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Alkenes and Alkynes I. Addition Reactions

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For simple alkenes, changing solvents has little or no effect on stereochemistry. Sect 8.7 Halohydrin Formation Mechanism Sect. 8.8: ... – PowerPoint PPT presentation

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Title: Alkenes and Alkynes I. Addition Reactions


1
Alkenes and Alkynes I. Addition Reactions
  • Chapter Eight

2
Chapter 8, Problem Assignments (up to Sect 8.6)
  • In Text problems 1 b, c, d, e ,g 2 a, b, d, e
    3 4 b, c, d, f , i, k 5
  • 6 c, d, e 7 a, b, c, e 8, 9, 10, 11
  • 14
  • End of Chapter problems 31, 32, 35, 36, 41b 43
    c, d

3
Complete set of problems
  • In Text problems 1 b, c, d, e ,g 2 a, b, d, e
    3 4 b, c, d, f , i, k 5
  • 6 c, d, e 7 a, b, c, e 8, 9, 10, 11
  • 14 16 17 a, b 18, 19 21- 28
  • End of Chapter problems 31, 32, 35, 36
  • 41 b 43 c, d 45 49, 50, 52, 53 a
  • 55, 56, 58, 70 b, d, e, h, k, m
  • 72 a, b, d, e, h

4
Sect 8.1 Addition Reactions
5
Addition of H-X
6
Addition of Water
7
Sect. 8.2 Introduction to Mechanisms
  • mechanism
  • two step
  • rate determining step (i.e. slow step)
  • energy diagram
  • transition states
  • intermediates

8
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
9
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
10
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)
11
Sect 8.3 Electrophilic Addition to a Double Bond
12
Hyperconjugation
13
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
14
Can you recognize the following carbocations? 1o,
2o, 3o
2 o
2 o
1 o
2 o
2 o
3 o
15
Sect 8.4 Addition of Hydrogen Halides
Markovnikovs rule
16
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)

17
Markovnikov
Markovnikov
Not Markovnikov
18
(No Transcript)
19
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).
20
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).
21
Mechanism (Markovnikov)
Secondary C
Major product
22
Mechanism (anti-Markovnikov)
Primary carbocation
Minor!
23
COMPETING PATHWAYS
rate-determining step
higher energy intermediate
slower
lower energy intermediate
faster
1 o
2 o
rate-determininng (slow) step
24
Markovnikov Addition to an Alkene
25
Mechanism
3 o C
26
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
27
Sect 8.5 Addition of Sulfuric Acid to an Alkene
28
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
29
Addition of Water to an Alkene
30
Mechanism of Hydration
31
Sect 8.6 Addition of Bromine to an Alkene
32
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
33
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
34
WHAT WOULD EXPLAIN FORMATION OF ONLY THE trans
PRODUCT ?
..... A BRIDGED OR CYCLIC INTERMEDIATE
35
CYCLIC BROMONIUM ION
note size of bromine

36
BRIDGED BROMONIUM ION
Br

Br
Br
bridging blocks approach from this side
37
FORMATION OF ENANTIOMERS
symmetric intermediate
Addition could also start from the top with
bromide attacking the bottom.

ENANTIOMERS
38
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?
39
NO ! THEY GIVE DIFFERENT PRODUCTS
cis
trans
enantiomers
meso
These results can only be explained by
stereospecific anti addition.
40
-
-
cis
ADD TO RIGHT
ADD TO LEFT
ROTATE
ROTATE
C
H
ENANTIOMERS
41
_
_
trans

ADD TO RIGHT
ADD TO LEFT
ROTATE
ROTATE
MESO
IDENTICAL (also meso)
42
Bromination of an Unsymmetrical Alkene
43
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

44
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.

45
Sect 8.7 Halohydrin Formation
Br2 H2O
HO-Br
HBr
46
Mechanism
47
Sect. 8.8 Carbocation Rearrangements
48
Sect 8.10 Free Radical Addition of HBr to
Alkenes (anti-Markovnikov!)
49
ADDITION OF HBr
Markovnikov Addition
Oxygen
Anti-Markovnikov Addition
50
(No Transcript)
51
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
( )
52
Sect. 8.11 and 8.12 Hydrogenation of Alkenes
and alkynes
53
Hydrogenation of Alkenes
Also PtO2 -- sometimes Ru, Rh, or Re
54
HYDROGEN ADSORBS ON THE SURFACE OF THE CATALYST
.
n H2 Pt
Pt(H )2n
finely-divided particles dispersed in solution
55
MECHANISM OF HYDROGENATION
CATALYST
56
MECHANISM OF HYDROGENATION
HYDROGEN ADSORBS
57
MECHANISM OF HYDROGENATION
ALKENE APPROACHES
58
MECHANISM OF HYDROGENATION
ALKENE PICKS UP TWO HYDROGENS
59
MECHANISM OF HYDROGENATION
R
R
R
R
ALKANE IS FORMED
.
.
60
BOTH HYDROGENS ADD TO THE SAME SIDE OF THE DOUBLE
BOND
not observed
anti addition
X
syn addition
61
EXAMPLES
62
Sect. 8.12 Addition to Alkynes
63
Hydration of an Alkyne
64
Mechanism of Alkyne Hydration
Hg2
65
Keto-Enol Tautomerism
  • Tautomers Two compounds with different
    structures but which exist in equilibrium.
  • Most often, the two structures differ by the
    position of a proton.
  • NOTE CAREFULLY This is equilibrium, not
    resonance!

66
Sect. 8.13 Addition of HCN and acetic acid to
an alkyne
67
Sect. 8.14 Addition Polymers
68
Polymerization of propene
69
Polymer Problems
  • Be sure to look at the mechanism of free radical
    polymerization of styrene (p. 733-735).
  • In-Text Problems 8-27, 8-28, and 8-29
  • End of chapter problems 59, 60, and 62

70
Highlights of Chapter 8
  • HX addition to alkenes/ Markovnikovs Rule
  • Addition of water to alkenes, H catalyst
  • Addition of halogens to alkenes/ stereochem.
  • Addition of X2 / H2O to alkenes
  • Carbocation rearrangements
  • Free-radical addition of HBr (Anti-Mark.)
  • Hydrogenation
  • Additions to alkynes
  • Addition polymers
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