Transcript Reactions of Alkanes

Reactions of Alkanes
IB
Combustion of alkanes
Alkanes are unreactive as a family because of the strong C–C and C–H
bonds as well as them being nonpolar compounds.
At room temperature alkanes do not react with acids, bases, or strong
oxidizing agents.
Alkanes do undergo combustion in air (making them good fuels):
2C2H6(g) + 7O2(g)  4CO2(g) + 6H2O(l)
H = –2855 kJ
Complete combustion produced carbon dioxide and water while
incomplete may produces a combination of carbon monoxide, carbon
and water in addition to carbon dioxide.
Carbon dioxide contributes to global warming while carbon monoxide is
toxic; hemoglobin binds to carbon monoxide in preference to oxygen
causing suffocation and even death.
Products of combustion
Complete combustion produces ONLY:
①
carbon dioxide
②
water vapour
while incomplete may produces a combination of :
water vapour
carbon dioxide
and carbon monoxide
and
carbon (soot)
Carbon dioxide contributes to global warming.
Carbon monoxide is toxic; hemoglobin binds to carbon monoxide in
preference to oxygen causing suffocation and even death.
Substitution Reaction
•
In the presence of light alkanes undergo substitution reaction with halogens (X).
RH + X2  RX + HX
•
In a substitution reaction, one atom of a molecule is removed
and replaced or substituted by another atom or group of atoms.
•
Mechanism of subtitution reaction involves free radicals.
Free Radical Substitution reaction
UV
CH 3 (CH 2 ) 4 CH 3  Br2 CH 3 (CH 2 ) 4 CH 2 Br  HBr
For a reaction between an alkane and bromine to occur, C-H and Br-Br bonds must break.
The C-H bond is stronger than Br-Br bond (check bond enthalpy in databooklet).
Therefore, the reaction proceeds by first the breakage of Br-Br bond, which is brought about by UV light.
Br-Br bond can be broken in one of two ways.
UV
Br2 
 2Br
or
UV

Br2 
 Br : Br

Free Radical Substitution reaction
When the bond is broken, either
① the bond pair can be equally shared between the two atoms producing two
bromine atoms (called free radicals),
or
②The bond pair goes with one atom producing a positive and a negatively charged
ions of bromine.
The first type of bond breakage producing free radicals is referred to as a homolytic fission
and the second heterolytic fission.
•Homolytic fission because the bond pairs are equally distributed, or particles that
are the same in every way is produced.
•Homolytic fission of the halogen takes place.
In the next step, the free radical removes a hydrogen atom from the alkane forming hydrogen
bromine and a free radical of the alkane.
CH3CH2CH2CH2CH2CH2-H + Br•  CH3CH2CH2CH2CH2CH2• + HBr
Free Radical Substitution reaction
•
The free radical goes on to react with a molecule of chlorine and regenerate another chlorine free
radical.
CH3CH2CH2CH2CH2CH2• + Br2  CH3CH2CH2CH2CH2CH2Br + Br•
And so on.
Because this reaction, once initiated, can keep itself going is referred to as a chain reaction.
The reaction can conducted with any halogen and the mechanism would be the same.
Not only that, more than one hydrogen can be substituted.
UV
CH3CH2CH2CH2CH2CH3  2Br2 
 CH3CH2CH2CH2CH2CHBr2  2HBr
1,1-dibromohexane
Mechanism of chlorination of methane
CHAIN REACTION
1. Initiation
..
: Cl
..
..
Cl :
..
light
2
..
: Cl .
..
a free radical
“dissociation”
2. Chain Propagation (first step)
REPEATING STEPS
CH3
+
H
..
: Cl .
..
H
..
Cl :
..
+
. CH3
methyl radical
“hydrogen abstraction”
3. Chain Propagation (second step)
. CH3
+
..
: Cl
..
..
Cl :
..
CH3
..
Cl :
..
+
..
: Cl .
..
feeds back into
step two
Mechanism of chlorination of methane
4. Termination Steps
..
2 : Cl .
..
CH3.
..
: Cl .
..
+
+
. CH
3
. CH
3
“recombinations”
..
: Cl
..
..
Cl :
..
CH3CH3
..
: Cl CH3
..
These steps stop
the chain reaction