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Alkenes and Alkynes I. Addition Reactions

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Title: Alkenes and Alkynes I. Addition Reactions


1
Alkenes and Alkynes I.Addition Reactions
  • Chapter Eight

2
Types of Additions
gt
3
Sect 8.1 Addition Reactions
4
Addition of H-X
5
Addition of Water
6
Sect. 8.2 Introduction to Mechanisms
  • mechanism
  • two step
  • rate determining step (i.e. slow step)
  • energy diagram
  • transition states
  • intermediates

7
MECHANISM
STEP-BY-STEP ACCOUNT OF WHAT HAPPENS
X-
X
step 1
step 2
C
C
C
C
C
C

E
E

E
intermediates are formed during a reaction but
are not products
Intermediate
8
ENERGY PROFILE
two step reaction
intermediate
TS1
E N E R G Y
TS2
-
X
C
C

E
step 1
step 2
DH
C
C
X
product
C
C

E
E
9
ACTIVATED COMPLEXES
correspond to transition states for each step
-
X
X
C
C
C
C
C
C

E
E
intermediate

E
?-
X
?
C
C
C
C
?
E
?
E
show bonds in the process of breaking or forming
ACTIVATED COMPLEXES
(bonds are half formed or half broken)
10
Sect 8.3 Electrophilic Addition to a Double Bond
11
Hyperconjugation
12
CARBOCATION STABILITY
HYPERCONJUGATION
H
..
electrons in an adjacent C-H s bond help to
stabilize the positive charge of the carbocation
by proximity (overlap)
R

C
C
H
R
H
R
C
R
C
H
R
R
C
H
R
R
Most stable
Least stable
2
gt
gt



tertiary
secondary
primary
13
Can you recognize the following carbocations? 1o,
2o, 3o
2 o
2 o
1 o
2 o
2 o
3 o
14
Sect 8.4 Addition of Hydrogen Halides
Markovnikovs rule
15
Markovnikovs Rule
  • In the ionic addition of an acid to the
    carbon-carbon double bond of an alkene, the
    hydrogen of the acid attaches itself to the
    carbon atom which already holds the greater
    number of hydrogens.
  • Them that has, gets!
  • The richer get richer! (V. W.
    Markovnikov -- 1838 - 1904)

16
Markovnikov
17
MARKOVNIKOV RULE
When adding HX to a double bond the hydrogen of
HX goes to the carbon which already has the most
hydrogens
C
H
C
H
3
2
C
l
HCl
..... conversely, the anion X adds to the
most highly substituted carbon ( the carbon
with most alkyl groups attached).
18
REGIOSELECTIVE REACTION
C
H
C
H
C
H
3
3
3
HCl
C
C
H
C
H
C
C
H
C
H

C
H
C
H
C
H2
3
2
3
3
3
C
l
C
l
major
minor
one of the possible products is formed in larger
amounts than the other one
Compare
REGIOSPECIFIC
only one of the possible products is formed
(100).
19
Mechanism (Markovnikov)
Secondary C
Major product
20
Mechanism (anti-Markovnikov)
Primary carbocation
Minor!
21
COMPETING PATHWAYS
rate-determining step
higher energy intermediate
slower
lower energy intermediate
faster
1 o
2 o
rate-determininng (slow) step
22
Markovnikov Addition to an Alkene
23
Mechanism
3 o C
24
SOME ADDITIONAL EXAMPLES
only major product is shown
C
H
C
H
3
3
C
l
HCl
C
H
C
H
3
2
C
l
HCl
C
H
C
H
C
H
C
H
3
2
HCl
C
l
25
Sect 8.5 Addition of Sulfuric Acid to an Alkene
26
ALKYL HYDROGEN SULFATES
O
-
O
S
O
H
O
S
O
H
O
3
SLOW
C
C
C
C
C
C

O
H
H
H
O
S
O
H
alkyl hydrogen sulfate
O
cold
water room temp
FOLLOWS MARKOVNIKOFF RULE
27
Addition of Water to an Alkene
28
Mechanism of Hydration
29
Sect 8.6 Addition of Bromine to an Alkene
30
ADDITION OF BROMINE
B
r
B
r
C
C
l
4
C
C
C
C
C
C
SLOW

B
r
B
r
B
r
B
r
d-
d

B
r
B
r
alkene polarizes bromine
31
THE REACTION IS STEREOSPECIFIC
ANTI ADDITION
-
anti
B
r
H
H
open carbocation would give both cis and trans

B
r
H
syn
B
r
H
B
r
syn
anti
H
H
H
B
r
B
r
B
r
B
r
H
cis compound
trans compound
NOT OBSERVED
ACTUAL PRODUCT
32
WHAT WOULD EXPLAIN FORMATION OF ONLY THE trans
PRODUCT ?
..... A BRIDGED OR CYCLIC INTERMEDIATE
33
CYCLIC BROMONIUM ION
note size of bromine

34
BRIDGED BROMONIUM ION
Br

Br
Br
bridging blocks approach from this side
35
FORMATION OF ENANTIOMERS
symmetric intermediate
Addition could also start from the top with
bromide attacking the bottom.

