Substituent rate effect

Deacylation of hydrophobic p-nitrophenyl alkanoates Hydroxamic acid and phenyl ester derivatives had alkyl or fluoroalkyl substituents. Rate effects depend on selectivity of binding of fluoro- and hydro-carbon derivatives Kunitake et ai, 1984... 

Almost two decades previous to the Doering papers a reasonable model for substituent rate effects was proposed that was based on a geometric model for the MOE-J energy surface for the 3,3-shift. Thus, a hyperbolic paraboloid surface equation could be differentiated to obtain coordinates and the activation free energy for the saddle point (the transition state) cast in terms of the relative free energies for formation of the diyl and the two allyl radicals, the same independent variables of Eqs. (7.1) and (7.2). Equation 7.3, which relates the independent variables by the harmonic mean is based on the simplest hyperbolic paraboloid surface, that is, one with linear edge potentials. Slightly more realistic models were also explored. 

As pointed out by Gajewski, The substituent rate effects can be explained by alteration in transition-state structure between the two extremes... in response to the nature and position of substituents [33]. In other words, one would expect that, for instance, the presence of radical stabilizing group at C-5 (R = Ph) would alter the nature of the transition state of the Ciaisen rearrangement toward an... 

The effect of substituents on the rate of the reaction catalysed by different metal ions has also been studied Correlation with resulted in perfectly linear Hammett plots. Now the p-values for the four Lewis-acids are of comparable magnitude and do not follow the Irving-Williams order. Note tlrat the substituents have opposing effects on complexation, which is favoured by electron donating substituents, and reactivity, which is increased by electron withdrawirg substituents. The effect on the reactivity is clearly more pronounced than the effect on the complexation equilibrium. 

Table 12 2 summarizes orientation and rate effects m electrophilic aromatic sub stitution reactions for a variety of frequently encountered substituents It is arranged m order of decreasing activating power the most strongly activating substituents are at the top the most strongly deactivating substituents are at the bottom The mam features of the table can be summarized as follows... 

The effects of the nucleophile on aromatic substitution which are pertinent to our main theme of relative reactivity of azine rings and of ring-positions are brought together here. The influence of a nucleophile on relative positional reactivity can arise from its characteristics alone or from its interaction with the ring or with ring-substituents. The effect of different nucleophiles on the rates of reaction of a single substrate has been discussed in terms of polarizability, basicity, alpha effect (lone-pair on the atom adjacent to the nucleophilic atom), and solvation in several reviews and papers. ... 

The reaction can be conveniently monitored by i.r. absorption spectroscopy by observing the intensity of the band at 1650 cm.-1 (C=N). The submitters observed almost complete disappearance of this band whereas the checkers found it still present in medium intensity in their product. In those instances where the a-carbon bears three alkyl substituents, steric effects retard the rate of addition, and in some cases (i.e., [Pg.15]

An extensive set of substituent rate factors has been derived for the N-methylation of pyridazines. They differ from those reported in Table II in that they represent differences between ortho and meta rate factors, rather than factors for individual positions. They should be consulted when considering pyridazines because the steric effects are likely to be more similar for ortho-substituted pyridazines than for pyridines.108 In some cases the pyridazine rate factors give calculated isomer ratios that are in better agreement with observed values than those using the data in Table II. 

If we compare the acid strengths Ka) of a series of substituted benzoic acids with the strength of benzoic acid itself (Table 26-4), we see that there are considerable variations with the nature of the substituent and its ring position, ortho, meta, or para. Thus all three nitrobenzoic acids are appreciably stronger than benzoic acid in the order ortho para > meta. A methoxy substituent in the ortho or meta position has a smaller acid-strengthening effect, and in the para position decreases the acid strength relative to benzoic acid. Rate effects also are produced by different substituents, as is evident from the data in Table 26-5 for basic hydrolysis of some substituted ethyl benzoates. A nitro substituent increases the rate, whereas methyl and methoxy substituents decrease the rate relative to that of the unsubstituted ester. 

It is well known that kH is similar for all alkyl-substituted radicals but rate constants for reaction of tin hydride with carbonyl-substituted radicals are not known. Substituents can effect the rate constant for hydrogen transfer. For example, the benzyl radical is about 50 times less reactive than a primary alkyl radical. 

