Enthalpy and Entropy of Activation
While the overall rate of a chemical reaction depends on the
!
G
,
the components of the free energy of activation (the
enthalpy and
entropy of activations
) often provide mechanistic information.
!
G
=
!
H
!
S
-
T
!
H
!
S
measures the enthalpy change as the reactants move towards
the
transition state.
Bonding changes
make important
contributions to this parameter.
Solvation effects
are also
important.
measures the change in entropy as the reactants move
towards the transition state. In most reactions, the
transition
state is more ordered
than the reactant state leading to a
negative value
for the entropy of activation, and an
unfavorable energy contribution to the free energy of
activation.
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Pyridine reacts rapidly with methyl iodide in a nucleophilic
substitution reaction yielding N-methylpyridinium iodide:
N
:
+
CH
3
I
pyridine
acetonitrile
25
o
C
N CH
3
+
+
I
-
N-methylpyridinium iodide
!
G
o
= -12 kcal/mol
Exergonic
A kinetic study of this reaction provides the overall rate of this
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- Summer '07
- Jung
- Organic chemistry, Thermodynamics, Enthalpy, Reaction, Energy, Entropy, Chemical reaction, Transition state theory
-
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