Topic 5E Reactions of alkenes and alkynes - PowerPoint PPT Presentation

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Topic 5E Reactions of alkenes and alkynes

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Acids are sources of electrophilic H ... Two electrons from bond form bond with H , an electrophile ... that addition of an electrophile gives the most stable ... – PowerPoint PPT presentation

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Title: Topic 5E Reactions of alkenes and alkynes


1
Topic 5E Reactions of alkenes and alkynes
2
Reaction Mechanisms
52
  • In an organic reaction
  • we break bonds and form bonds, and
  • these bonds are covalent electron pairs are
    involved
  • A mechanism describes the sequence in which bond
    breaking and bond formation occurs as well as how
    the energy of the system changes during a process
  • Use arrow notation to depict movement of
    electrons and energy profile diagrams to depict
    energy changes during that process.

3
Reaction Energy Profiles
53
  • Exothermic reaction pathway

4
Reaction Energy Profiles
53
  • Endothermic reaction pathway

5
A two-step reaction
54
  • Intermediates lie in shallow energy wells
  • Rate-determining step step with highest EA

6
Addition reactions summary
55
RCH
X
H
RCH
7
Addition reactions summary
37
RCH
X
H
RCH
8
Addition of Hydrogen Halides
55
H
Br
Br
Ethyl bromide
  • Hydrogen halides react with alkenes
  • Alkyl halides are formed

9
Addition of Hydrogen Halides
56
  • Two products are possible
  • Only 2-chloropropane is formed
  • H always adds to side of double bond with most
    hydrogens the Markovnikov rule

10
Mechanism
57
Acids are sources of electrophilic H H is
attacked by the ? electon pair of???bond leaving
a carbocation a positive carbon
(Double headed arrows for electron pairs)
11
Mechanism animation
57
Addition of HCl to 2-methylpropene
Ocol, Bcorgchm CD ROM McMurry 2.0
12
Mechanism in detail
57
H
H
H
H
H
H
  • Two electrons from ? bond form ? bond with H, an
    electrophile
  • This leaves sp2 carbon with five electrons, a
    carbocation

13
Mechanism in detail
58
H
Cl
H

H
H
H
H
CH3
H
CH3
H
  • Finally Cl , a nucleophile,donates a pair of
    electrons to carbocation forming a CCl ? bond
  • Both carbons are sp3 hybridised.
  • HCl has added across the double bond

14
Mechanism in detail
58
secondary carbocation

primary carbocation
tertiary carbocation
primary carbocation
Why?
15
Classification of carbocations
58




C
C
C
H
H
H
Primary
Secondary
Tertiary
  • Three Groups attached to cation centre
  • Two Groups attached to cation centre
  • One Group attached to cation centre

16
Stability of carbocations
59
  • Alkyl groups push electrons through sigma bonds,
    they are electron donating
  • They are positively INDUCTIVE (I) and charge is
    stabilised through delocalisation or dispersion
  • The more alkyl groups the greater the stability
  • Inductive effects operate over only one to two
    bonds


17
Mechanism in detail
59
secondary carbocation

primary carbocation
tertiary carbocation
primary carbocation
Why?
18
Stability of carbocations
59

Energy
Reaction Coordinate)
  • More stable carbocation is formed more easily
  • It is a lower activation energy process

19
Other additions of HX
60
An addition of HBr

C
C
H
20
Other additions of HX
61
An addition of HI

I

C
C
3-iodo-3-methylpentane
21
Summary
61

Markovnikov's Rule
  • Positive part adds to the carbon of the double
    bond which has the greater number of hydrogens
    attached to it
  • Today this is better stated that addition of an
    electrophile gives the most stable carbocation
  • This is a general rule for addition to alkenes
  • All alkenes can be expected to react in this
    manner

22
Alkene addition reactions
61
RCH
X
H
RCH
23
Hydration of alkenes
61
  • An example of Markovnikov addition of water

24
Hydration of alkenes
61
Mechanism

25
Alkene addition reactions
63
RCH
X
H
RCH
26
Addition of halogen molecules
63
RCH
27
General mechanism
63

Br
Br
X
A "bridged bromonium ion" is formed
Addition is trans.
28
Bromination of bacon fat
63
Bacon fat contains unsaturated fats which add
bromine
Movie from Saunders General Chemistry CD-ROM
29
Addition of bromine, Br2
63
Addition is trans.
30
Br2 addition to a ring
63
H
H

Br
  • A "bridged bromonium ion" is formed
  • Addition gives the trans product by anti addition

