Methane. Hydrocarbons – compounds containing only carbon and hydrogen. hydrocarbons aliphatic...
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Transcript of Methane. Hydrocarbons – compounds containing only carbon and hydrogen. hydrocarbons aliphatic...
Hydrocarbons – compounds containing only carbon and hydrogen.
hydrocarbons
aliphatic aromatic
alkanes alkenes 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 sp3 tetrahedral 109.5o bond angles | H
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
CH4 + conc. H2SO4
CH4 + conc. NaOH
CH4 + sodium metal
CH4 + KMnO4
CH4 + H2/Ni
CH4 + Cl2
NR (no reaction)
NR
NR
NR
NR
NR
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 CO + H2 + H2C2 (acetylene)
CH4 + H2O , Ni, 850o 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
CH4 + Cl2
CH4 + I2, heat
CH4 + Br2, hv
NR (requires heat or uv light)
NR (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 CI4
triiodomethane tetraiodomethane
iodoform carbon tetraiodide
Can proceed further:
CH4 + Cl2, heat CH3Cl + CH2Cl2 + CHCl3 + CCl4 + HCl
Control?
(xs) CH4 + Cl2, heat CH3Cl + HCl
bp –162o bp –24o
CH4 + (xs) Cl2, heat CCl4 + 4 HCl
Cl Cl Cl + Cl
Mechanism
step 1 : initiating step = homolytic bond dissociation
step 2
Cl H C H
H
HC H
H
H++
Cl Cl Cl+ Cl Cl + Cl
possible but non-productive
step 3
CH
H
H
+ Cl Cl H C
H
H
Cl + Cl
.
.
.
.
.
.
.
.
Cl H
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.
Eact/RTePZrate **
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 HX + CH3•
3) CH3• + X2 CH3X + X•
4) 2 X• X2
5) X• + CH3• CH3X
6) 2 CH3• CH3CH3
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•