Title: Ch. 8 - 1
1Chapter 8
- Alkenes and Alkynes II
- Addition Reactions
2About The Authors
- These PowerPoint Lecture Slides were created and
prepared by Professor William Tam and his wife,
Dr. Phillis Chang. -
- Professor William Tam received his B.Sc. at the
University of Hong Kong in 1990 and his Ph.D. at
the University of Toronto (Canada) in 1995. He
was an NSERC postdoctoral fellow at the Imperial
College (UK) and at Harvard University (USA). He
joined the Department of Chemistry at the
University of Guelph (Ontario, Canada) in 1998
and is currently a Full Professor and Associate
Chair in the department. Professor Tam has
received several awards in research and teaching,
and according to Essential Science Indicators, he
is currently ranked as the Top 1 most cited
Chemists worldwide. He has published four books
and over 80 scientific papers in top
international journals such as J. Am. Chem. Soc.,
Angew. Chem., Org. Lett., and J. Org. Chem. -
- Dr. Phillis Chang received her B.Sc. at New York
University (USA) in 1994, her M.Sc. and Ph.D. in
1997 and 2001 at the University of Guelph
(Canada). She lives in Guelph with her husband,
William, and their son, Matthew.
3- Addition Reactions of Alkenes
41A. How To Understand Additions to Alkenes
- This is an addition reaction ENu added across
the double bond
Bonds broken
Bonds formed
5- Since p bonds are formed from the overlapping of
p orbitals, p electron clouds are above and below
the plane of the double bond
p electron clouds
6- Electrophilic
- electron seeking
- CC and CC p bonds are particularly susceptible
to electrophilic reagents (electrophiles) - Common electrophile
- H, X (X Cl, Br, I), Hg2, etc.
7- In an electrophilic addition, the p electrons
seek an electrophile, breaking the p bond,
forming a s bond and leaving a positive charge on
the vacant p orbital on the adjacent carbon.
Addition of B to form a s bond provides an
addition product
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9- Electrophilic Addition ofHydrogen Halides to
Alkenes Mechanism and MarkovnikovsRule
10- Mechanism
- Sometimes do not go through a free carbocation,
may go via
11- Markovnikovs Rule
- For symmetrical substrates, no problem for
regiochemistry
12- Markovnikovs Rule
- But for unsymmetrical substrates, two
regioisomers are possible
13- Markovnikovs Rule
- In the electrophilic addition of an unsymmetrical
electrophile across a double bond of an alkene,
the more highly substituted and more stabilized
carbocation is formed as the intermediate in
preference to the less highly substituted and
less stable one
14Note carbocation stability ? 3o gt 2o gt 1o
15- Addition of Hydrogen Halides
- Addition of HCl, HBr and HI across a CC bond
- H is the electrophile
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172A. Theoretical Explanation ofMarkovnikovs Rule
- One way to state Markovnikovs rule is to say
that in the addition of HX to an alkene, the
hydrogen atom adds to the carbon atom of the
double bond that already has the greater number
of hydrogen atoms
18?
Step 1
Step 2
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20 212B. Modern Statement of Markovnikovs Rule
- In the ionic addition of an unsymmetrical reagent
to a double bond, the positive portion of the
added reagent attaches itself to a carbon atom of
the double bond so as to yield the more stable
carbocation as an intermediate
22 23 242C. Regioselective Reactions
- When a reaction that can potentially yield two or
more constitutional isomers actually produces
only one (or a predominance of one), the reaction
is said to be regioselective
252D. An Exception to Markovnikovs Rule
- Via a radical mechanism (see Chapter 10)
- This anti-Markovnikov addition does not take
place with HI, HCl, and HF, even when peroxides
are present
26- Stereochemistry of the IonicAddition to an Alkene
attack from top
racemate
achiral trigonal planar carbocation
attack from bottom
274. Addition of Sulfuric Acid to Alkenes
more stable 3o cation
less stable 1o cation
- Addition of HOSO3H across a CC bond
284A. Alcohols from Alkyl HydrogenSulfates
- The overall result of the addition of sulfuric
acid to an alkene followed by hydrolysis is the
Markovnikov addition of H and OH
29- Addition of Water to AlkenesAcid-Catalyzed
Hydration
- Overall process
- Addition of HOH across a CC bond
- H is the electrophile
- Follow Markovnikovs rule
305A. Mechanism
315B. Rearrangements
- Rearrangement can occur with certain carbocations
32- Alcohols from Alkenes throughOxymercurationDemer
curationMarkovnikov Addition
336A. Regioselectivity of Oxymercura-tionDemercura
tion
- Oxymercurationdemercuration is also highly
regioselective and follows Markovnikovs rule
346B. Rearrangements Seldom Occur
inOxymercurationDemercuration
- Recall acid-catalyzed hydration of some alkenes
leads to rearrangement products
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36- Rearrangements of the carbon skeleton seldom
occur in oxymercurationdemercuration
no rearrangement
376C. Mechanism of Oxymercuration
- Does not undergo a free carbocation
38- Stereochemistry
- Usually anti-addition
39- Although attack by water on the bridged
mercurinium ion leads to anti addition of the
hydroxyl and mercury groups, the reaction that
replaces mercury with hydrogen is not
stereocontrolled (it likely involves radicals).
