Methane
Hydrocarbons – compounds containing only carbon and
hydrogen.
hydrocarbons
aliphatic
alkanes
alkenes
aromatic
alkynes
Alkanes – hydrocarbons with the general formula
CnH2n+2
(four bonds to each carbon and only single bonds)
CH4
methane
C2H6 ethane
C3H8 propane
Etc.
Methane = CH4
H
|
H—C—H
|
H
sp3
tetrahedral 109.5o bond angles
Non-polar – van der Waals (London forces)
Gas at room temperature mp = -183oC bp = -161.5oC
Water insoluble
Colorless and odorless gas
“swamp gas” ; fossil fuel found with petroleum & coal
Important fuel/organic raw material
Chemistry of methane (reactions)?
CH4
+ H2O

NR (no reaction)
CH4
+ conc. H2SO4

NR
CH4
+ conc. NaOH

NR
CH4
+ sodium metal 
NR
CH4
+ KMnO4

NR
CH4
+ H2/Ni

NR
CH4
+ Cl2

NR
Methane is typically unreactive. It does not
react with water, acids, bases, active metals,
oxidizing agents, reducing agents, or
halogens.
Reactions of methane:
1. Combustion (oxidation;complete & partial)
2. Halogenation
Reactions of Methane
1. Combustion (oxidation)
a) complete oxidation
CH4 + 2 O2 , flame or spark  CO2 + H2O + energy
b) partial oxidation
6 CH4 + O2 , 1500o
CH4 + H2O , Ni, 850o
 CO + H2 + H2C2 (acetylene)

CO + H2
2. Halogenation
CH4
+ X2 , Δ or hυ

CH3X
+ HX
X2 = Cl2 or Br2
 a) Requires heat (Δ) or uv light (hυ)
b) May proceed further
 c) Cl2 reacts faster than Br2
 d) No reaction with I2
“Substitution” reaction

NR
CH4
+ Cl2
CH4
+ I2, heat  NR
CH4 + Br2, hv

(requires heat or uv light)
(does not react with I2)
CH3Br + HBr
CH4
+ Cl2, hv
 CH3Cl
+ HCl
methyl chloride
chloromethane
CH3Cl
+
Cl2, hv

CH2Cl2
+ HCl
methylene chloride
dichloromethane
CH2Cl2
+ Cl2, hv  CCl3H + HCl
chloroform
trichloromethane
CCl3H
+
Cl2, hv  CCl4 + HCl
carbon tetrachloride
tetrachloromethane
CH4
+ Br2, hv

CH3Br
+ HBr
methyl bromide
bromomethane
CH3Br
+ Br2, hv

CH2Br2
+
HBr
methylene bromide
dibromomethane
CH2Br2
+
Br2, hv
 CBr3H
+
HBr
bromoform
tribromomethane
CBr3H
+ Br2, hv

