Transcript PowerPoint Presentation - 3D Alkenes and alkynes

Topic 5E
Reactions of alkenes and alkynes
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.
Reaction Energy Profiles
Exothermic reaction pathway:
Transition state
A
Activation energy barrier
B
E
n
er
yg
Reactants
C
Products
Reaction Coordinate
53
Reaction Energy Profiles
Endothermic reaction pathway:
Transition state
A
Activation energy barrier
B
E
n
er
g
y
Products
C
Reactants
Reaction Coordinate
53
A two-step reaction
54
• Intermediates lie in shallow energy wells
• Rate-determining step— step with highest EA
Transition state 1
A
Transition state 2
B
C
E
n
er
g
y
Intermediate
D
Products
E
Reactants
Reaction Coordinate
Addition reactions summary
RCH
HO
CH2
OH
RCH
CH2
X
H
HX
+
H /HOH
OsO4
or KMnO4
RCH
CH2
H2
X2
RCH
CH2
RCH
CH2
H
H
X
X
RCH
CH2
HO
H
55
Addition reactions summary
RCH
CH2
X
H
HX
RCH
CH2
37
Addition of Hydrogen
Halides
CH2
CH2
H Br
CH3
55
CH2
Ethyl bromide
• Hydrogen halides react with alkenes
• Alkyl halides are formed
Br
Addition of Hydrogen
Halides
CH3–CH=CH2 + HCl
CH3CH2CH2Cl
Not formed
CH3CHCH3
Cl
• Two products are possible
• Only 2-chloropropane is formed
• H always adds to side of double bond with
most hydrogens — the Markovnikov rule
56
Mechanism
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)
57
Mechanism animation
57
Addition of HCl to 2-methylpropene
QuickTime™ and a
Cinepak Codec by SuperMatch decompressor
are needed to see this picture.
Ocol, Bcorgchm
CD ROM
McMurry 2.0
Mechanism in detail
H+
H
CH 3
H
H
57
vacant pz
orbital
H
CH 3
H
H
sp3
• Two electrons from  bond form  bond with H+, an
electrophile
• This leaves sp2 carbon with five electrons, a carbocation
Mechanism in detail
Cl
H
CH3
–
sp3
H
+
sp2
H
H
Cl
H
H
CH3
H
H
sp3
• Finally Cl– , a nucleophile,donates a pair of electrons to carbocation
forming a C—Cl  bond
• Both carbons are sp3 hybridised.
• H—Cl has added across the double bond
58
Mechanism in detail
H+
CH3 CH
H+
CH3
C
CH3
CH2
58
secondary
carbocation
+
CH3 CHCH3 (not CH3 CH2 CH2 + ) primary
carbocation
tertiary
carbocation
CH3
CH2
+
C
CH3
Why?
CH3
H
not CH3 C
CH3
primary
+
CH2 carbocation
Classification of
carbocations
CH 3
+
C
H
H
Primary
• One Group
attached to
cation centre
CH3
+
C
CH3
58
CH3
+
C
CH3
CH3
H
Secondary
Tertiary
• Two Groups
attached to
cation centre
• Three Groups
attached to
cation centre
Stability of carbocations
59
RCH2 + << R2 CH+ < R3 C+
increasing carbocation stability
+
CH 3
•
•
•
•
+
C
+
H
CH3
+
C
+
CH3
+
CH3
+
C
+
CH3
H
+CH3
H
Primary
Secondary
Tertiary
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
Mechanism in detail
H+
CH3 CH
CH2
C
CH3
secondary
carbocation
+
CH3 CHCH3 (not CH3 CH2 CH2 + ) primary
carbocation
H+
CH3
59
tertiary
carbocation
CH3
CH2
CH3
+
C
CH3
H
not CH3 C
CH3
Why?
