to check your answer.
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I. Hydrogenation of Alkenes
Answer 6-1. Give the product of the following
reaction. Click on the arrow to check your answer.
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II. Electrophilic Addition
Loose p electrons are nucleophilic (Lewis
bases), react with electrophiles (Lewis acids).
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II. Electrophilic Addition
A. Addition of hydrogen halides
(X Cl, Br, I)
Reactivity HI gt HBr gt HCl gtgt HF (stronger acid
better electrophile)
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II. Electrophilic Addition
A. Addition of hydrogen halides
1. Markovnikovs rule
In the addition of HX to an alkene, the H goes to
the carbon with more Hs.
Question 6-2. Draw the products. Click on the
arrow to check answers.
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II. Electrophilic Addition
A. Addition of hydrogen halides
1. Markovnikovs rule
In the addition of HX to an alkene, the H goes to
the carbon with more Hs.
Answer 6-2.
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II. Electrophilic Addition
A. Addition of hydrogen halides
2. mechanism
Mechanistic interpretation of Markovnikovs rule
The reaction proceeds through the more stable
carbocation intermediate.
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II. Electrophilic Addition
A. Addition of hydrogen halides
2. mechanism
lower Ea ? faster rate of formation
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II. Electrophilic Addition
A. Addition of hydrogen halides
3. carbocation rearrangements
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II. Electrophilic Addition
A. Addition of hydrogen halides
3. carbocation rearrangements
Question 6-3. Give the expected major product
for each reaction. Click on the arrow to check
answers.
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II. Electrophilic Addition
A. Addition of hydrogen halides
3. carbocation rearrangements
Answer 6-3. Give the expected major product for
each reaction. Click on the arrow to check
answers.
forms tertiary carbocation- no need
for rearrangement.
forms secondary carbocation, rearranges to a
tertiary carbocation via a hydride shift
forms a secondary carbocation, rearranges to a
tertiary carbocation via a methide shift
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II. Electrophilic Addition
A. Addition of hydrogen halides
4. free-radical addition of HBr
Markovnikov orientation
antiMarkovnikov orientation (peroxide effect)
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II. Electrophilic Addition
A. Addition of hydrogen halides
4. free-radical addition of HBr
Free radical chain mechanism
Initiation
Propagation
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II. Electrophilic Addition
A. Addition of hydrogen halides
4. free-radical addition of HBr
Reaction proceeds through more stable radical
intermediate.
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II. Electrophilic Addition
A. Addition of hydrogen halides
4. free-radical addition of HBr
Compare addition of HBr with and without
peroxides
Markovnikov orientation
More stable intermediate in both cases.
antiMarkovnikov orientation
Regiochemical control
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II. Electrophilic Addition
A. Addition of hydrogen halides
Question 6-5. Give the major products of the
following reactions. Click on the arrow to check
your answer.
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II. Electrophilic Addition
A. Addition of hydrogen halides
Answer 6-5. Give the major products of the
following reactions.
goes Markovnikov via electrophilic reaction goes
anti-Markovnikov via radical reaction rearranges
to more stable carbocation goes anti-Mark via
free radical, no rearrangement
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II. Electrophilic Addition
B. Addition of sulfuric acid (industrial)
alkyl hydrogen sulfate
overall hydration
Markovnikov orientation
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II. Electrophilic Addition
C. Acid-catalyzed hydration
reverse of dehydration (Le Châtelier)
Markovnikov
Principle of microscopic reversibility
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II. Electrophilic Addition
D. Addition of halogens
(X Cl or Br)
a vicinal dihalide
Stereoselective anti addition
trans only
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II. Electrophilic Addition
D. Addition of halogens
Mechanism halonium ions
cyclic bromonium ion
trans product
anti addition
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II. Electrophilic Addition
E. Formation of halohydrins
vicinal halohydrin
anti addition
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II. Electrophilic Addition
E. Formation of halohydrins
Regioselective
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II. Electrophilic Addition
E. Formation of halohydrins
resonance hybrid
more substituted C carries greater d, has
stronger attraction for nucleophile
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II. Electrophilic Addition
E. Formation of halohydrins
Other nucleophiles
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II. Electrophilic Addition
E. Formation of halohydrins
Question 6-8. Give the products, showing
stereochemistry where applicable. Click on the
arrow to check your answers.
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II. Electrophilic Addition
E. Formation of halohydrins
Answer 6-8. Give the products, showing
stereochemistry where applicable. Click on the
arrow to check your answers.
Br- is nucleophile H2O is nucleophile Ethano
l is nucleophile Interesting! Br is
electrophile, since it is less electronegative
Cl- is nucleophile.