ENANTIOMERS
36
ADDITION OF BROMINE TO 2-BUTENE


C
H
C
H
C
H
C
H
C
H
C
H
C
H
C
H
3
3
3
3

B
r
B
r
B
r
2
2n possible stereoisomers
C
H
H
C
H
C
H
3
3
3
H
C
H
H
H
3
cis-2-butene
trans-2-butene
WILL THESE STEREOISOMERS GIVE THE SAME PRODUCTS?
37
NO ! THEY GIVE DIFFERENT PRODUCTS
cis
trans
enantiomers
meso
These results can only be explained by
stereospecific anti addition.
38
-
-
cis
ADD TO RIGHT
ADD TO LEFT
ROTATE
ROTATE
C
H
ENANTIOMERS
39
_
_
trans

ADD TO RIGHT
ADD TO LEFT
ROTATE
ROTATE
MESO
IDENTICAL (also meso)
40
Bromination of an Unsymmetrical Alkene
41
Stereochemistry of Bromination of Alkenes
  • Simple alkenes Addition of bromine or chlorine
    goes exclusively anti, with the formation of a
    bridged ion
  • If a resonance-stabilized open-chain carbocation
    is possible, there may be a mixture of
    mechanisms, with some molecules reacting via a
    bridged ion and some molecules reacting via an
    open-chain carbocation

42
Stereochemistry of Bromination of Alkenes--Part
Two
  • In cases where a resonance-stabilized carbocation
    is possible, if the solvent is made more polar
    (acetic acid or nitromethane), the proportion of
    molecules reacting via an open-chain carbocation
    increases.
  • For simple alkenes, changing solvents has little
    or no effect on stereochemistry.

43
Sect 8.7 Halohydrin Formation
Br2 H2O
HO-Br
HBr
44
Mechanism
45
Sect. 8.8 Carbocation Rearrangements
46
Sect 8.10 Free Radical Addition of HBr to
Alkenes (anti-Markovnikov!)
47
ADDITION OF HBr
Markovnikov Addition
Oxygen
Anti-Markovnikov Addition
48
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49
STABILITY OF CARBON RADICAL INTERMEDIATES
Radicals are electron-deficient just like
carbocations and have the same stability order.
lowest energy
highest energy
tertiary
secondary
primary
methyl
and they are stabilized by resonance and / or
hyperconjugation.
(
)
( )
etc.
( )
C
H
2
( )
50
Sect. 8.11 and 8.12 Hydrogenation of Alkenes
and alkynes
51
Hydrogenation of Alkenes
Also PtO2 -- sometimes Ru, Rh, or Re
52
HYDROGEN ADSORBS ON THE SURFACE OF THE CATALYST
.
n H2 Pt
Pt(H )2n
finely-divided particles dispersed in solution
53
MECHANISM OF HYDROGENATION
CATALYST
54
MECHANISM OF HYDROGENATION
HYDROGEN ADSORBS
55
MECHANISM OF HYDROGENATION
ALKENE APPROACHES
56
MECHANISM OF HYDROGENATION
ALKENE PICKS UP TWO HYDROGENS
57
MECHANISM OF HYDROGENATION
R
R
R
R
ALKANE IS FORMED
.
.
58
BOTH HYDROGENS ADD TO THE SAME SIDE OF THE DOUBLE
BOND
not observed
anti addition
X
syn addition
59
EXAMPLES
60
Sect. 8.14 Addition Polymers
61
Polymerization of propene
62
Indirect Hydration
  • Oxymercuration-Demercuration
  • Markovnikov product formed
  • Anti addition of H-OH
  • No rearrangements
  • Hydroboration
  • Anti-Markovnikov product formed
  • Syn addition of H-OH

63
Hydroboration
  • Borane, BH3, adds a hydrogen to the most
    substituted carbon in the double bond.
  • The alkylborane is then oxidized to the alcohol
    which is the anti-Mark product.

64
Borane Reagent
  • Borane exists as a dimer, B2H6, in equilibrium
    with its monomer.
  • Borane is a toxic, flammable, explosive gas.
  • Safe when complexed with tetrahydrofuran.

65
Predict the Product
Predict the product when the given alkene reacts
with borane in THF, followed by oxidation with
basic hydrogen peroxide.
66
Epoxidation
  • Alkene reacts with a peroxyacid to form an
    epoxide (also called oxirane).
  • Usual reagent is peroxybenzoic acid.

67
One-Step Reaction
  • To synthesize the glycol without isolating the
    epoxide, use aqueous peroxyacetic acid or
    peroxyformic acid.
  • The reaction is stereospecific.

68
Syn Hydroxylation of Alkenes
  • Alkene is converted to a cis-1,2-diol,
  • Two reagents
  • Osmium tetroxide (expensive!), followed by
    hydrogen peroxide or
  • Cold, dilute aqueous potassium permanganate,
    followed by hydrolysis with base

    gt

69
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