Methane

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Methane
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Hydrocarbons compounds containing only carbon
and hydrogen.
hydrocarbons
aromatic
aliphatic
alkanes
alkenes
alkynes
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Alkanes hydrocarbons with the general
formula CnH2n2 (four bonds to each
carbon and only single bonds) CH4 methane C2H6 et
hane C3H8 propane Etc.
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Methane CH4 H HCH 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
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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
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• Methane is typically unreactive. It does not
  react with water, acids, bases, active metals,
  oxidizing agents, reducing agents, or halogens.
• Reactions of methane
• Combustion (oxidationcomplete partial)
• Halogenation

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• Reactions of Methane
• 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

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• Halogenation
• CH4 X2 , ? or h? ? CH3X
  HX
• X2 Cl2 or Br2
• ? a) Requires heat (?) or uv light (h?)
• b) May proceed further
• ? c) Cl2 reacts faster than Br2
• ? d) No reaction with I2

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Substitution reaction CH4 Cl2
? CH4 I2, heat ? CH4 Br2, hv
?
NR (requires heat or uv light)
NR (does not react with I2)
CH3Br HBr
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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
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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
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CH3I CH2I2 iodomethane diiodomethane
methyl iodide methylene iodide CHI3 CI4
triiodomethane tetraiodomethane
iodoform carbon tetraiodide
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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
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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

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• Energy Changes? ?H
• Homolytic bond dissociation energies (see inside
  the front cover of MB)
• HCl 103 Kcal/mole
• ClCl 58 Kcal/mole
• CH3H 104 Kcal/mole
• CH3Cl 84 Kcal/mole

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• We need only consider those bonds that are broken
  or formed in the reaction.
• CH3H ClCl ? CH3Cl HCl
• 104 58 -84
  -103
• PE 162? -187?
• ?H 162 187 -25 Kcal/mole
• (exothermic, gives off heat energy)

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• ?H for each step in the mechanism?
• ClCl ? 2 Cl
• 58 ?H 58
• Cl CH3H ? HCl CH3
• 104 -103
  ?H 1
• CH3 ClCl ? CH3Cl Cl
• 58 -84
  ?H -26
• Cl Cl ? ClCl
• -58
  ?H -58

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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.
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Z collision frequency P probability
factor e-Eact/RT fraction of collisions with E
gt Eact Note rate decreases exponentially as
the Eact increases!
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_at_ 275oC Eact Collisions gt 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!
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Eact cannot be easily calculated like ?H, but we
can estimate a minimum value for Eact If ?H gt
0, then Eact gt ?H If ?H lt 0, then Eact gt 0
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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) ?H 1 Kcal/mole Eact gt 1 Kcal
(estimated) Step 3) ?H -26 Kcal/mole Eact gt 0
Kcal (estimated) Step 2 is estimated to be slower
than step 3 and is the RDS
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• An alternate mechanism
• Cl CH4 ? CH3Cl H
• H Cl2 ? HCl Cl
• Why not this mechanism?
• Step 2 ?H 104-84 20 Kcal/mole Eact gt 20
  Kcal
• Step 3 ?H 58-103 -45 Kcal/mole Eact gt 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!

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• Halogenation
• ? or h?
• CH4 X2 ? CH3X HX
• requires heat or light
• X2 Cl2 gt Br2 ? I2
• why?how?mechanism

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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. FF 38
Kcal/mole ClCl 58 Kcal/mole BrBr 46
Kcal/mole II 36 Kcal/mole Once initiated the
reaction may or may not continue based on the
Eact for the RDS.
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• 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

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?H 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
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Estimation of Eact for the propagating
steps Eact (est.) F Cl Br I 2 gt0 gt1
gt16 gt33 3 gt0 gt0 gt0 gt0 Step 2 is the
RDS Rate Cl2 gt Br2 because in the RDS Eact(Cl2)
lt Eact(Br2) NR with I2 because RDS Eact(I2) gt 33
Kcal/mole only 1/1012 collisions would have E gt
33 at 275o
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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
d d
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Hammonds 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 gt 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
d d
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• Reactions of Methane
• 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
• Halogenation
• CH4 X2, heat or hv ? CH3X
  HX
• requires heat or light
• Cl2 gt Br2 NR with I2