Substituent constant primary

In contrast to the steric effoits, the purely electronic influences of substituents are less clear. They are test documented by linear free-energy relationships, which, for the cases in question, are for the most part only plots of voltammetrically obtained peak oxidation potentials of corresponding monomers against their respective Hammett substituent constant As a rule, the linear correlations are very good for all systems, and prove, in aax>rdance with the Hammett-Taft equation, the dominance of electronic effects in the primary oxidation step. But the effects of identical substituents on the respective system s tendency to polymerize differ from parent monomer to parent monomer. Whereas thiophenes which receive electron-withdrawing substituents in the, as such, favourable P-position do not polymerize at all indoles with the same substituents polymerize particularly well... 

Elimination reactions of ( )- and (Z)-benzaldehyde Opivaloyloximes (19a) and (19b) with DBU in MeCN have been found to occur by a nitrile-forming E2 mechanism which is ca 2000-fold faster for the latter isomer in each case.15 The corresponding Hammett substituent constants, activation parameters, and primary deuterium isotope effects, suggest that the anti elimination from (19b) (for which p = 2.4 0.1, H/ D = 2.7 0.3, A/H = 12.5 0.2 kcal mol-1, and A= —31.0 0.6eu) proceeds to (20) via a more symmetrical transition state with a smaller degree of proton transfer, less charge development at the jS-carbon and greater extent of triple bond formation than for syn elimination from (19a) (for which p = 1.4 0.1, kn/kn = 7.8 0.3, AH = 8.8 0.1 kcal mol 1 and A= -23.6 0.4 eu). 

Steric requirements, hydrogen and deuterium, 299 Stem-Volmer plot, 181 Stiff differential equations, 109 Stochastic simulation, 109 Stoichiometric coefficients, 11 Stokes-Einstein equation, 135 Stopped flow, 179 Stmetured water, 395 Structure-reactivity relationships, 311 Sublimation energy, 403 Substituent, 313 Substituent constant, 323 alkyl group, 341 electrophilic, 322 Hammett, 316 inductive, 325, 338 normal. 324 polar, 339 primary, 324 resonance, 325... 

Several amplifications of the results of Kuwana have since been published. Free-energy relationships between values and various substituent constants have been employed in an attempt to characterize more quantitatively the nature of the electronic effect transferred from the substituent to the reaction site, in this case the iron atom. Such studies have brought conclusions that the primary mode of transmittance of these effects involves inductive 38) and combined inductive-resonance parameters 25, 28, 29,... 

Several electronic substituent constants were defined so as to represent both global and particular electronic effects. The a values obtained unambiguously from experimentally accessible data or from the many possible reaction series are called primary values and the corresponding set the primary standard. The o values derived from the primary values, by rescaling with modified q constants or correlation equations, are called secondary values and the corresponding set the secondary standard. 

The AD parameter is a spectroscopic measure of radical stabilization through spin delocalization by aryl substituents. A priori there is no reason to justify their use as radical substituent constants (ffrad) in linear free energy relationships, since for the latter the correlation of kinetic (rate constants) or thermodynamic (equilibrium constants) and not of spectroscopic data is of primary interest. Nevertheless, the linear correlation with the calculated RSE (Fig. 13) and other established substituent constants (cf. Section III.D) strongly suggests that the AD parameter may be appropriately employed to assess electronic effects in benzyl-type radicals. 

The symbol k0 is an intercept term that is equal to k for the parent (unsubstituted) compound. The reaction constant p depends on reaction conditions such as solvent and temperature, representing the susceptibility of the reaction to environmental effects. In contrast, the substituent constant up is a measure of the electronic effect of replacing hydrogen by a given substituent, and is assumed to be independent of the reaction conditions. By defining p = 1 for the room temperature ionization of substituted benzoic acids in water, Hammett calculated op values directly for 13 substituents, and predicted those for a further 17 substituents by applying the primary rrP values to other reactions. Later work increased the number of aP values to 44 and the number of reaction series to 51 [35]. 

Rate-retarding effects of ca 105 were observed in hydrolyses of sodium alkyl sulphates in basic solution60, where Z is the strongly electron-donating substituent O-. Primary substrates showed second-order kinetics, but first-order rate constants were obtained for secondary alkyl substrates e.g. for i-propyl at 100 °C, the (interpolated) rate constant is 3.0 x 10 6 s-1 for the sulphate salt, whereas for hydrolysis of i-propyl tosylate the (extrapolated) rate constant is 0.7633. 

Steric effects in oxidation are found primarily in the H-atom donor as, for example, isobutane versus 2,4-dimethylpentane where both co-oxidation and added hydroperoxide measurements indicate isobutane to be 1.4 times as reactive toward either R02 radical [38,39], For a series of branched alkanes, the value of kp was more sensitive to changes in steric bulk on the adjacent carbon than on the same carbon but all values were within a factor three [39], Increasing size in f-R02 radicals has little effect on the value of kp for H-atom transfer from primary, secondary or tertiary sites [69]. However, kp for a given hydrocarbon does increase by a factor of about 5—10 as R02 is changed from a tertiary to secondary or primary ROa [78], Moreover, in the oxidation of aromatic compounds, the ratio of kp s for H-atom transfer to parent R02- and to f-Bu02-correlate with meta substituent constants which suggests that this difference in reactivity is due mainly to polar effects [86]. 