This step scrambles the overall stereochemistry - The net result of oxymercurationdemercuration is
a mixture of syn and anti addition of H and OH
to the alkene
40- Solvomercuration-Demercuration
41- Alcohols from Alkenes throughHydroborationOxidat
ionAnti-Markovnikov Syn Hydration
- Addition of HBH2 across a CC bond
42- BH3 exists as dimer B2H6 or complex with
coordinative solvent
43syn addition
Anti-Markovnikov addition of H OH
44- Compare with oxymercuration-demercuration
anti addition
Markovnikov addition of H OH
45- Hydroboration Synthesis ofAlkylboranes
468A. Mechanism of Hydroboration
47 488B. Stereochemistry of Hydroboration
49- Oxidation and Hydrolysis ofAlkylboranes
B always ends up on the least hindered carbon
50 51 52 53- Overall synthetic process of hydroboration-oxidati
on-hydrolysis
- Overall anti-Markovnikov addition of HOH across
a CC bond - Opposite regioisomers as oxymercuration-demercurat
ion
54anti-Markovnikov syn addition
This oxidation step occurs with retention of
configuration
55- Summary of Alkene HydrationMethods
Summary of Methods for Converting Alkene to Alcohol Summary of Methods for Converting Alkene to Alcohol Summary of Methods for Converting Alkene to Alcohol Summary of Methods for Converting Alkene to Alcohol
Reaction Regiochemistry Stereochemistry Occurrence of Rearrangements
Acid-catalyzed hydration Markovnikov addition Not controlled Frequent
Oxymercuration-demercuration Markovnikov addition Not controlled Seldom
Hydroboration-oxidation Anti-Markovnikov addition Stereospecific syn addition of H and OH Seldom
56with rearrangement
Markovnikov addition of H2O without rearrangement
anti-Markovni-kov, syn addition of H2O
57- Protonolysis of Alkylboranes
- Protonolysis of an alkylborane takes place with
retention of configuration hydrogen replaces
boron where it stands in the alkylborane - Overall stereochemistry of hydroborationprotonoly
sis syn
58 59- Electrophilic Addition of Bromine and Chlorine
to Alkenes
- Addition of XX (X Cl, Br) across a CC bond
60 6112A. Mechanism of Halogen Addition
BrBr bond becomes polarized when close to alkene
(vincinal Dibromide)
(bromonium)
62- Stereochemistry
- Anti addition
63- Stereospecific Reactions
- A reaction is stereospecific when a particular
stereoisomeric form of the starting material
reacts by a mechanism that gives a specific
stereoisomeric form of the product
64 65- Addition of bromine to cis-2-Butene
(a)
(b)
66- Addition of bromine to trans-2-Butene
(a)
(b)
67- Halohydrin Formation
- Addition of OH and X (X Cl, Br) across a CC
bond - X is the electrophile
- Follow Markovnikovs rule
68 69- Other variation
- If H2O is replaced by ROH, RÖH will be the
nucleophile
70- Divalent Carbon CompoundsCarbenes
15A. Structure and Reactions of Methylene
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7215B. Reactions of Other Carbenes
Dihalocarbenes
- CX2 (e.g. CCl2)
- Generation by a-elimination of chloroform
73- Usually a syn (cis) addition across a CC bond
74 7515C. Carbenoids The Simmons-Smith
Cyclopropane Synthesis
76- A stereospecific syn (cis) addition across a CC
bond
77- Oxidation of AlkenesSyn 1,2-Dihydroxylation
- Overall addition of 2 OH groups across a CC bond
- Reagents dilute KMnO4 / OH? / H2O / cold or
OsO4, pyridine then NaHSO3, H2O
7816A. Mechanism for Syn Dihydroxylation of
Alkenes
79- Both reagents give syn dihydroxylation
80- Comparison of the two reagents
- KMnO4 usually lower yield and possibly side
products due to over-oxidation
(oxidative cleavage of CC)
- OsO4 usually much higher yield but OsO4 is
extremely toxic
81- Oxidative Cleavage of Alkenes
17A. Cleavage with Hot Basic Potassium
Permanganate
82 8317B. Cleavage with Ozone
84 85 86- Electrophilic Addition of Bromine Chlorine to
Alkynes
87- Addition of Hydrogen Halidesto Alkynes
- Regioselectivity
- Follow Markovnikovs rule
88 89- Anti-Markovnikov addition of hydrogen bromide to
alkynes occurs when peroxides are present in the
reaction mixture
90- Oxidative Cleavage of Alkynes
OR
91- How to Plan a SynthesisSome Approaches
Examples
- In planning a synthesis we often have to consider
four interrelated aspects - Construction of the carbon skeleton
- Functional group interconversions
- Control of regiochemistry
- Control of stereochemistry
9221A. Retrosynthetic Analysis
93Markovnikov addition of H2O
94anti-Markovnikov addition of H2O
9521B. Disconnections, Synthons, and Synthetic
Equivalents
- One approach to retrosynthetic analysis is to
consider a retrosynthetic step as a
disconnection of one of the bonds - In general, we call the fragments of a
hypothetical retrosynthetic disconnection Synthons
96(gem-dibromide came from addition of HBr across a
CC bond)
97synthons
disconnection
synthetic equivalent
98 9921C. Stereochemical Considerations
100- Retrosynthetic analysis
- The precursor of a vicinal dibromide is usually
an alkene - Bromination of alkenes are anti addition
(rotate 180o)
(anti addition of Br2)
(anti addition of H2)
101(anti addition of H2)
(anti addition of Br2)
102? END OF CHAPTER 8 ?