1. Methane

2. Hydrocarbons – compounds containing only carbon and hydrogen.
aromatic
aliphatic
alkanes
alkenes
alkynes

3. Alkanes – hydrocarbons with the general formula
CnH2n+2
(four bonds to each carbon and only single bonds)
CH4 methane
C2H6 ethane
C3H8 propane
Etc.

4. Methane = CH4 H |
H—C—H sp tetrahedral o bond angles
Non-polar – van der Waals (London forces)
Gas at room temperature mp = -183oC bp = oC
Water insoluble
Colorless and odorless gas
“swamp gas” ; fossil fuel found with petroleum & coal
Important fuel/organic raw material

5. Chemistry of methane (reactions)?
CH H2O 
CH conc. H2SO4 
CH conc. NaOH 
CH sodium metal 
CH KMnO 
CH H2/Ni 
CH Cl 
NR (no reaction) NR NR NR NR NR NR

6. Methane is typically unreactive
Methane is typically unreactive. It does not react with water, acids, bases, active metals, oxidizing agents, reducing agents, or halogens.
Reactions of methane:
Combustion (oxidation;complete & partial)
Halogenation

7. Reactions of Methane
Combustion (oxidation)
a) complete oxidation
CH O2 , flame or spark  CO H2O energy
b) partial oxidation
6 CH O2 , 1500o  CO H H2C2 (acetylene)
CH H2O , Ni, 850o  CO H2

8. Halogenation
CH 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

9. “Substitution” reaction
CH Cl2 
CH I2, heat 
CH Br2, hv 
NR (requires heat or uv light)
NR (does not react with I2)
CH3Br HBr

10. CH4 + Cl2, hv  CH3Cl + HCl CH3Cl + Cl2, hv  CH2Cl2 + HCl
methyl chloride
chloromethane
CH3Cl Cl2, hv  CH2Cl HCl
methylene chloride
dichloromethane
CH2Cl Cl2, hv  CCl3H HCl
chloroform
trichloromethane
CCl3H Cl2, hv  CCl HCl
carbon tetrachloride
tetrachloromethane

11. CH4 + Br2, hv  CH3Br + HBr CH3Br + Br2, hv  CH2Br2 + HBr
methyl bromide
bromomethane
CH3Br Br2, hv  CH2Br HBr
methylene bromide
dibromomethane
CH2Br Br2, hv  CBr3H HBr
bromoform
tribromomethane
CBr3H Br2, hv  CBr HBr
carbon tetrabromide
tetrabromomethane

12. CH3I CH2I2
iodomethane diiodomethane
methyl iodide methylene iodide
CHI3 CI4
triiodomethane tetraiodomethane
iodoform carbon tetraiodide

13. CH4 + Cl2, heat  CH3Cl + CH2Cl2 + CHCl3 + CCl4 + HCl
Can proceed further:
CH Cl2, heat  CH3Cl + CH2Cl CHCl CCl HCl
Control?
(xs) CH Cl2, heat  CH3Cl HCl
bp –162o bp –24o
CH (xs) Cl2, heat  CCl HCl

15. Mechanism for the monochlorination of methane
initiating step:
Cl2  2 Cl•
propagating steps:
Cl• CH4  HCl CH3•
CH3• Cl2  CH3Cl Cl•
then 2), then 3), then 2), etc.
terminating steps:
Cl• Cl•  Cl2
Cl• CH3•  CH3Cl
CH3• CH3•  CH3CH3

16. 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

17. We need only consider those bonds that are broken or formed in the reaction.
CH3—H + Cl—Cl  CH3—Cl + H—Cl
PE:  
ΔH = –187 = -25 Kcal/mole
(exothermic, gives off heat energy)

18. ΔH for each step in the mechanism?
Cl—Cl  2 Cl•
ΔH = +58
Cl• + CH3—H  H—Cl + CH3•
ΔH = +1
CH3• + Cl—Cl  CH3—Cl + Cl•
ΔH = -26
Cl• + Cl•  Cl—Cl
ΔH = -58

20. 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.

22. Z = collision frequency
P = probability factor
e-Eact/RT = fraction of collisions with E > Eact
Note: rate decreases exponentially as the Eact increases!

23. @ 275oC
Eact Collisions > Eact
5 Kcal 10,000/1,000,000
10 Kcal /1,000,000
15 Kcal /1,000,000
If the Eact is doubled, the rate is decreased by a factor of 100 times!

24. If ΔH > 0, then Eact > ΔH If ΔH < 0, then Eact > 0
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

25. 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

26. An “alternate mechanism:
Cl• CH4  CH3Cl H•
H• Cl2  HCl Cl•
Why not this mechanism?
Step 2: ΔH = = +20 Kcal/mole; Eact > +20 Kcal
Step 3: ΔH = = -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!

27. Halogenation
Δ or hυ
CH X2  CH3X HX
requires heat or light
X2: Cl2 > Br2  I2
why?…how?…mechanism

28. 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.

29. “generic” mechanism for the halogenation of methane
(free radical substitution mechanism)
X2  2 X•
X • + CH4  HX CH3•
CH3• + X2  CH3X X•
2 X•  X2
X• + CH3•  CH3X
2 CH3•  CH3CH3

30. ΔH for each step in the mechanism by halogen:
F Cl Br I

31. 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

34. 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•

35. 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•

36. Reactions of Methane
Combustion (oxidation)
a) complete oxidation
CH O2 , flame or spark  CO H2O heat
b) partial oxidation
6 CH O2, 1500oC  CO H H2C2
CH H2O, 850o, Ni  CO H2
Halogenation
CH X2, heat or hv  CH3X HX
requires heat or light
Cl2 > Br NR with I2