Download Chapter 9. Addition Reactions of Alkenes

C h a p t e r N i n e:
Addition Reactions of Alkenes
.
O
CO2H
O
H
Biosynthesis of a prostaglandin from arachidonic acid:
intermediate intramolecular radical addition
Note: Problems with italicized numbers are more challenging.
You may want to try them last.
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CHM 321: Summary of Important Concepts
YConcepts for Chapter 9 : Addition Reactions of Alkenes
I.
Reactions of Alkenes
A. addition reactions
1. electrophilic additions (all by very similar mechanisms)
a. when carbocations are intermediates, remember to anticipate
rearrangement
b. Hammond postulate
c. hydrohalogenation: addition of HX to yield haloalkanes
i.
Markovnikov
– In an addition of HX to C=C, HX adds so as to
produce the thermodynamically more stable
carbocation intermediate
I. Bimolecular electrophilic addition
II. Carbocations are intermediates
III. Stabilities: 3E > 2E > 1E> methyl carbocation
IV. Carbocations that can rearrange do rearrange
– 1,2 hydride shifts
– 1,2 alkyl shifts
V. Most common rearragements:
– 2E to 2E
– 2E to 3E
– 3E to 3E
ii. antiMarkovnikov
I. Radical addition of HBr to C=C
d. hydration: addition of H2O to yield alcohols
i.
Markovnikov
I.
addition of H2SO4, then H2O
II.
50:50 H2SO4:H2O
III.
oxymercuration-demercuration
ii.
antiMarkovnikov
I.
hydroboration-oxidation
A. syn-addition stereochemistry
e. dihalogenation: addition of X2 to yield 1,2-dihaloalkanes
i.
halonium ions
ii.
anti-addition stereochemistry
f. halohydroxylation: addition of X–OH to yield halohydrins
i.
halonium ions
ii.
anti-addition stereochemistry
iii.
mechanism similar to dihalogenation
g. methylenation: addition of :CH2 to yield cyclopropanes
i. almost any carbene, :CR2, will work
ii. most commonly used reaction: the Simmons-Smith reaction
2 CH2I2 + Zn(Cu) ÷ ICH2ZnI + “:CH2”
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h. dihydroxylation: addition of OH and OH to C=C to yield 1,2dihydroxyalkanes (common name: glycols)
i. cold, dilute, aqueous alkaline KMnO4 (yields only fair)
ii. OsO4 (yields are good)
iii. syn-addition stereochemistry
i. epoxidation: addition of O to yield epoxides
i. typical to use peracids like peracetic acid
ii. basic hydrolysis causes net anti-dihydroxylation
2. hydrogenation: addition of H2 with a noble metal catalyst to yield alkanes
a. syn-addition stereochemistry
3. C=C cleavage reactions
a. ozonolysis: addition of 2 O to yield formaldehyde, aldehydes, or
ketones
b. KMnO4 in base or acid: addition of $ 2O to yield CO2, carboxylic
acids, or ketones
4. Cleavage of 1,2-diols
a. HIO4 yields two carbonyl campounds
II. Synthesis strategies
A. One-step
1. Substitutions
2. Eliminations
3. Additions
4. Rearrangements
B. Multistep
1. Combine II.A.1. - II.A.4. in linear sequences
a. Constitutional rearrangements
i. Changing leaving group location
> Elimination – Addition
ii. Pi bond position
> Addition – Elimination
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1.
Provide the principal organic reaction product for each of the following reactions. Include
stereochemistry of the product when appropriate. If you believe there will be no reaction
given the starting materials and the reaction condition, indicate so by writing "NR" for "no
reaction".
a.
HCl
23 EC
b.
O
1. Hg(OCCH3)2
2. NaBH4, HO-
Br
c.
O
CH3COOH
O
Cl
CH3COH
Cl
d.
xs H2
Pt
e.
OH
Br2
H2O, dark
0 EC
f.
H2
Pd/C
5
2.
Provide brief answers for each of the following questions:
a. Circle which compound undergoes catalytic hydrogenation faster:
H
CH3
E
F
b. Draw the structure of the intermediate bromonium ion formed when Br2 reacts with (Z)-2butene to provide 2,3-dibromobutane:
c. Rank the following cations in order of relative thermodynamic stability, from least stable
(1) to most stable (3):
Br
Br
+
+
+
________
________
6
________
Br
d. Each of the following isomeric compounds of formula C8H14 release heat energy when
hydrogenated with excess H2 using Pt as catalyst. Rank the compounds in order of relative
heat of hydrogenation, from largest (1) to smallest (4) heat of hydrogenation:
e. Draw the structure of the only primary alcohol of molecular formula C5H12O which cannot
be prepared from an alkene.
3.
Muscalure is the common name given to a hydrocarbon pheromone of molecular formula
C23H46 which the female housefly Musca domestica secretes to attract male houseflies.
Ozonolysis of this compound, followed by workup using Zn and H2O, yields two aldehydes
with formulas CH3(CH2)7CHO and CH3(CH2)12CHO. What is the structure of muscalure?
