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Reactions introduces the nature of free-radical reactions
and addresses such topics as chlorination, bromination, energy factors,
the Hammond Postulate, isotopic effects, and other reactive intermediates.
After a brief overview of the way in which the Chemistry Professor
intends to address chemical reactions, the unit directs attention
to the chlorination of methane and the nature of the free radical.
An explanation of how the free radical mechanism occurs is accompanied
by animation to promote understanding of this fundamental reaction.
The three phases of the reaction are discussed: initiation, propagation,
and termination with detailed examples and explanations of each.
Positioning of chlorination on hydrocarbons with non-identical carbons
is introduced by comparing percent of possible products based on
probability of attack and the range of products based on observed
percent. The concept of free radical stability is introduced and
students address the product ratios that experience teaches. The
concept of stability is treated using the Arrhenius equation and
appropriate energy diagrams so that students can understand the
actual reasons for the unexpected differences between the probability
approach and the free radical approach. The method of reasonably
predicting accurate product distribution is demonstrated.
approach to transition state as the rate determining step is explained
using energy diagrams stressing Ea. Multiple transition states are
noted and the rate determining step for these situations noted.
The program then moves to the bromination of propane and the many
ways in which it is similar to and different from chlorination of
propane. The familiar series of free-radical steps is noted, reminding
the student of the basic principles of the free-radical mechanism.
Superimposed energy diagrams are used to compare the ease with which
second degree and first degree free radicals are produced. Chemical
reactions then compares the chlorination vs. bromination processes
by using comparative energy diagrams. The endothermic nature of
one and exothermic characteristic of the other are compared, as
are the energy differences in the transition states.
The Hammond Postulate is introduced by denoting differences in the
reactive states of the two processes. The student is led to note
positional energy relationships among the states: reactants, transition
state, and products, hence clarifying the Hammond Postulate. From
these discussions, the concept of reactivity vs. selectivity is
The need to selectively designate certain hydrogens for attack via
free-radicals gives rise to the need for isotopic labeling. The
characteristics of deuterium and deuterium-to-carbon bonds play
important roles in studying chemical reactions. The ways in which
deuterium labeling are used is described in this section. Brief
mention is made of other reactive intermediates such as the
carbocation, the carbanion, and the methylene free radical. The
structures are presented for comparison, but the actual topics are
addressed in later, more appropriate, units.