Chemical Reactions

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Chemical 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.

The 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 introduced.

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.

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