The Study of Chemical Reactions

The Study of Chemical Reactions
 Writing and balancing the overall
equation for the reaction barely
scrapes the surface…You must
understand the
 mechanism,
 thermodynamics, and
 kinetics
of the reaction as well.
The Chlorination of Methane
 We will see how to investigate
each of these areas by studying
the following gas-phase reaction:
∆ or hν
CH4 + Cl2
HCl + CH3Cl
(+ CH2Cl2 + CHCl3 + CCl4)
The Chlorination of Methane
 Does not occur at room
temperature in the absence of
 The most effective light for the
reaction is blue and is absorbed
by the Cl2.
 The light-initiated reaction has a
high quantum yield.
Free-Radical Chlorination of
 Proceeds by a chain reaction.
 Steps in any chain reaction:
 Initiation
 the generation of a reactive intermediate
 Propagation
 Products form. Reaction continues until
reactants or intermediates are depleted.
 Termination
 removes the reactive intermediates
Initiation: Generation of a Free Radical
 With either heat (∆) or the appropriate
wavelength of light (hν), Cl2 undergoes
homolytic cleavage, one electron in the
bond going to each of the Cl atoms:
 Propagation refers to the steps in the reaction
that generate the products and regenerate the
reactive intermediates.
step 1
reactive intermediates
step 2
 Propagation continues until
 a reactant is used up, or
 the reactive intermediates get
depleted by nonproductive reactions.
These are some of the termination reactions.
Thermodynamics of the FreeRadical Chlorination of Methane
 Thermodynamics tell a lot about a
system at equilibrium.
 ∆G°(25°C) = -108.6 kJ
 KP = e-∆G°/RT =
CH4 + Cl2 HCl + CH3Cl
KP 
PCH 4 PCl 2
Thermodynamics of the FreeRadical Chlorination of Methane
 The large value of K and the large
negative value of ∆G° say the
reaction goes to completion.
 In general, a reaction goes to >99%
completion if its ∆G is < -12kJ.
Thermodynamics of the FreeRadical Chlorination of Methane
The free energy change depends on the enthalpy
change for the reaction, the temperature at which the
reaction takes place, and the entropy change:
∆G = ∆H - T∆S
Examining ∆H and ∆S will show what drives the
reaction and, consequently, how the reaction will
behave at different temperatures.
∆H°(25°C) = -105 kJ The reaction is very exothermic,
which favors products.
∆S°(25°C) = 12.16 J/K The entropy change is positive,
which also favors products.
At 25°C, the bigger influence on ∆G° is the fact that the
reaction is so exothermic.
Bond-Dissociation Enthalpies
 A measure of the strength of a bond is
how much energy it takes to break it
CH3-H + 435* kJ/mol  ▪CH3 + H▪
 The weaker the bond, the less energy
is needed to break it.
Cl-Cl + 242 kJ/mol  2Cl▪
 This explains why blue light initiates
the formation of Cl free radicals but
not methyl radicals.
*From Table 4-2
Bond-Dissociation Enthalpies
 Since we know the mechanism of the
chlorination reaction, we can calculate ∆H based
on bond dissociation enthalpies (BDEs).
To break bonds
431 kJ
435 kJ
351 kJ
242 kJ
To form bonds
Bond-Dissociation Enthalpies
 ∆H° = BDE(bonds broken)-BDE(bonds
= 677-782 = -105 kJ/mol
 This value tells us the reaction is very
exothermic, primarily because the Cl-Cl
bond is fairly weak.
 Thermodynamics tell a lot about a
system at equilibrium.
 Kinetics tell how fast a system
will reach equilibrium.
 Reaction rates are determined
 rate = k[reactant 1]a[reactant 2]b …
 [reactant 1] is the molarity of
reactant 1.
 a is the order of the reaction for
reactant 1.
 a+b+… is the overall order of the
 k is the rate constant.
 k is the rate constant.
 k is given by the Arrhenius equation
 A is a constant that incorporates collision
frequency and orientation
 Ea is the activation energy, the energy
needed to form the transition state.
 R is the gas constant (8.3145 J/mol K)
Reaction-Energy Diagram for a
Single-Step Reaction
transition state
Ea, the activation energy
ΔE, the energy
change for the
Reaction-Energy Diagram for the Two
Propagation Steps of the Chlorination of
rate equation for step 1:
Temperature Dependence of the
Rate Constant
k increases with T.
At a higher temperature, more reactant molecules
will have kinetic energies ≥ Ea.
Estimation: Rate doubles for every 10°C the
temperature increases.
Chlorination of Other Alkanes
 For ethane and the cycloalkanes, the
mechanism is very similar to that of
propagation step 1
Chlorination of Ethane
propagation step 2
overall reaction
Chlorination of Other Alkanes
 Can you write the complete mechanism
for the chlorination of cyclopentane?
propagation step 1
propagation step 2
overall reaction

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