Organic Chemistry
13 – Reaction Mechanisms
Leaving Certificate Chemistry
The conversion of an
alkane to a chloroalkane
A free radical mechanism
chemical reactions of the alkanes
2003 Q. 6 (b) (3)
Reaction with Chlorine
2005 Q. 7(b) (3)
Formation of the Chloroalkanes
The Alkanes
e.g. Ethane
C2H6
Free Radical Mechanism
Ultraviolet Light
Substitution
A substitution
reaction
involving
ultraviolet light
The Chloroalkanes
e.g. Chloroethane
C2H5Cl
Mechanism of the reaction
How does this
reaction happen
and
How do we know?
The initiation (starting) step
• Fact: The reaction needs heat of over 300oC or
the presence of UV light to happen
This input of energy allows the reaction:
Cl2
.
2Cl Chlorine free radicals
This is called homolytic fission ( equal splitting)
Two chlorine free radicals (chlorine atoms with
an unpaired electron each) are made for each
photon of light.
Cl
Cl
Initiation
Ultraviolet light splits chlorine molecules into
extremely reactive chlorine free atoms
The propagation (repeating middle
steps)
• Fact: Hydrogen chloride is a major product
of this reaction
Reaction 1:
.
.
Cl + CH4
HCl + CH3
• Hydrogen chloride is formed
• A methyl free radical is formed
Cl
Cl
Hydrogen Chloride
formed
H
H
C
H
H
Methyl Radical
The propagation ( repeating middle
steps)
• Fact: Chloromethane is a major product of
this reaction.
Reaction 2:
.
.
CH3 + Cl2
CH3Cl + Cl
• Chloromethane is formed
• A new chlorine free radical is formed –
which can go on to cause more
propagation steps to happen.
• These middle steps are called a chain
reaction
Initiation
Ultraviolet light splits chlorine molecules into
extremely reactive chlorine free atoms
Cl
Cl
Hydrogen Chloride
formed
H
H
C
H
H
Chloromethane
Methyl Radical
Cl
formed
Cl
2007 Q. 11(b) (19)
The termination (ending) steps
Fact:
• The reaction can’t go on forever as the reactants run out
• Ethane is a minor product of the reaction
How the reaction eventually stops…
.
.
1. Cl + CH3
CH3Cl
.
.
2. CH3 + CH3
C 2H 6
.
.
3. Cl + Cl
Cl2
•
•
More chloromethane is formed
A small amount of ethane is also formed
Termination Steps
Cl
Cl
Chlorine formed
H3 C
CH3
Ethane (C2H6) formed
H3 C
Cl
Chloromethane formed
Evidence for this mechanism
1.
No reaction will occur in the absence of
ultraviolet light. If irradiation is stopped the
speed of the reaction slows down
2.
The formation of ethane (C2H6) in the
reaction
of
methane
with
chlorine
provides evidence of the formation of
methyl radicals (CH3 )
3.
Addition of a radical promoter (like
tetraethyl lead) speeds up the reaction.
Recap - The reaction of methane with
chlorine
In the presence of ultraviolet light
H
H
C
H
H
Chloromethane
Methane
Cl
Cl
Hydrogen
Chlorine
chloride
Initiation
Ultraviolet light splits chlorine molecules into
extremely reactive chlorine free atoms
Cl
Cl
Hydrogen Chloride
formed
H
H
C
H
H
Chloromethane
Methyl Radical
Cl
formed
Cl
2007 Q. 11(b) (19)
Termination Steps
Cl
Cl
Chlorine formed
H3 C
CH3
Ethane (C2H6) formed
H3 C
Cl
Chloromethane formed
Class activity
Try to work out the mechanism for the reaction
between ethane and chlorine. Include the
iniation, propagation and termination steps.
Facts:
• It is a substitution reaction
• UV light is needed for the reaction to happen at
room temperature
• The main products are Chlorine and
chloroethane.
• A minor product is butane
The reaction of ethane with chlorine
2003 Q. 6 (d) (9)
H
H
H
C
C
H
In the presence of ultraviolet light
H
H
Chloroethane
Ethane
Cl
Cl
Hydrogen
Chlorine
chloride
The initiation (starting) step
• Fact: The reaction needs heat of over 300oC or
the presence of UV light to happen
This input of energy allows the reaction:
Cl2
.
2Cl Chlorine free radicals
This is called homolytic fission ( equal splitting)
Two chlorine free radicals (chlorine atoms with
an unpaired electron each) are made for each
photon of light.
The propagation ( repeating middle
steps)
• Fact: Hydrogen chloride is a major product
of this reaction
Reaction 1:
.
.
Cl + C2H6
HCl + C2H5
• Hydrogen chloride is formed
• An ethyl free radical is formed
The propagation ( repeating middle
steps)
• Fact: Chloroethane is a major product of
this reaction.
Reaction 2:
.
.
C2H5 + Cl2
C2H5Cl + Cl
• Chloroethane is formed
• A new chlorine free radical is formed –
which can go on to cause more
propagation steps to happen.