31
Anti-addition
63
Br
Br
Br
H

Br
H
H
Br
H
H
H
H
H

trans
Br
cis
  • A "bridged bromonium ion" explains why only trans
    is formed since only anti addition is possible
  • Stepwise addition would give the cis and trans
    product

32
Addition of chlorine
63
  • A bridged chloronium ion is formed
  • Cl attacks to give the trans addition product
  • The reaction is general for alkenes with halogens

33
Addition reactions summary
65
RCH
X
H
RCH
34
Cis addition to alkenes
64
Cis diol (glycol) formation with osmium tetroxide
  • A cyclic osmic ester is first formed
  • Water converts this to the cis diol

35
Diol (glycol) formation
65
Permanganate reacts similarly
  • A cyclic manganese ester is formed
  • Water converts this to the cis diol
  • Brown MnO2 is generated (purple colour of KMnO4
    lost)

36
Diol formation cyclic alkenes
65
H
H
H
H
O
OH
O
OH
  • Syn-addition to cyclic alkenes affords hydroxyl
    groups on the same face
  • Cyclic alkenes afford the cis-1,2-diol

37
Addition reactions summary
65
RCH
X
H
RCH
38
Addition of hydrogen, H2
65
R''
R
R''
R
High EA
R'
R'''
C
C
C
C
R'''
R'
H
H
  • Addition of hydrogen is exothermic by 120kJmol1
  • Addition has a very high activation energy though
  • With catalysts, addition occurs via a low energy
    path
  • Pt, Pd, Ni, Rh and Ru

39
Catalytic action
66
uncatalysed high EA
Energy
Catalysed (multistep, low EA each step)
Progress of reaction
  • Catalytic pathways may be multistep
  • Overall activation energy is lower

40
Schematic of catalytic hydrogenation
66
H
H
H
H
  • Hydrogen is absorbed onto the surface
  • HH bond weakened and hydrogens become atom-like

41
Schematic of catalytic hydrogenation
66
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
H
H
H
H
H
H
H
  • Hydrogens react with ?-bond (stepwise)
  • Hydrogens attach on same face (syn addition) to
    give cis product.

42
Cis addition to cyclic alkenes
67
H
H
  • Hydrogens add to one face of the alkene
  • The result is cis addition

43
Reaction of alkynes
  • Very similar to alkenes
  • They add halogens twice, X2
  • They add hydrogen halides twice, HX
  • They add hydrogen twice, H2
  • They add water with acid (hydration)
  • Markovnikov's rule applies
  • They are slightly acidic and react with strong
    bases

44
Addition of bromine

H
Br
H
C
C
H
C
C
Br
H
Br
H
C
C
H
Br
1,1,2,2-tetrabromoethane
  • Two molecules of bromine add successively

45
Addition of HBr

Br
HBr
HBr
CH
Br
Br
2,2-dibromopropane
  • Two molecules of HBr add successively
  • Markovnikov addition in both steps

46
Addition of H2


R
special catalyst
R
C
C
C
C
H
H
normal catalyst
R'
R
  • Normal catalysis leads to double addition
  • Less active catalysts allow syn addition of one
    molecule

47
Reactions as an acid
Liq. NH3


R
C
C
H
R
C
C

Strong base
Alkynide ion
  • Very strong base required

48
Alkynide formation
  • Anions (electron pairs) in sp hybrid orbitals
    are closer to the carbon nucleus
  • More stable than anions in sp2 or sp3 orbitals

49
Index of hydrogen deficiency (Double bond
equivalents)
67
C6H12
  • Hexane C6H14
  • Hydrogen deficiency is TWO
  • 2H is equivalent to either a double bond or a ring

50
Index of hydrogen deficiency
68
Equivalent to either
  • TWO double bond or
  • ONE ring and ONE double bond or
  • TWO rings

51
Combustion analysis
68
C 88.16
H 11.84
88.16
Then
C
7.341
12.01
11.84
H
11.75
1.008
  • Microanalysis gives the percentages of carbon and
    hydrogen
  • These, when divided by the atomic numbers, give
    the relative proportion of each element

52
Combustion analysis
69
Divide by lowest number
CH
x 2
C
H
1.6
2
3.2
x 3
C
H
3
4.8
x 4
C
H
4
6.4
molar mass 68
x 5
C
H
5
8
  • This the empirical formula
  • Compare to the weight of the compound
  • If the same, this is the molecular formula
  • If not the molecular formula will be a factor of
    the empirical formula
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