Methane

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•

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 + H2 + H2C2

CH4 + H2O, 850o, Ni CO + H2

2. Halogenation

CH4 + X2, heat or hv CH3X + HX

requires heat or light

Cl2 > Br2 NR with I2