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III. Other Reactions of Alkenes
A. Hydroboration-oxidation
a boron hydride
an organoborane
antiMarkovnikov orientation stereoselective syn
addition no rearrangements
)3
anti-Markovnikov syn addition
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III. Other Reactions of Alkenes
A. Hydroboration-oxidation
Mechanism
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III. Other Reactions of Alkenes
A. Hydroboration-oxidation
Regiochemical control in synthesis
Markovnikov
antiMarkovnikov
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III. Other Reactions of Alkenes
A. Hydroboration-oxidation
Question 6-9. Give the products of the following
reactions. Click on the arrow to check your
answers.
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III. Other Reactions of Alkenes
A. Hydroboration-oxidation
Answer 6-9. Give the products of the following
reactions. Click on the arrow to check your
answers.
anti-Markovnikov Markovnikov
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III. Other Reactions of Alkenes
B. Epoxidation
epoxyethane (ethylene oxide) 1,2-epoxypropane
(propylene oxide) 1,2-epoxycyclohexane
(cyclohexene oxide)
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III. Other Reactions of Alkenes
B. Epoxidation
Stereospecific syn addition
cis-2,3-epoxybutane
trans-2,3-epoxybutane
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III. Other Reactions of Alkenes
C. Ozonolysis
oxidative cleavage
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III. Other Reactions of Alkenes
C. Ozonolysis
Synthesis
Analysis
Question 6-10. What is the structure of the
alkene if ozonolysis produced the following
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III. Other Reactions of Alkenes
C. Ozonolysis
Answer 6-10. What is the structure of the unknown
alkene if ozonolysis produced the following
The fragments are colored to make it easier to
see how they go together. The fragment with two
carbonyls must contain two double bonds. The
alkene is 2,7-dimethyl-2,4-octadiene. It is
impossible to determine whether the diene is E or
Z.
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III. Other Reactions of Alkenes
D. Polymerization
n 1000s 10,000s
monomer
polymer
radical initiators O2
di-t-butyl peroxide
benzoyl peroxide
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III. Other Reactions of Alkenes
D. Polymerization
monomer polymer
polyethylene polypropylene (poly) poly(vinyl
chloride) (PVC, vinyl) polystyrene
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III. Other Reactions of Alkenes
D. Polymerization
monomer polymer
Saran Orlon polytetrafluoroethylene (PTFE,
Teflon)
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III. Other Reactions of Alkenes
D. Polymerization
Free-radical chain polymerization
Initiation
Propagation
head to tail polymerization
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Summary Reactions of Alkenes
Question 6-11. Draw the structures of the
products in the following reactions, including
stereochemistry where appropriate.
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Summary Reactions of Alkenes
Answer 6-11. Draw the structures of the products
in the following reactions, including
stereochemistry where appropriate.
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Summary Reactions of Alkenes
Question 6-12. What reagents would be used to
carry out the following conversions?
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Summary Reactions of Alkenes
Answer 6-12. What reagents would be used to carry
out the following conversions?
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IV. Introduction to Organic Synthesis
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IV. Introduction to Organic Synthesis Work
backwards! And know the reactions!
What different methods do you know for making an
alkene? 1. Dehydrating an alcohol with strong
acid OR 2. Dehydrohalogenating an alkyl halide
with a strong base. So, you could either make an
alcohol and dehydrate it, or you could make
an alkyl halide and dehydrohalogenate it. Which
seems easier to make? It will be easier to make
an alkyl halide since you know how to convert an
alkane to an alkyl halide with free radical
halogenation. Since free radical bromination is
more selective for tertiary positions, you should
use Br2 and heat or light rather than free
radical chlorination, which would give
mixtures. Solution
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IV. Introduction to Organic Synthesis Question
6-13. Convert the starting material to the
indicated product. Show all necessary reagents.
Write the product of each step. Then check your
answers.
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IV. Introduction to Organic Synthesis Answer
6-13. Convert the starting material to the
indicated product. Show all necessary reagents.
Write the product of each step. Then check your
answers.
1. Cl2, hv (or Br2, heat) 2. KOH, ethanol 1.
Br2, heat 2. KOH, EtOH 3. a) B2H6 b)H2O2,
OH- 1. KOH, EtOH 2. HBr, peroxides 1.
Cl2, hv 2. KOH, EtOH 3. Cl2, CCl4
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IV. Introduction to Organic Synthesis Answer
6-13. Convert the starting material to the
indicated product. Show all necessary reagents.
Write the product of each step. Then check your
answers.
1. H2SO4, heat 2. Br2, H2O 1. Br2, CH3OH
2. KOH, EtOH 1. H2SO4, heat
2. A) O3 b) H2O, Zn 1. KOH, EtOH 2. CH3CO3H