C341/Spring 2010
Chapter 6
Chapter 6 Outline: Alkenes
1. Structure of Carbocations
2. Addition Reactions to Alkenes
A. Addition of HX (hydrohalogenation)
B. Addition of H2O (hydration)
C. Addition of X2 (halogenation)
D. Addition of X2/H2O (halohydrin)
E. Addition of H2O without rearrangement (oxymercuration)
F. Hydroboration-Oxidation
3. Oxidation/Reduction Reactions
A. Reduction of Alkenes
B. Oxidative Cleavage of Alkenes
C. Forming Cis Diols
There are only 5 primary types of reactions in organic chemistry
1. Acid-Base (chapter 4) – involves identifying the acids & bases
2. Oxidation/reduction (ch. 6) – change in oxidation number at C
3. Addition (ch. 6-7) – involves identifying NuӨ, E+
4. Substitution (ch. 9) – involves identifying NuӨ, E+ & LG
5. Elimination (ch. 9)– involves identifying the base & LG
Your success in organic chemistry will be based on whether you can understand and
can predict the reactions we do, NOT JUST MEMORIZE THEM!
Although there may be some memorization, I will try and minimize this and teach
you to understand why reactions occur as they do.
It would be BEST if you did ALL the textbook problems, but at the very least
do these: 6.14-6.20, 6.27, 6.29-6.33, 6.34a, 6.35, 6.36, 6.39, 6.41, 6.44, 6.47, 6.49
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C341/Spring 2010
Chapter 6
Road Map to where we are going:
Addition Reactions:
X
OH (OR)
(+ E)
(+ E)
X
X
Chapter 9
strong base
OH
X2
CCl4
X
X2
H2O or ROH
H3PO4, H2O
H3PO4
H2O
heat
OH
(+ E)
Chapter 10
POCl3, pyr
OH
1.BH3(THF)
2. NaOH, H2O2
1.Hg(OAc)2, H2O
2.NaBH4
(+ E)
OH
(+ E)
OH
Oxidation/Reduction Reactions:
H
O
H2
Pd, high P
1. O3
2. Me2S
H
O
H
KMnO4, NaOH, H2O
or
1. OsO4
2. NaHSO3, H2O
OH
OH
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+ E
C341/Spring 2010
Chapter 6
1. Structure of Carbocations –
“Intermediates” are the species formed on the way to products. They are less stable
than the reactants and products; hence, they are short-lived and always proceed to
products.
What is the geometry/hybridization around a carbocation?
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C341/Spring 2010
Chapter 6
Resonance in the carbocation adds stability (I think you could have guessed this!)
Allylic carbons are identified as carbocations that are adjacent to a double bond.
For example:
What kind of carbocations are these and which form is more stable?
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Chapter 6
Skeletal Rearrangements
Intermediates can and will undergo hydride and methyl
rearrangements during the reactions:
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C341/Spring 2010
2.
Chapter 6
Addition Reactions - Electrophilic Addition Reactions
The bulk of this chapter is dictated by Addition Reactions:
SYN
ANTI
Two modes of addition are possible – syn or anti addition
Alkenes are nucleophiles, so we will react them with electrophilic reagents.
What is the driving force of addition reactions to alkenes?
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C341/Spring 2010
Chapter 6
A. Hydrohalogenation – Electrophilic Addition of HX
This is a Regioselective Reaction:
H
Br
+
H
HBr
Regioselective reaction =
Markovnikov’s Rule =
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Br
C341/Spring 2010
Chapter 6
Mechanism?
+
HBr
+
HBr
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C341/Spring 2010
Chapter 6
Predict the products for the following reactions:
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C341/Spring 2010
Chapter 6
Provide the mechanism explaining the following reaction:
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C341/Spring 2010
B.
Chapter 6
Hydration—Electrophilic Addition of Water
o In the presence of a non-halide containing acid, like H2SO4, water adds
to an alkene to give an alcohol. (This reaction occurs in equilibrium
and is heat dependent.)
o Mechanism occurs through a carbocation; hence, the most substituted
alkene product is always favored (rearrangements can occur).
General Reaction:
Could you use these conditions to make primary alcohols?
Practice drawing products before we try the mechanism:
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C341/Spring 2010
Mechanism?
Chapter 6
Acid-catalyzed hydration
H2SO4
H 2O
OH
A catalyzed reaction mechanism always regenerates the catalyst.
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C341/Spring 2010
Chapter 6
C. Halogenation—Addition of X2
o Usually Br2 and Cl2 is added to alkenes at room temperature by
adding across the double bond.
o The reaction can be run “neat” or in an “inert” solvent like CCl4 or
CH2Cl2.
General Reaction:
o Halogens add to π-bonds because halogens are polarizable (can
become electrophiles).
o The electron rich double bond induces a dipole in an approaching
halogen molecule, making one halogen atom electron deficient and
the other electron rich
o The electrophilic halogen atom is then attracted to the nucleophilic
double bond, making addition possible.
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C341/Spring 2010
Chapter 6
The reaction is stereoselective yielding the halogens anti addition.