• These middle steps are called a chain
reaction
The termination (ending) steps
Fact:
• The reaction can’t go on forever as the reactants run out
• Ethane is a minor product of the reaction
How the reaction eventually stops…
.
.
1. Cl + C2H5
C2H5Cl
.
.
2. C2H5 + C2H5
C4H10
.
.
3. Cl + Cl
Cl2
•
•
More chloroethane is formed
A small amount of ethane is also formed
Cl
Cl
Hydrogen Chloride
formed
H
H
C
H
C
H
H
Ethyl free radical formed
H
Cl
Cl
Chloroethane
formed
Termination Steps
Cl
Cl
Chlorine formed
H5 C2
C2 H5
Butane (C4H10) formed
H 5 C2
Cl
Chloroethane formed
Evidence for this mechanism
1.
No reaction will occur in the absence of
ultraviolet light. If irradiation is stopped the
speed of the reaction slows down
2.
The formation of butane (C4H10) in the
reaction
of
ethane
with
chlorine
provides evidence of the formation of
ethyl radicals (C2H5 )
3.
Addition of a radical promoter (like
tetraethyl lead) speeds up the reaction.
The conversion of an
alkene to a chloroalkane
Ionic Addition
Addition of
Hydrogen Chloride
H
H
C
C
H
H
H
H
H
C
C
H
Cl
Ethene
Chloroethane
HCl
Hydrogen Chloride
H
H
C
Finally
As a The
result
A
the
pair
polar
negatively
of
of this
electrons
Hydrogen
two
charged
ions
chlorine
are formed
moves
Chloride
ionout
bonds
– amolecule
from
negatively
with
thethe
charged
carbonium
double
becomes
chloride
bond
ionfurther
to
toion
form
form
andaa
H positively
polarised
covalent
chloroethane
charged
on
bond
approach
carbon
with ion
to
(carbonium
the
hydrogen
double
ion)
bond
∂+
H
Cl
∂―
C
H
H
chloroethane
Ethene formed
Region of high electron density
(negative charge)
How do we
represent this
on paper?
H
H
C
H
∂+
Cl
C
H
Ethene
H
Chloroethane
∂H formed
Cl
Addition of Chlorine
H
H
Industrially
Important
H
H
C
C
As 1, 2 – dichloroethane is used to make
C
C
H
H
chloroethene
which
is theHraw material
for
H
Ethene
the important plastic PVC
(polyvinylchloride)Cl
Cl
Cl2
Chlorine
1, 2 - dichloroethane
H
C
Finally
As aThe
result
the
Anonpolar
pair
negatively
ofof
this
electrons
chlorine
two
charged
ions
chlorine
are formed
molecule
moves
ion bonds
–out
abecomes
negatively
from
withthe
the
charged
carbonium
polarised
double
chloride
bond
on
ionapproach
to
ion
toform
form
and aa
H positively
covalent
tochloroethane
the
charged
double
bondcarbon
bond
with one
ion
(carbonium
of the chlorines
ion)
Cl
Cl
C
H
H
1, 2 -Ethene
dichloroethane
formed
Region
of high electron density
(negative charge)
How do we
represent this
on paper?
1, 2 - dichloroethane
formed
H
H
C
Cl
∂+
Cl
C
H
Ethene
H
Cl
∂-
Cl
Addition of Bromine
H
H
Industrially
H
C
H
Important
Ethene
C
H
C
H
Br
Br2
H
C
H
Br
1, 2 - dibromoethane
As 1,Bromine
2 – dibromoethane is used to prevent
lead accumulating in car engines
H
C
Finally
As aThe
result
the
Anonpolar
pair
negatively
ofof
this
electrons
bromine
two
charged
ions
bromide
are formed
molecule
moves
ion bonds
–out
abecomes
negatively
from
withthe
the
charged
carbonium
polarised
double
bromide
bond
ion
on approach
toion
toform
form
and aa
H positively
covalent
tochloroethane
the
charged
double
bondcarbon
bond
with one
ion
of
(carbonium
the bromine
ion) atoms
Br
Br
C
H
H
2006 Q. 9(c) (9)
1, 2 -Ethene
dibromoethane
formed
Region
of high electron density
(negative charge)
How do we
represent this
on paper?
1, 2 - dibromoethane
formed
H
H
C
Br
∂+
Br
C
H
Ethene
H
Br
∂-
Br
2003 Q. 6(e) (6)
Evidence for
this
mechanism
2006 Q. 9(c) (6)
When bromine reacts
with ethene only one
product is formed:
1, 2 - dibromoethane
When bromine in salt
water reacts with
ethene three products
are formed – and this
indicates the formation
+
of the C ion:
H
2-bromoethanol
formed
1-bromo-2-chloroethane
1, 2 - dibromoethane
formed
formed
H
C
Br
∂+
C
H
v
Ethene
OH
Cl
Br
H
Br
∂-
OH
Cl
Br
Bromine salt
water is a source
of
ofOH
Cl- -ions
ions