RCH2 + << R2 CH+ < R3 C+
increasing carbocation stability
primary
+
CH2 carbocation
Stability of carbocations
Energy
CH3CH2CH2 not formed
Ea (1°)
Ea (2°)
CH3CHCH3 observed reaction
CH3CH=CH2 + H +
Reaction Coordinate)
• More stable carbocation is formed more easily
• It is a lower activation energy process
59
Other additions of HX
60
An addition of HBr:
CH3
CH3
C C
CH3
H
H
+
CH3
CH3
Br–
C CH2 CH3
+
CH3
Br
2-bromo-2-methylbutane
CH3
CH CH
+
CH2 CH3
C
CH3
3 carbocation
not
CH3
CH3
2carbocation
Other additions of HX
61
An addition of HI:
CH3 CH2
C CH2
H+
CH3 CH2
CH3 CH2
CH3 CH2
+
C CH3
–
I
I
CH3 CH2 CCH2 CH3
CH3
3 carbocation
(not 1 )
3-iodo-3-methylpentane
An addition of HCl:
CH3
H+
+
CH3
CH3
Cl –
Cl
H
3 carbocation
1-Chloro-1-methylcyclohexane
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
61
Alkene addition reactions
RCH
CH2
X
H
HX
RCH
+
H /HOH
CH2
RCH
CH2
HO
H
Hydration of alkenes
H+
CH3 CH
CH2
H2 O
61
OH
CH3 CHCH3
2-propanol
(an alcohol)
•An example of Markovnikov addition of water
Hydration of alkenes
Mechanism:
61
63
Alkene addition reactions
RCH
CH2
X
H
HX
RCH
+
H /HOH
CH2
X2
RCH
CH2
X
X
RCH
CH2
HO
H
Addition of halogen
molecules
RCH
63
CH2
X2
CH2
CH2
Br2
BrCH2 CH2 Br
1,2-dibromoet hane
CH3 CH
CH2 + Cl2
CH3 CH
Cl
CH2
Cl
RCH
CH2
X
X
General mechanism
Br
CH2
63
Br
CH2
+
CH2CH2Br + Br –
X
H
but
Br
Br+
H
H
H
H
H
then Br – H
A "bridged bromonium ion" is formed
Addition is trans.
H
Br
Bromination of bacon fat
63
Bacon fat contains unsaturated fats which
add bromine
Movie from Saunders
General Chemistry
CD-ROM
QuickTime™ and a
Graphics decompressor
are needed to see this picture.
Addition of bromine, Br2
Br2
63
H
Br
Br
H
Br
H
trans-1,2-dibromocyclohexane
Addition is trans.
Br
H
Diequat orial
conformat ion
Br2 addition to a ring
Br+
H
Br–
63
Br
H
H
Br
H
trans-1,2-dibromocyclohexane
• A "bridged bromonium ion" is formed
• Addition gives the trans product by anti addition
Anti-addition
Br–
+
H
Br–
H
Br
Br Br
H
H H
cis
Br H
trans
63
Br
Br+
H
Br–
H
cis and trans-1,2-dibromocyclohexane
• 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
Addition of chlorine
Cl2
Cl
–
Cl
• A bridged chloronium ion is formed
• Cl– attacks to give the trans addition product
• The reaction is general for alkenes with halogens
63
H
Cl
Cl H
Addition reactions summary
RCH
HO
CH2
OH
RCH
CH2
X
H
HX
+
H /HOH
OsO4
or KMnO4
RCH
CH2
X2
RCH
CH2
X
X
RCH
CH2
HO
H
65
Cis addition to alkenes
64
Cis diol (glycol) formation with osmium tetroxide:
OsO2
R
R'
C
C
O
O
Os
O
O
R''
R'''
OsO4
R
R' C
R''
C R''' H2 O
O
O
Os
O
O
• A cyclic osmic ester is first formed
• Water converts this to the cis diol
R
R'
C
HO
C
R''
R'''
OH
cis-1,2-diol
Diol (glycol) formation
65
Permanganate reacts similarly:
R
R'
C
O
C
O
Mn
O
O
R''
R'''
MnO4
–
R
R' C
O
R''
C R''' H2 O
O
–
Mn
O
O
MnO2
R
R'
C
HO
C
R''
R'''
OH
cis-1,2-diol
• A cyclic manganese ester is formed
• Water converts this to the cis diol
• Brown MnO2 is generated (purple colour of KMnO4 lost)
Diol formation — cyclic