We now have available a small group of primary aj values (from set 01) which are as close to being properly scaled as is possible at the present time. This small set of primary constants is insufficient for the needs of those workers who are interested in applying correlation analysis to a very wide range of structural types. It is therefore necessary for us to make use of secondary sources. As was pointed out by McDaniel and Brown (28), the use of secondary sources may lead to large errors in the values of the (Tm and Op substituent constants. 

State [46,125,126], This has been suggested as an explanation for the observation of extended linear Bronsted plots [46,126], and it would be of interest to know if it could also lead to constant primary isotope effects. Most examples studied pertain to elimination or addition at a carbonyl group [46, 126] where proton transfer occurs between oxygen atoms. However, Kemp and Casey [75] have measured both Bronsted coefficients and isotope effects for eliminations of substituted benzisoxazoles, in which proton transfer occurs from carbon. In this case it was found that Bronsted exponents were substantially insensitive to substituents in either the base or the substrate, and for... 

The constants 3 are almost independent of the substituents on the benzene ring (H, 4-C1, and 4-N02). The identification of the primary and secondary products 6.20 and 6.21 as the (Z)- and ( >isomers respectively is based solely on analogies with diazoates and diazocyanides with respect to UV spectra, etc. The conclusion may be incorrect, although that is unlikely. The reduction to the hydrazinedisul-fonate (6.22) becomes kinetically significant only in the presence of excess sulfite. 

The hydrogen abstraction from the Si-H moiety of silanes is fundamentally important for these reactions. Kinetic studies have been performed with many types of silicon hydrides and with a large variety of radicals and been reviewed periodically. The data can be interpreted in terms of the electronic properties of the silanes imparted by substituents for each attacking radical. In brevity, we compared in Figure 1 the rate constants of hydrogen abstraction from a variety of reducing systems by primary alkyl radicals at ca. 80°C. ... 

Schrauzer and co-workers have studied the kinetics of alkylation of Co(I) complexes by organic halides (RX) and have examined the effect of changing R, X, the equatorial, and axial ligands 148, 147). Some of their rate constants are given in Table II. They show that the rates vary with X in the order Cl < Br < I and with R in the order methyl > other primary alkyls > secondary alkyls. Moreover, the rate can be enhanced by substituents such as Ph, CN, and OMe. tert-Butyl chloride will also react slowly with [Co (DMG)2py] to give isobutylene and the Co(II) complex, presumably via the intermediate formation of the unstable (ert-butyl complex. In the case of Co(I) cobalamin, the Co(II) complex is formed in the reaction with isopropyl iodide as well as tert-butyl chloride. Solvent has only a slight effect on the rate, e.g., the rate of reaction of Co(I) cobalamin... 

The fact that we have three olefinic hydrogens means that our compound is a primary olefin, the fact that the other two carbons are both methylene carbons means that our substituent, bromine, is terminal. Thus the only possibility we have is that we are dealing with 4-bromo-1-butene (try to find another isomer that fits ). But this simple molecules has a highly complex proton spectrum, which can only be interpreted completely (exact chemical shift, coupling constants) by spectrum simulation. 

As a result of the inductive and hyperconjugative effects it is to be expected that tertiary carbonium ions will be more stable than secondary carbonium ions, which in turn will be more stable than primary ions. The stabilization of the corresponding transition states for ionization should be in the same order, since the transition state will somewhat resemble the ion. Thus the first order rate constant for the solvolysis of tert-buty bromide in alkaline 80% aqueous ethanol at 55° is about 4000 times that of isopropyl bromide, while for ethyl and methyl bromides the first order contribution to the hydrolysis rate is imperceptible against the contribution from the bimolecular hydrolysis.217 Formic acid is such a good ionizing solvent that even primary alkyl bromides hydrolyze at a rate nearly independent of water concentration. The relative rates at 100° are tertiary butyl, 108 isopropyl, 44.7 ethyl, 1.71 and methyl, 1.00.218>212 One a-phenyl substituent is about as effective in accelerating the ionization as two a-alkyl groups.212 Thus the reactions of benzyl compounds, like those of secondary alkyl compounds, are of borderline mechanism, while benzhydryl compounds react by the unimolecular ionization mechanism. 

The constants of ion pair formation of 33 amines with 2,4-dinitrophenol in benzene (Ab) have been compared with the pAa in water56. The effects of structural variations on basicity are larger in water than in benzene for primary and secondary cyclic amines, but similar for tertiary amines. The Taft and Hancock equation [where <7 has the usual meaning and E° (R ) is the steric effect of a component substituent]... 

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