Transition energy diagram

Figure Bl.1.3. State energy diagram for a typical organic molecule. Solid arrows show radiative transitions A absorption, F fluorescence, P phosphorescence. Dotted arrows non-radiative transitions.

A potential energy diagram for nng inversion m cyclohexane is shown m Figure 3 18 In the first step the chair conformation is converted to a skew boat which then proceeds to the inverted chair m the second step The skew boat conformation is an inter mediate in the process of ring inversion Unlike a transition state an intermediate is not a potential energy maximum but is a local minimum on the potential energy profile... 

Sketch a potential energy diagram for the reaction of 1 heptanol with hydrogen bromide paying careful attention to the positioning and structures of the intermediates and transition states... 

Section 4 9 The potential energy diagrams for separate elementary steps can be merged into a diagram for the overall process The diagram for the reac tion of a secondary or tertiary alcohol with a hydrogen halide is charac terized by two intermediates and three transition states The reaction is classified as a ummolecular nucleophilic substitution, abbreviated as SnI... 

Potential energy diagram (Section 4 8) Plot of potential en ergy versus some arbitrary measure of the degree to which a reaction has proceeded (the reaction coordinate) The point of maximum potential energy is the transition state Primary alkyl group (Section 2 13) Structural unit of the type RCH2— in which the point of attachment is to a pnmary carbon... 

Fig. 5.11. Contrasting potential energy diagrams for stable and unstable bridged norbomyl cation. (A) Bridged ion is a transition state for rearrangement between classical structures. (B) Bridged ion is an intermediate in rearrangement of one classical structure to the other. (C) Bridged nonclassical ion is the only stable structure.

Z7. The cotr arison of activation parameters for reactions in two different solvents requires consideration of differences in solvation of both the reactants and the transition states. This can be done using a potential energy diagram such as that illustrated below, where A and B refer to two different solvents. By thermodynamic methods, it is possible to establish values which correspond to the enthalpy... 

There is another useiiil way of depicting the ideas embodied in the variable transition state theory of elimination reactions. This is to construct a three-dimensional potential energy diagram. Suppose that we consider the case of an ethyl halide. The two stepwise reaction paths both require the formation of high-energy intermediates. The El mechanism requires formation of a carbocation whereas the Elcb mechanism proceeds via a caibanion intermediate. 

Three-dimensional potential energy diagrams of the type discussed in connection with the variable E2 transition state theory for elimination reactions can be used to consider structural effects on the reactivity of carbonyl compounds and the tetrahedral intermediates involved in carbonyl-group reactions. Many of these reactions involve the formation or breaking of two separate bonds. This is the case in the first stage of acetal hydrolysis, which involves both a proton transfer and breaking of a C—O bond. The overall reaction might take place in several ways. There are two mechanistic extremes ... 

There is an intermediate mechanism between these extremes. This is a general acid catalysis in which the proton transfer and the C—O bond rupture occur as a concerted process. The concerted process need not be perfectly synchronous that is, proton transfer might be more complete at the transition state than C—O rupture, or vice versa. These ideas are represented in a three-dimensional energy diagram in Fig. 8.1. 

For each reaction, plot energy (vertical axis) vs. the number of the structure in the overall sequence (horizontal axis). Do reactions that share the same mechanistic label also share similar reaction energy diagrams How many barriers separate the reactants and products in an Sn2 reaction In an SnI reaction Based on your observations, draw a step-by-step mechanism for each reaction using curved arrows () to show electron movements. The drawing for each step should show the reactants and products for that step and curved arrows needed for that step only. Do not draw transition states, and do not combine arrows for different steps. 

The Hammond Postulate implies that the transition stah of a fast exothermic reaction resembles the reactants (se( reaction energy diagram at left). This means that it wil be hard to predict the selectivity of competing exothermi( reactions both barriers may be small and similar even i one reaction is more exothermic than the other. 

Use of the Hammond Postulate requires that the reverse reactions both be fast. Obtain energies for the transition states leading to 1-propyl and 2-propyl radicals ipropane+Br end and propane+Br center), and draw a reaction energy diagram for each (place the diagrams on the same axes). Is use of the Hammond Postulate justified Compare the partial CH and HBr bond distances in each transition state to the corresponding distances in propane and hydrogen bromide, respectively. Does the Hammond Postulate correctly predict which bond distances will be most similar Explain. 

The different phase behaviors are evidenced in the corresponding free energy diagrams, which have been estimated for both polymers [15]. These diagrams are shown in Fig. 10 (due to the different approximations used in the calculation of the free energy differences, these diagrams are only semiquantitative [15]). It can be seen that the monotropic transition of the crystal in... 

Describing a Reaction Energy Diagrams and Transition States 157... 

Figure 5.4 An energy diagram for the first step in the reaction of ethylene with HBr. The energy difference between reactants and transition state, AG, defines the reaction rate. The energy difference between reactants and carbocation product, AG°, defines the position of the equilibrium.

Figure 5.7 An energy diagram for the overall reaction of ethylene with HBr. Two separate steps are involved, each with its own transition state. The energy minimum between the two steps represents the carbocation reaction intermediate.

Draw an energy diagram for a one-slep reaction with /Cet) < 1. Label the parts of the diagram corresponding to reactants, products, transition state, AG°, and ACT Is AG° positive or negative ... 

Figure 6.14 Energy diagrams for endergonic and exergonic steps, [a) In an endergonic step, the energy levels of transition state and product are closer.

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