+ Br2
Br
CCl4
Br
+
Br
Br
“anti-addition”
What is the relationship between these two?
Mechanism?
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C341/Spring 2010
D.
Chapter 6
Halohydrin Reaction - Addition of X2/H2O
General Reaction:
What observation(s) do you notice about how the substituents added?
Mechanism?
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C341/Spring 2010
E.
Chapter 6
Oxymercuration-Reduction (NOT IN THE TEXTBOOK)
Reaction is:
Compare to:
Reagents are:
1. Hg(OAc)2 is:
2. NaBH4 is:
The reaction brings about a hydration of an alkene, with the –OH moiety
on the most substituted carbon.
This reaction is regiospecific:
o The product is a result of Markovnikov addition
o These reactions occur WITHOUT rearrangement.
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Chapter 6
Mehcanism?
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C341/Spring 2010
F.
Chapter 6
Hydroboration-Oxidation (last type of hydration of an alkene)
Reaction:
1. BH3(THF)
2. NaOH, H2O2
OH
o These reactions occur with syn addition and WITHOUT rearrangement due
to a concerted mechanism that does not go through a carbocation.
What is BH3(THF)? BH3 does not exists as a single species, but it exists as a
Lewis acid/Lewis base adduct:
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C341/Spring 2010
Chapter 6
Mechanism?
First step is syn addition; the second step (oxidation) shows retention of
configuration (we will not discuss the mechanism):
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C341/Spring 2010
Chapter 6
Compare all three hydration reaction conditions:
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C341/Spring 2010
Chapter 6
3. Oxidation/Reduction Reactions
How to Recognize Redox Reactions:
• Oxidation:
loss of C—H, the gain of C—Z, or both
• Reduction:
the gain of C—H, the loss of C—Z, or both
What is Z?
One direction of a reaction will be oxidation and the opposite direction will be
reduction:
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C341/Spring 2010
Chapter 6
Identify the following reactions as oxidation (O), reduction (R) or neither (N).
a.
b.
c.
d.
e.
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C341/Spring 2010
Chapter 6
Recognizing Oxidizing & Reducing Reagents
Reducing agents contain hydrogen or hydride:
o H2 in the presence of a heavy metal, Pt, Pd Ni
Oxidizing agents contain O, an O—O, or an M—O bond:
o O2, O3 (ozone), H2O2 (hydrogen peroxide), (CH3)3COOH (tert-butyl
hydroperoxide), and peroxyacids.
o The most common oxidizing agents with metal-oxygen bonds are H2Cr2O7,
Na2Cr2O7 and K2Cr2O7 and pyridinium chlorochromate (PCC)
o Also common reagents are KMnO4, OsO4 and Ag2O.
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C341/Spring 2010
Chapter 6
A. Reduction of Alkenes
Most alkenes react with H2 in the presence of a late transition metal catalyst like Pd,
Pt, Ru or Ni.
o Yields are nearly quantitative (i.e. ca. 99%).
o Process is catalytic, needing only small amounts of metal
What kind of addition do you observe below?
ca. 85%
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15%
C341/Spring 2010
Chapter 6
B. Oxidative Cleavage of Alkenes
o Oxidative cleavage of an alkene breaks both the σ and π bonds of the
double bond to form two carbonyl compounds.
o Cleavage with ozone (O3) is called ozonolysis.
Second step can be:
o Reductive workup: Zn (in H2O) or dimethylsulfide (CH3SCH3) are
two common reagents used are used in the second step to afford the
carbonyl compounds.
o Oxidative workup: H2O2 in ROH
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C341/Spring 2010
Chapter 6
Ozonolysis – cleavage of a C=C bond to form carbonyls
1. O3
2. (CH3)2S
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C341/Spring 2010
C.
Chapter 6
Dihydroxylation of alkenes –
o Depending on the reagent, the two new OH groups can be added to the
opposite sides (anti addition) or the same side (syn addition) of the
double bond.
o Only syn hydroxylation is learned now and anti addition will be
addressed in chapter 9.
Syn hydroxylation results when an alkene is treated with either KMnO4 or
OsO4.
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C341/Spring 2010
Chapter 6
Alkene Reactions. Provide correct product(s) for the following reactions
demonstrating correct stereochemistry in your products. If the product has an
enantiomer or diastereomer, then just write +E or +D as necessary. Finally, circle if
the reaction undergoes ANTI or SYN addition (if a mechanism undergoes both then
circle both words). Provide reagents for reactions A and B.
SYN
or
ANTI ?
SYN
HBr
HO
or
ANTI ?
SYN
Br2, CCl4
or
ANTI ?
1. BH3(THF)
2. NaOH, H2O2
OH
B
A
H
O
CH3CO3H
SYN
or
ANTI ?
Br2, H2O
SYN
or
ANTI ?
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O
H2SO4, CH3OH
SYN
or
ANTI ?
C341/Spring 2010
Chapter 6
Mechanisms. Using mechanistic arrows provide step-by-step mechanisms
for BOTH of the following reaction to prepare the major product. As
always, correct placement of arrows and formal charges are necessary for
full credit.
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