Effects on reactant stability

In a seminal work, Koga and Morokuma suggested that the high activation energy of the Bergman cyclization is due to the strong electron repulsion between the 

Recently, we analyzed the role of electron repulsion relative to bond breaking and antiaromaticity effects on a quantitative basis using Natural Bond Orbital (NBO) analysis.24 Two other destabilizing factors were considered at the initial stage of the cyclization in addition to four-electron repulsion between the filled in-plane acetylenic re-orbitals - distortion/breaking of the acetylenic bonds as a result of their bending, and the fact that, at a distance of ca. 3 A, the in-plane re-orbitals become parallel and reach a geometry that resembles the antiaromatic TS of the symmetry forbidden [2S + 2S] cycloaddition (vide infra). 

An extensive computational analysis expanded the range of the c-d distances for reactive cyclic enediynes to 2.9-3.4 A.38 By comparing unsubstituted enediynes with dialkyl-substituted enediynes, it was found that the activation enthalpy is dependent on factors other than the c-d distance and that reactivity hinges on a subtle interplay of steric and electronic effects that accompany distortion caused by incorporation into a macrocycle. For example, since alkyl substituents stabilize acetylenic bonds to a greater extend than olefinic bonds,39 such substituents stabilize the starting material, thus increasing both the activation barrier and the reaction endothermicity. 

Despite the contribution of the above factors, it is clear that bending of the two alkyne moieties toward each other increases the energy of the system as illustrated by the three plots in Fig. 7.24 These plots illustrate the relation between the ring size and calculated cyclization parameters in more detail and dependence of the total energy of the system from the c-d distance. Interestingly, simple bending of alkyne moiety reproduced the effect of cyclic restraints reasonably well. A similar conclusion has been reached even earlier by Kraka and Cremer.40 

It is clear that although initially the effect is relatively small and can be easily masked by other steric or electronic factors, its importance increases as the bending 

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Transition states (Continued) in hydrogen abstraction, 25 in phosphodiester hydrolysis, 190 reactant-like vs product-like, 96 solvation energy of, 211, 213,214 solvent effects on, 46 stabilization of charge distribution, 91, 225-227... 

As mentioned in the preceding section, Ji-effects on the stability of the reactants are going to be rather subtle in thermal cyclizations, since the determining factor in the activation barrier for this reaction is the formation of the bond between in-plane orbitals. One way to accelerate this reaction would be to destabilize the reactant ji-system. The challenge is in designing a system where the reactant destabilization is not transferred to the transition state and product as well. An elegant approach to... 

In C and D are situations in which the effects on reactants and on the transition state are balancing, so that the magnitude of the rate enhancement is dependent on the relative stabilization in case of C, or the relative destabilization in case of D, of the initial state and the transition state. Hence C and D could in principle lead to retardation in a dipolar aprotic medium relative to protic. 

Reactions such as catalytic hydrogenation that take place at the less hindered side of a reactant are common m organic chemistry and are examples of steric effects on reactivity Previously we saw steric effects on structure and stability m the case of CIS and trans stereoisomers and m the preference for equatorial substituents on cyclo hexane rings... 

What about solvent Do solvents have the same effect in S>g 1 reactions that they have in S j2 reactions The answer is both yes and no. Yes, solvents have a large effect on S l reactions, but no, the reasons for the effects on S jl and SN2 reactions are not the same. Solvent effects in the SN2 reaction are due largely to stabilization or destabilization of the nucleophile reactant. Solvent effects in the Sjsjl reaction, however, are due largely to stabilization or destabilization of the transition state. 

If it is assumed that penultimate unit effects on the reaction entropy are insignificant, the terms in eqs. 18 and 19 corresponding to the stabilization energy of the reactant propagating radical will cancel and rVli=ryly There should be no explicit penultimate unit effect on copolymer composition. On the other hand, the radical reactivity ratio j (eq. 20) compares two different propagating radicals so... 

One of the key challenges for this process is dealing with the wide range of contaminants in the waste HBr stream. Both inorganic and organic contaminants may be present. These contaminants are typically reactants and products of the upstream bromination process which generated the waste HBr. In addition, they may include corrosion products of upstream equipment or ionic materials present in the water used to scrub the gaseous bromination process effluent. The main concerns about contaminants in the feed streams are their effect on catalyst activity and stability and their effect on bromine product quality. 

An increase in absorbance at 351 nm and a concomitant decrease in absorbance at 380 nm in the ultraviolet visible spectrum of methylcobalamin during the abiotic transfer of the methyl group to Hg2+ are characteristic for the loss of the methyl group and formation of aquocobalamin. In experiments monitored by both analytical techniques, gas chromatographic measurements of methylmercury formation were in good agreement with the spectropho-tometric measurement of aquocobalamin formation from methylcobalamin at 351 nm. Aerobic versus anaerobic reaction conditions had no measurable effect on either the methyl transfer rates, the stability of the reactants, or on the reaction products. 

Generally speaking, the influence of solvent on reaction rates (equilibria) is determined by the difference between the effects < n the stability of transition states (products) and reactants. According to what Leffler and Grunwald (1963) call the first approximation, the free energy of a solute molecule RX is given by the sum of internal and solvent contributions, as shown in (59). The... 

Aromaticity remains a concept of central importance in chemistry. It is very useful to rationalize important aspects of many chemical compounds such as the structure, stability, spectroscopy, magnetic properties, and last but not the least, their chemical reactivity. In this chapter, we have discussed just a few examples in which the presence of chemical structures (reactants, intermediates, and products) and TSs with aromatic or antiaromatic properties along the reaction coordinate have a profound effect on the reaction. It is clear that many more exciting insights in this area, especially from the newly developed aromatic inorganic clusters, can be expected in the near future from both experimental and theoretical investigations. 

TV-Ethyl substitution had very little effect on the measured rate constant, whereas a 4-methyl substituent increased the rate constant by a factor of ca 100. In this case the initial product (identified by Bamberger) is the iminocyclohexadienol 39, which slowly hydrolyses to the quinone 40. These substituent effects suggest that in the transition state the developing positive change is located mostly at the 4-position (stabilized by the 4-Me substituent) and very little on the nitrogen atom (no stabilization by a TV-Et substituent), so that the intermediate is more properly described by the iminium ion. This is supported by an earlier observation46 that whilst full incorporation of 180 from the solvent H2180 occurs in the product, there is no detectable 180 incorporation into the reactant phenylhydroxylamine. 

As expected, the terminal functional groups mainly determine the reactivity of these siloxane oligomers towards other reactants. The variations in the backbone composition have critical effects on the glass transition temperature, solubility parameter, thermal stability and surface behavior of the resulting oligomers(12,13). 

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