Resonance, reaction rate

Reaction Rate Resonance in the Concentration Cycling of the Catalytic Oxidation of Carbon Monoxide... 

McConnell H M 1958 Reaction rates by nuclear magnetic resonance J. Chem. Phys. 28 430-1... 

Let us illustrate this with the example of the bromination of monosubstituted benzene derivatives. Observations on the product distributions and relative reaction rates compared with unsubstituted benzene led chemists to conceive the notion of inductive and resonance effects that made it possible to explain" the experimental observations. On an even more quantitative basis, linear free energy relationships of the form of the Hammett equation allowed the estimation of relative rates. It has to be emphasized that inductive and resonance effects were conceived, not from theoretical calculations, but as constructs to order observations. The explanation" is built on analogy, not on any theoretical method. 

Decades of work have led to a profusion of LEERs for a variety of reactions, for both equilibrium constants and reaction rates. LEERs were also established for other observations such as spectral data. Furthermore, various different scales of substituent constants have been proposed to model these different chemical systems. Attempts were then made to come up with a few fundamental substituent constants, such as those for the inductive, resonance, steric, or field effects. These fundamental constants have then to be combined linearly to different extents to model the various real-world systems. However, for each chemical system investigated, it had to be established which effects are operative and with which weighting factors the frmdamental constants would have to be combined. Much of this work has been summarized in two books and has also been outlined in a more recent review [9-11]. 

It can be seen from Table 1 that there are no individual steps that are exothermic enough to break carbon—carbon bonds except the termination of step 3a of —407.9 kJ/mol (—97.5 kcal/mol). Consequentiy, procedures or conditions that reduce the atomic fluorine concentration or decrease the mobiUty of hydrocarbon radical intermediates, and/or keep them in the soHd state during reaction, are desirable. It is necessary to reduce the reaction rate to the extent that these hydrocarbon radical intermediates have longer lifetimes permitting the advantages of fluorination in individual steps to be achieved experimentally. It has been demonstrated by electron paramagnetic resonance (epr) methods (26) that, with high fluorine dilution, various radicals do indeed have appreciable lifetimes. 

The differenee in reaction rates of the amino alcohols to isobutyraldehyde and the secondary amine in strong acidic solutions is determined by the reactivity as well as the concentration of the intermediate zwitterions [Fig. 2, Eq. (10)]. Since several of the equilibrium constants of the foregoing reactions are unknown, an estimate of the relative concentrations of these dipolar species is difficult. As far as the reactivity is concerned, the rate of decomposition is expected to be higher, according as the basicity of the secondary amines is lower, since the necessary driving force to expel the amine will increase with increasing basicity of the secondary amine. The kinetics and mechanism of the hydrolysis of enamines demonstrate that not only resonance in the starting material is an important factor [e.g., if... 

When an azine-nitrogen and a leaving group are in the 2,3-relation to each other in monoaza- and polyaza-naphthalenes, there is a dramatic effect on the reaction rate (for 3-chloroisoquLnoline lO -lO -fold less than for its 1-chloro isomer and for 2-chloroquinoline 200-400-fold less than for 2-chloropyridine) due to restrictions imposed on the resonance stabilization of charge in the transition state by the bicyclic system ... 

The last comprehensive review of reactions between carbon-centered radicals appeared in 1973.142 Rate constants for radical-radical reactions in the liquid phase have been tabulated by Griller.14 The area has also been reviewed by Alfassi114 and Moad and Solomon.145 Radical-radical reactions arc, in general, very exothermic and activation barriers are extremely small even for highly resonance-stabilized radicals. As a consequence, reaction rate constants often approach the diffusion-controlled limit (typically -109 M 1 s"1). 

The effect of a methoxy group in the -phenyl ring, 146, X = OCH3, Y = H, upon the reaction rate was minimal (about 40%), which indicates that resonance contributions from the carbene species 148b are insignificant. Such... 

R. Schubert, H. Waalkens, and S. Wiggins, Efficient computation of transition state resonances and reaction rates from a quantum normal form, Phys. Rev. Lett. 96, 218302 (2006). 

The most frequently encountered hydrolysis reaction in drug instability is that of the ester, but curtain esters can be stable for many years when properly formulated. Substituents can have a dramatic effect on reaction rates. For example, the tert-butyl ester of acetic acid is about 120 times more stable than the methyl ester, which, in turn, is approximately 60 times more stable than the vinyl analog [16]. Structure-reactivity relationships are dealt with in the discipline of physical organic chemistry. Substituent groups may exert electronic (inductive and resonance), steric, and/or hydrogen-bonding effects that can drastically affect the stability of compounds. A detailed treatment of substituent effects can be found in a review by Hansch et al. [17] and in the classical reference text by Hammett [18]. 

More advanced scale was proposed by Kamlet and Taft [52], This phenomenological approach is very universal as may be successfully applied to the positions and intensities of maximal absorption in IR, NMR (nuclear magnetic resonance), ESR (electron spin resonance), and UV-VS absorption and fluorescence spectra, and to many other physical or chemical parameters (reaction rates, equilibrium constant, etc.). The scale is quite simple and may be presented as ... 

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