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Transition state of lowest energy

Another example is the attack by an electrophile on the carboxylate anion, an allylic system bearing more than one lone pair (Fig. 2.5). Since the allylic system stabilizes both starting material and product, the lowest-energy route would maintain that allylic system throughout the reaction path. In general, the transition state of lowest energy maximizes the extent of bonding. The loss of the electrophile (a proton, for example) would create a new lone pair in the plane of the carboxylate, perpendicular to the allylic pi system. The reverse reaction must be electrophilic attack on a lone pair that is not part of the allylic pi system. Because of the principle of microscopic reversibility, we use either the forward or reverse reaction to decide the best route. [Pg.38]

Principle of microscopic reversibility The forward and reverse reactions follow the same lowest-energy route but in opposite directions. There is only one energetically best pathway. The transition state of lowest energy maximizes the extent of bonding. [Pg.57]

Considerably less certainty is attached to the rationalization of enantioselec-tivity in the hydrogenation, which must involve the difference in free energy AG between the two diastereomeric transition states of lowest energy. For simplicity, these can be assumed to be of similar structure for routes to the two enantiomers (not necessarily correct ). For cases where substrate binding is strong. [Pg.139]

According to X-ray analysis alkene 64 adopts conformation 64A. The face anti to the bulky siloxy group is open for a tetrahedral attack of radical R", and 66A is the transition state of lowest energy. [Pg.397]

For an elimination reaction, the transition state has some double bond character, and so the factors known to stabilize an alkene will also stabilize the transition state. As a consequence, the pathway that leads to the more stable alkene usually also passes through the transition state of lowest energy thus, the reaction leading to the most stable alkene is not only the most exothermic but also the fastest. [Pg.1288]

Thus, to define the mechanism of either reaction under given experimental conditions is to define the mechanisms of both, since the transition state or states of lowest energy are necessarily the same in either direction. In practice, however, ester hydrolysis and formation are not carried out under the same conditions. Hydrolysis is carried out in water, but esterification reactions... [Pg.125]

Conformational analysis of the AZ-28 bound hapten suggests that it is not a true mimic of the oxy-Cope transition state. The lowest energy conformation for the hapten is with the cyclohexyl moiety in the chair-... [Pg.243]

Below the glass transition temperature (Tg ca 100 °C) the depicted mechanism is assumed to be the main response of polyacrylonitrile fibers to stress. It may account for an elongation of the fiber of up to W%. As it may be expected, the extension is almost entirely reversible, if the stress is removed, and the kinetic conditions are provided permitting the macromolecules to restore the state of lowest energy (e. g. by steam treatment above Tg). [Pg.129]

The four different transition states in Fig. 8.10 were considered with BF3 as a model for the BLA catalyst in the theoretical calculations. It was found that the lowest transition-state energy for the BF3-catalyzed reactions was calculated to be 21.3 kcal mol for anti-exo transition state, while only 1.5 kcal mol higher in energy the syn-exo transition state, was found. The uncatalyzed reaction was calculation to proceed via an exo transition state having an energy of 37.0 kcal mol . The calculations indicated that the reaction proceeds predominantly by an exo transition-state structure and that it is enhanced by the coordination of the Lewis acid. [Pg.313]

The orbital energies are useful when comparing the stability of complexes in different structures. As usual, electrons are placed in the orbitals starting with the orbitals of lowest energy. We will have opportunity to make use of the orbital energies when we consider reactions of complexes in which the transition state has a different structure than that of the starting complex (see Chapter 20). [Pg.624]


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See also in sourсe #XX -- [ Pg.38 , Pg.57 ]




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Energies of transitions

Energy, transition energies

Lowest energy

Lowest energy transition state

Lowest state

Lowest-energy state

Transition energies

Transition-state energies

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