Chapter 9. Addition Reactions of Alkenes

18 Pages·2012·0.06 MB·English

C h a p t e r N i n e:
Addition Reactions of Alkenes
.
CO H
O 2
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 H O to yield alcohols
2
i. Markovnikov
I. addition of H SO , then H O
2 4 2
II. 50:50 H SO :H O
2 4 2
III. oxymercuration-demercuration
ii. antiMarkovnikov
I. hydroboration-oxidation
A. syn-addition stereochemistry
e. dihalogenation: addition of X to yield 1,2-dihaloalkanes
2
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 :CH to yield cyclopropanes
2
i. almost any carbene, :CR , will work
2
ii. most commonly used reaction: the Simmons-Smith reaction
2 CH I + Zn(Cu) ÷ ICH ZnI + “:CH ”
2 2 2 2
3
h. dihydroxylation: addition of OH and OH to C=C to yield 1,2-
dihydroxyalkanes (common name: glycols)
i. cold, dilute, aqueous alkaline KMnO (yields only fair)
4
ii. OsO (yields are good)
4
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 H with a noble metal catalyst to yield alkanes
2
a. syn-addition stereochemistry
3. C=C cleavage reactions
a. ozonolysis: addition of 2 O to yield formaldehyde, aldehydes, or
ketones
b. KMnO in base or acid: addition of $ 2O to yield CO , carboxylic
4 2
acids, or ketones
4. Cleavage of 1,2-diols
a. HIO yields two carbonyl campounds
4
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(OCCH )
3 2
2. NaBH , HO-
4
Br
c.
O
CH COOH
3
O
Cl
Cl CH COH
3
d.
xs H
2
Pt
e.
OH
Br
2
H O, dark
2
0 EC
f.
H
2
Pd/C
5
2. Provide brief answers for each of the following questions:
a. Circle which compound undergoes catalytic hydrogenation faster:
CH H
3
E F
b. Draw the structure of the intermediate bromonium ion formed when Br reacts with (Z)-2-
2
butene 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
+
Br
+
________ ________ ________
6
d. Each of the following isomeric compounds of formula C H release heat energy when
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hydrogenated with excess H using Pt as catalyst. Rank the compounds in order of relative
2
heat of hydrogenation, from largest (1) to smallest (4) heat of hydrogenation:
e. Draw the structure of the only primary alcohol of molecular formula C H O which cannot
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be prepared from an alkene.
3. Muscalure is the common name given to a hydrocarbon pheromone of molecular formula
C H which the female housefly Musca domestica secretes to attract male houseflies.
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Ozonolysis of this compound, followed by workup using Zn and H O, yields two aldehydes
2
with formulas CH (CH ) CHO and CH (CH ) CHO. What is the structure of muscalure?
3 2 7 3 2 12
7
4. When (6Z)-2,6-dimethyl-2,6-octadiene is heated with H SO for two hours, a new compound,
2 4
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 H SO : H O to produce three isomeric alcohols, A, B, and C:
2 4 2
OH
OH OH
+ +
A 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.
8
b. Would analogous oxymercuration–demercuration using aqueous Hg(CO CH ) ,
2 3 2
followed by NaBH in aqueous base, yield the same mixture of isomeric alcohol products as
4
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 Cl in the dark at 0 EC, a single isomer of molecular
2
formula C H Cl is produced.
6 12 2
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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