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4.
When (6Z)-2,6-dimethyl-2,6-octadiene is heated with H2SO4 for two hours, a new compound,
identified as 1,2,3,3-tetramethyl-1-cyclohexene, is isolated in good chemical yield.
Provide a clear, reasonable, detailed stepwise reaction mechanism for this chemical
transformation.
5.
3-Methyl-3-(1-methylethyl)-1-cyclohexene is found to undergo acid-catalyzed hydration
using 1 : 1 H2SO4 : H2O to produce three isomeric alcohols, A, B, and C:
OH
OH
+
A
OH
+
B
C
a.
Provide a detailed, clear, accurate reaction mechanism that explains how the starting
alkene provides product C. It is not necessary to show how the other two products form.
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b.
Would analogous oxymercuration–demercuration using aqueous Hg(CO2CH3)2,
followed by NaBH4 in aqueous base, yield the same mixture of isomeric alcohol products as
the above reaction?
c.
Briefly, explain the reasoning that lead to your answer in part b.
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6.
When (E)-3-hexene is reacted with Cl2 in the dark at 0 EC, a single isomer of molecular
formula C6H12Cl2 is produced.
a.
Draw the structural formula of the product that forms, using wedge and hatched lines.
b.
Draw the highest and the lowest energy Newman projections of the product, viewed
along the C3–C4 sigma bond.
highest
lowest
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7.
Like water, alkanols can add to double bonds in the presence of acid catalysts. For instance:
OH
H2SO4
+
CH2Cl2
O
O
O
a.
Acount for the formation of the product of this reaction using an accurate, clear,
detailed reaction mechanism.
b.
Would this be considered to be a Markovnikov or an antiMarkovnikov addition?
Briefly explain the reasoning that led you to your answer.
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8.
Unlike alkenes, dacarbazine, an antineoplastic drug, undergoes facile cis-trans isomerization
of its N=N bond in the presence of an acid catalyst:
O
O
N
N
H
NH2
N
N
N
H+
N
H
N(CH3)2
NH2
N N
N(CH3)2
Provide a reasonable, clear, detailed mechanism that accounts for this isomerization.
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9.
The following mechanism describing the electrophilic addition of hydrogen chloride to an alkene has an
awful error in each step:
. . ..
O
+
. . ..
O
..
H Cl :
..
. . ..
O
+
.. Cl H
..
a.
: Cl :
..
Briefly but clearly, describe the two errors in the mechanism. Then, correct the errors by redrawing
the correct, error-free mechanism.
Error 1.
Error 2.
Correct mechanism:
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10.
Each of the following reactions provides a major product in good chemical yield. Provide a structural
formula for each reaction’s major product, including stereochemistry where appropriate.
(a)
Cl
HIO4
HO
OH
(b)
1. BH3 @ THF
2. H2O2, -OH
(c)
Cl
HBr
dark, 0 EC
Cl
(d)
1. Hg(CO2CH3)2, H2O
O
2. NaBH4, -OH
O
(e)
Cl
H2
Pt, acetic acid
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11.
∆3-Carene is a major component of turpentine. It has molecular formula C10H16. When it is
hydrogenated at room temperature using excess hydrogen gas, compound Y with molecular
formula C10H18 is isolated. Ozonolysis of ∆3-carene provides compound Z:
O
O
CH3CCH2
CH2CH
H
H
H3C
CH3
1. O3
2. Zn, H2O
)3-Carene
xs. H2
Y
Pd/C
C10H16
Z
Draw the structural formulas of ∆3-carene and Y.
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C10H18
12.
For the reactions shown below, predict the major organic product. If you decide that no
reaction occurs for a given set of reaction conditions, write NR. Be sure to indicate the
correct stereochemistry of the product when appropriate.
a.
Na+ CNDMSO
b.
HBr
peroxides
Br2
CCl4
KI
16
acetone
13.
The reaction below, which provides compound M as its major product, appears to defy the
principles that we discussed in class. Draw the structures of the intermediate carbocations
that form in this reaction, then clearly but briefly explain why M, and not L, is the major
product of this reaction. Hint: draw a complete Lewis structure for the starting material.
Cl
HCl
+
Cl
NO2
NO2
NO2
M
14.
L
Consider the following series of carbocations:
+
+
+
+
+
+
A
B
C
D
a. Which carbocation is the thermodynamically least stable?
E
____________
b. Which carbocation is an intermediate in the Markovnikov addition of HCl to
4-ethyl-1,4-dimethyl-1-cyclopentene?
____________
+
c. Which carbocation rearranges to form
17
?
____________
F
15. (24 points) Treatment of 4-penten-1-ol with aqueous Br2 yields a cyclic bromoether as the major product:
OH
O
Br2
Br
+ HBr
H2O
4-penten-1-ol
a. Propose a clear, detailed, correct reaction mechanism that explains how this product forms.
b. Draw the transition state of the first step of the mechanism you provided in part a of this question.
c. Yes or no: is this a regioselective reaction?
____________
d. Yes or no: is this a stereoselective reaction?
____________
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