CBr4
+ HBr
carbon tetrabromide
tetrabromomethane
CH3I
CH2I2
iodomethane
diiodomethane
methyl iodide
methylene iodide
CHI3
triiodomethane
iodoform
CI4
tetraiodomethane
carbon tetraiodide
Can proceed further:
CH4 + Cl2, heat  CH3Cl +
CH2Cl2
+ CHCl3
Control?
(xs) CH4
bp –162o
CH4
+
Cl2, heat 
CH3Cl
+
HCl
bp –24o
+ (xs) Cl2, heat  CCl4
+ 4 HCl
+ CCl4
+ HCl
Mechanism
step 1 : initiating step = homolytic bond dissociation
Cl Cl
step 2
H
+ H C H
H
.
Cl
Cl
Cl
.
+
Cl Cl
.
+
. Cl
Cl H
Cl Cl
.
+
+
H
C H
H
. Cl
possible but non-productive
step 3
H
H C
H
.
+
Cl Cl
H
H C Cl
H
+
.Cl
Mechanism for the monochlorination of methane
initiating step:
1) Cl2  2 Cl•
propagating steps:
2) Cl• + CH4  HCl + CH3•
3) CH3• + Cl2  CH3Cl + Cl•
then 2), then 3), then 2), etc.
terminating steps:
4) Cl• + Cl•  Cl2
5) Cl• + CH3•  CH3Cl
6) CH3• + CH3•  CH3CH3
Energy Changes? ΔH
Homolytic bond dissociation energies
(see inside the front cover of M&B)
H—Cl
103 Kcal/mole
Cl—Cl
58 Kcal/mole
CH3—H 104 Kcal/mole
CH3—Cl 84 Kcal/mole
We need only consider those bonds that are
broken or formed in the reaction.
CH3—H + Cl—Cl  CH3—Cl + H—Cl
+104
+58
-84
-103
PE:
+162
-187
ΔH = +162 –187 = -25 Kcal/mole
(exothermic, gives off heat energy)
ΔH for each step in the mechanism?
1) Cl—Cl  2 Cl•
+58
ΔH = +58
2) Cl• + CH3—H  H—Cl + CH3•
+104
-103
ΔH = +1
3) CH3• + Cl—Cl  CH3—Cl + Cl•
+58
-84
ΔH = -26
4) Cl• + Cl•  Cl—Cl
-58
ΔH = -58
Rates of chemical reactions depend on three factors:
Collision frequency
(collision per unit time)
Probability factor
(fraction of collisions with correct geometry)
Energy factor
(fraction of collisions with sufficient energy)
“sufficient energy” = Energy of activation, minimum
energy required for a collision to go to the product.
rate  Z * P * e
 Eact/RT
Z = collision frequency
P = probability factor
e-Eact/RT = fraction of collisions with E > Eact
Note: rate decreases exponentially as the Eact
increases!
@ 275oC
Eact
Collisions > Eact
5 Kcal
10,000/1,000,000
10 Kcal
100/1,000,000
15 Kcal
1/1,000,000
If the Eact is doubled, the rate is decreased by a factor of 100
times!
Eact cannot be easily calculated like ΔH,
but we can estimate a minimum value for
Eact:
If ΔH > 0, then Eact > ΔH
If ΔH < 0, then Eact > 0
Rate determining step (RDS) = the step in the mechanism
that determines the overall rate of a reaction. In a “chain
reaction” this will be the slowest propagating step.
For chlorination of methane, which propagating step is
slower?
Step 2) ΔH = +1 Kcal/mole
Eact > +1 Kcal (estimated)
Step 3) ΔH = -26 Kcal/mole
Eact > 0 Kcal (estimated)
Step 2 is estimated to be slower than step 3 and is the RDS
An “alternate mechanism:
2) Cl• + CH4  CH3Cl + H•
3) H•
+ Cl2 
HCl + Cl•
Why not this mechanism?
Step 2: ΔH = +104-84 = +20 Kcal/mole; Eact > +20 Kcal
Step 3: ΔH = +58-103 = -45 Kcal/mole; Eact > 0 Kcal
RDS for this mechanism is step 2 and requires a minimum of
20Kcal/mole! Unlikely compared to our mechanism
where the RDS only requires an estimated minimum of 1
Kcal!
2. Halogenation
Δ or hυ
CH4
+ X2
 CH3X + HX
requires heat or light
X2: Cl2 > Br2  I2
why?…how?…mechanism
This reaction requires heat or light because the first step in
the mechanism involves the breaking of the X-X bond. This
bond has to be broken to initiate the chain mechanism.
F—F
38 Kcal/mole
Cl—Cl
58 Kcal/mole
Br—Br
46 Kcal/mole
I—I
36 Kcal/mole
Once initiated the reaction may or may not continue based on
the Eact for the RDS.
“generic” mechanism for the halogenation of methane
(free radical substitution mechanism)
1) X2
 2 X•
2) X • + CH4

3) CH3• + X2
 CH3X
HX
4) 2 X•  X2
5) X• + CH3• 
CH3X
6) 2 CH3•  CH3CH3
+
CH3•
+ X•
ΔH for each step in the mechanism by halogen:
F
Cl
Br
I
1
+38
+58
+46
+36
2
-32
+1
+16
+33
3
-70
-26
-24
-20
4
-38
-58
-46
-36
5
-108
-84
-70
-56
6
-88
-88
-88
-88
Estimation of Eact for the propagating steps:
Eact (est.)
F
Cl
Br
I
2
>0
>+1
>+16
>+33
3
>0
>0
>0
>0
Step 2 is the RDS
Rate Cl2 > Br2 because in the RDS Eact(Cl2) < Eact(Br2)
NR with I2 because RDS Eact(I2) > +33 Kcal/mole
only 1/1012 collisions would have E > +33 at 275o
The transition state (‡) or “activated complex” is the
unstable structure that is formed between reactants and
products in a step in a mechanism. It corresponds to the
energy at the top of the energy barrier between reactants
and products.
step 2 in the chlorination of methane:
Cl• + CH4  HCl + CH3•
Transition state:
[ Cl--------H-------CH3 ]‡
δ•
δ•
Hammond’s Postulate: the higher the Eact of a step in a
mechanism, the later the transition state is reached and the
more the transition state will look like the products.
In step 2 of the mechanism for the bromination of methane,
the Eact is estimated to be > +16 Kcal/mole. Since the Eact
is high, the transition state is reached later in this step than
it is in chlorination and will look more like the products:
[ Br----H-----------CH3 ]‡
δ•
δ•
Reactions of Methane
1. Combustion (oxidation)
a) complete oxidation
CH4 + 2 O2 , flame or spark 
CO2 + H2O + heat
b) partial oxidation
6 CH4 + O2, 1500oC  CO
CH4
+ H2O, 850o, Ni

+ H2
CO
+
+ H2C2
H2
2. Halogenation
CH4
+ X2, heat or hv
requires heat or light
Cl2 > Br2
NR with I2
 CH3X + HX