The influence of both the steric and electronic properties of the silyl group on the rate of epoxidation have been examined experimentally [104], Two rate effects were considered. First, the overall rate of epoxidation of the silyl allylic alcohols was found to be one-fifth to one-sixth that of the similar carbon analogs. This rate difference was attributed to electronic differences between the silicon and carbon substituents. Second, the increase in k[el to 700 for silyl allylic alcohols compared with carbon analogs (e.g., 104 for entry 3, Table 6A.8) was attributed to the steric effect of the large trimethylsilyl group. As expected, when abulky (-butyl group was placed at C-3, k[e] increased to 300 [104],... 

Then the differences in rate caused by the electronic effect of the substituent are correlated by the Hammett equation log(kz/kH) = poz, where kz is the rate constant obtained for a compound with a particular meta or para substituent, ku is the rate constant for the unsubstituted phenyl group, and crz is the substituent constant for each substituent used. The proportionality constant p relates the substituent constant (electron donating or wididrawing) and the substituent s effect on rate. It gives information about the type and extent of charge development in the activated complex. It is determined by plotting log(kz/kQ) versus ov for a series of substituents. The slope of the linear plot is p and is termed the reaction constant. For example, the reaction shown above is an elimination reaction in which a proton and the nosy late group are eliminated and a C-N n bond is formed in their place. The reaction is second order overall, first order in substrate, and first order in base. The rate constants were measured for several substituted compounds ... 

Apparently, the discrepancies detected for the substitution data are largely the consequence of a multiplicity of minor influences operative in the transition state. The deviations are sufficiently diverse in character to require the significance of additional influences on the stability of the transition state. Four other important factors are complexing of the substituent with the electrophilic reagent or catalyst, the involvement of 7r-complex character in the transition state for the reaction, rate effects originating in the rupture of carbon-hydrogen bonds, and differential solvation of the electron-deficient transition states. 

If the other reactant is an electrophile and a strong Lewis acid or proton acid is present, then the aromatic ring acts as the nucleophile and the reaction is one of the electrophilic aromatic substitution reactions listed in Table 17.2. Do not forget to consider the directive and rate effects of substituents on the aromatic ring. 

The rate of hydrolysis of polysaccharides is affected by several factors. Because of substituent interaction effects, furanosides are hydrolyzed much more rapidly than the pyranoside analogues. Differences in the hydrolysis rates of diastereomeric glycosides are significant. For example, the relative hydrolysis rates of methyl-a-D-gluco-, manno-, and galactopyranosides are 1.0 2.9 5.0. This can be related to the stabilities of the respective conjugate acids, which are transformed into the half-chair carbonium ions at different rates. Also, substituents bound to the C-2 position obviously prevent the formation of the half-chair conformation. 

The dependence of the relative reaction rates on olefin geometry can be discussed with reference to Equation 14. As pointed out by Murray and co-workers (25), the complex formation occurs with cis-olefins rather than with their trans isomers for steric reasons hence, Kc (cis) > Kc (trans). However, during the formation of the primary ozonide by either path the olefinic carbon atoms change in hybridization, from sp2 to spz. The bond angles thus decrease from 120° to 109° in the cis isomers, this results in a compression of the substituents van der Waals radii. The repulsion between the substituents is increased, and so is the activation energy. Consequently, ki (trans) > ki (cis) and k2 (trans) > k2 (cis). In the final analysis, the geometry of the olefin has opposite effects (a) onKc and (b) on ki and k2. Present results seem to indicate that for large substituents the effect on Kc predominates since k (cis) > k (trans). 

In general, polymerizations with these salts are much more efficient than those with the unsubstituted salt. Considering that the structure of the propagating species and the rate of the polymerization (kp) are expected to be the same in all three cases, the enhanced activity may be attributed to the stabilization of the benzyl cation by the substituents. Similar effects were observed with the benzylic sulphonium salts. However, more detailed studies [30] of polymerization and hydrolytic properties of various p-substituted... 

Table 5-10. Expected substituent and solvent rate effects for elementary radical reactions [15, 213],...

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