Download Chapter 4 The Study of Chemical Reactions

Chapter 4
The Study of Chemical Reactions
1
4.1 Introduction
If we want to understand a reaction, we must also know the
mechanism, the step-by-step pathway from reactants
to products.
• To know how well the reaction goes to products, we study its
thermodynamics, the energetics of the reaction at equilibrium.
• To use a reaction in a realistic time period (and to
keep the reaction from becoming violent), we study its kinetics,
the variation of reaction rates with different conditions and
concentrations of reagents
Reference: Organic Chemistry", L.G. Wade, Printice Hall, 8th Edition
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4.2 Chlorination of Methane
4.3 The Free-Radical Chain Reaction
The chain reaction consists of three kinds of steps:
1. The initiation step, which generates a reactive intermediate.
2. Propagation steps, in which the reactive intermediate reacts with a
stable molecule to form a product and another reactive intermediate
3. Termination steps, side reactions that destroy reactive
intermediates and tend to slow or stop the reaction.
Reference: Organic Chemistry", L.G. Wade, Printice Hall, 8th Edition
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Example:
Chlorination of methane (substitution reaction)
4.31 The Initiation Step: Generation of Radicals
The unpaired electron is called the odd electron or the radical
electron. Species with unpaired electrons are called radicals or free
radicals. Radicals are electron-deficient because they lack an octet
Reference: Organic Chemistry", L.G. Wade, Printice Hall, 8th Edition
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Examples of free radicals
4-32 Propagation Steps
Reference: Organic Chemistry", L.G. Wade, Printice Hall, 8th Edition
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Problem 1:
Draw Lewis structures for the following free radicals.
(a)The ethyl radical,
(b)The tert-butyl radical,
(c) The isopropyl radical (2-propyl radical)
(d) The iodine atom
Problem 2:
Write the propagation steps leading to the formation of
dichloromethane CH2Cl2 from chloromethane.
Reference: Organic Chemistry", L.G. Wade, Printice Hall, 8th Edition
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4.33 Termination Reactions
Reference: Organic Chemistry", L.G. Wade, Printice Hall, 8th Edition
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4.4 Equilibrium Constants and Free Energy
Thermodynamics is the branch of chemistry that deals with the
energy changes accompanying chemical and physical transformations.
These energy changes are most useful for describing the properties of
systems at equilibrium.
The equilibrium concentrations of reactants and products are
governed by the equilibrium constant of the reaction.
Example : The chlorination of methane
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Reference: Organic Chemistry", L.G. Wade, Printice Hall,
8th
Edition
From the value of Keq we can calculate the change in free energy
(sometimes called Gibbs free energy)
The relationship between Keq and
is given by
where
R = 8.314 J kelvin-mol (1.987 cal kelvin-mol), the gas constant
T = absolute temperature, in kelvins.
e = 2.718 the base of natural logarithms
Notice:
Absolute temperatures (in kelvins) are correctly given without
a degree sign, as in the equation 25 °C = 298 K
Reference: Organic Chemistry", L.G. Wade, Printice Hall,
8th
Edition
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Problem :
Calculate the value of
f or the chlorination of methane
Solution :
Reference: Organic Chemistry", L.G. Wade, Printice Hall, 8th Edition
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4.5 Enthalpy and Entropy
Two factors contribute to the change in free energy: the change in
enthalpy and the change in entropy multiplied by the temperature.
4.51 Enthalpy
The change in enthalpy
is the heat of reaction
 Exothermic (negative value of
)
 Endothermic (positive value of
)
4.52 Entropy
Is often described as randomness, disorder, or freedom of
motion. Reactions tend to favour products with the greatest
entropy
Reference: Organic Chemistry", L.G. Wade, Printice Hall, 8th Edition
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Notice:
 The negative sign in the entropy
term of the free-energy
expression.
 The positive value of the entropy
change indicating that the
products have more freedom of motion than the reactants, makes
a favorable (negative) contribution to
Problem :
Reference: Organic Chemistry", L.G. Wade, Printice Hall, 8th Edition
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4.6 Bond-Dissociation Enthalpies
The bond-dissociation enthalpy (BDE, also called bond-dissociation
energy) is the amount of enthalpy required to break a particular
bond homolytically—that is, in such a way that each bonded atom
retains one of the bond’s two electrons.
In contrast, when a bond is broken heterolytically, one of the atoms
retains both electrons.
Reference: Organic Chemistry", L.G. Wade, Printice Hall, 8th Edition
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4.7 Enthalpy Changes in Chlorination
Problem :
a) Using bond-dissociation enthalpies from Table 4-2 (page 143),
calculate the heat of reaction for each step in the free-radical
bromination of methane.
b) Calculate the overall heat of reaction
Reference: Organic Chemistry", L.G. Wade, Printice Hall, 8th Edition
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