alkenes
65
H
H
H
H
O
O
–
MnO
2
OH
OH
cis-1,2-cyclohexanediol
• Syn-addition to cyclic alkenes affords hydroxyl
groups on the same face
• Cyclic alkenes afford the cis-1,2-diol
Addition reactions summary
RCH
HO
CH2
OH
RCH
CH2
X
H
HX
+
H /HOH
OsO4
or KMnO4
RCH
CH2
H2
X2
RCH
CH2
RCH
CH2
H
H
X
X
RCH
CH2
HO
H
65
Addition of hydrogen, H2
R
C
R'
•
•
•
•
High EA
R''
+ H2
C
R
R'
R'''
Addition of hydrogen is exothermic by 120kJmol–1
Addition has a very high activation energy though
With catalysts, addition occurs via a low energy path
Pt, Pd, Ni, Rh and Ru
H
C
65
C
R''
R'''
H
Catalytic action
Energy
uncatalysed high EA
Catalysed (multistep, low EA each step)
Progress of reaction
• Catalytic pathways may be multistep
• Overall activation energy is lower
66
Schematic of catalytic
hydrogenation
H
H
metal
surface
H
66
H
metal
surface
• Hydrogen is absorbed onto the surface
• H—H bond weakened and hydrogens become
atom-like
Schematic of catalytic
hydrogenation
R
R
R
R
RR
H
H
RR
H cis-addition
H
metal
surface
metal
surface
• Hydrogens react with -bond (stepwise)
• Hydrogens attach on same face (syn addition) to give cis
product.
66
Cis addition to cyclic
alkenes
CH3
CH3
H2 /Pt
CH3
H
H
CH3
cis-1,2-dimethylcyclohexane
• Hydrogens add to one face of the alkene
• The result is cis addition
67
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
Addition of bromine
H
C
C H + Br2
Br
C C
H
Br
Br
H
C
H
H
C
+ Br2
Br
Br2CH
CHBr2
1,1,2,2-tetrabromoethane
• Two molecules of bromine add successively
Addition of HBr
CH3 C
CH
HBr
CH3 C
Br
CH2
HBr
Br
CH3 CCH3
Br
2,2-dibromopropane
• Two molecules of HBr add successively
• Markovnikov addition in both steps
Addition of H2
R
C
C
R
+ H2
special catalyst
R
R
C
H
C
H
normal catalyst
R
CH2 CH2
R'
• Normal catalysis leads to double addition
• Less active catalysts allow syn addition of one
molecule
Reactions as an acid
R C
C H
–
NH2
Liq. NH3
Strong base
• Very strong base required
R
C
C
–
+
Alkynide ion
NH3
Alkynide formation
s
% s charact er: 100
sp
s p2
s p3
50
33
25
Elect rons further from positive nucleus
• Anions (electron pairs) in sp hybrid orbitals are
closer to the carbon nucleus
• More stable than anions in sp2 or sp3 orbitals
Index of hydrogen
deficiency
(Double bond equivalents)
1-pentene
cyclohexane
C6H12
• Hexane C6H14
• Hydrogen deficiency is TWO
• 2H is equivalent to either a double bond or a
ring
67
Index of hydrogen
deficiency
68
H2 C
H
C
CH 2
1-pentyne
cyclopentene
C5H12 – C5H8 = 2 x H2
Equivalent to either
• TWO double bond or
• ONE ring and ONE double bond or
• TWO rings
H2 C
C
H
Combustion analysis
C: 88.16%
H: 11.84%
Then
88.16
C = 12.01 = 7.341
11.84
H = 1.008 = 11.75
• Microanalysis gives the percentages of carbon
and hydrogen
• These, when divided by the atomic numbers,
give the relative proportion of each element
68
Combustion analysis
69
Divide by lowest number:
CH1.6 x 2 = C2H3.2
x3
C 3H4.8
x4
C4H6.4
•
•
•
•
molar mass 68
x5
C5H8
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