Integrated SequencePhysics Chemistry Organic Biology

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Purdue University - Activation Energy
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Work, Energy, and Power


The Chemical Bond

Chemical Kinetics

Under transition state theory, a reaction coordinate diagram is a way of visualizing a reaction pathway. A reaction coordinate diagram represents a pathway that traces saddle points along a hypothetical three-dimensional energy surface describing the vibrations, rotations, and translations of bonded atoms, leading from the configuration of the reagents through that of the transition state to the configuration of the products.

The reaction coordinate follows the path of free energy from reactants to products. The pathway corresponds to breaking and forming new bonds. The rate of passage over the barrier is influenced by the height of the height of the barrier (activation energy) as well as the energy distribution (temperature).

Chemical Kinetics

Reactions of Alkyl Halides

The ability to generate reaction coordinate diagrams for classic reactions from organic chemistry is an important skill. For example, the reaction coordinate diagram for SN2 substitution has one maximum because there is only one activated complex. The leaving group departure coincides with nucleophile approach.

With SN1 substitution, however, the reaction coordinate diagram has two maxima. Leaving group departure occurs first followed by nucleophile approach. Because the activation energy corresponding to leaving group departure is typically higher than the activation energy of nucleophile approach in SN1 substitution, leaving group departure is the rate determining step. Therefore, only the concentration of the alkyl halide substrate appears in the rate expression.

Chemical Kinetics

Reactions of Alkanes

An MCAT favorite, Hammond's postulate provides the conceptual basis to analyze the structure of transition states based on the structures of reagents and products (or intermediates in multistep reactions). The postulate stipulates that transition state structure will resemble the form to which it is most close in energy. The increased regioselectivity in halogenation with bromination over chlorination is the example most of us learn in 1st year organic chemistry of the interpretive power of Hammond's postulate.

The first step in free radical halogenation is hydrogen removal, producing an alkyl radical intermediate. Because hydrogen atom removal in bromination is more endothermic than with chlorination, Hammond's postulate tells us that the transition state with bromination will have more alkyl radical character. The reason for this is that the more endothermic pathway places the transition state closer in energy to the alkyl radical intermediate. Hammond's postulate takes the proximity in energy and makes the connection to state that the transition state in bromination must have greater alkyl radical character.

For the transition state in bromination to have more alkyl radical character means that substitution effects that stabilize alkyl radicals will play more of a role in determining regioselectivity in bromination compared to chlorination. Because greater substitution decreases the energy of alkyl radicals, and because the transition state in bromination has more radical character than with chlorination, bromination will be more selective. A greater portion of the product leads to tertiary alkyl halides with bromination.

Chemical Kinetics


The tools of transition state theory provide a powerful analytical framework for interpreting the role of catalysts.

In transition state theory, we view progress along a reaction pathway as following the reaction coordinate where intermediates and stable forms are separated by transition states, energy barriers which must be crossed. The rate of the reaction is determined by the fraction of reagent possessing sufficient energy to achieve the transition state.

The rate of a reaction may be increased by increasing the concentration of reagents present at the rate determining step, increasing the temperature, or lowering the transition state barrier. The last represents the function of a catalyst, to lower the transition state barrier. The activity of protein enzymes is much more coherent if you understand that the enzyme is lowering the energy of the transition state.

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