Substituent constant alkyl group

Fig. 4 Correlation of constants for transition state stabilization (pKxs) and substrate binding (pKs) for the cleavage of meta- and para-substituted phenyl acetates by /3-CD. The substituents are alkyl groups and the four halogens. The two deviant points are for longish p-alkyl groups (n-butyl and n-pentyl). Data from Tables A5.2...

In recent years a large number of jS-keto esters, and some /5-diketones have been investigated in an attempt to discover some of the smaller effects of the substituents, especially alkyl groups on the keto-enol equilibrium. In Table 34 are given the values for the equilibrium constant... 

Ultraviolet photoelectron spectroscopy allows the determination of ionization potentials. For thiazole the first experimental measurement using this technique was preformed by Salmona et al. (189) who later studied various alkyl and functional derivatives in the 2-position (190,191). Substitution of an hydrogen atom by an alkyl group destabilizes the first ionization potential, the perturbation being constant for tso-propyl and heavier substituents. Introduction in the 2-position of an amino group strongly destabilizes the first band and only slightly the second. 

Instead of stabilizing the carbonyl group by electron donation as alkyl substituents do trifluoromethyl groups destabilize it by withdrawing electrons A less stabilized carbonyl group IS associated with a greater equilibrium constant for addition... 

It was found (32) that in the acid range (pH 4—6) the alkyl group does not influence the rate of decomposition, which is similar for all xanthates. In the alkaline range the rates are markedly influenced by the substitutional group, and the rates could be correlated with the Taft polar substituent constants estabhshed for the various groups. 

The nonspecialist reading Table 7-7 will probably be impressed by the substantial consistency among cti values evaluated by different methods, but the specialist tends to concentrate on the differences. There is one very interesting difference in Table 7-7, that for cti of alkyl groups based on Eq. (7-33) compared with cti based on the ionization of 3, the latter values showing practically no effect of inductive electron release and certainly no trend with increased branching. (The uncertainties associated with these substituent constants can be found in the original literature.) Swain... 

Few other reactions of series of substituted pyridines have been investigated extensively. Dondoni, Modena, and Todesco have measured the rate of N-oxidation of a limited series of pyridines and found a good correlation with normal u-values with a p-value of — 2.23. The A-alkylation of pyridines with alkyl iodides in nitrobenzene has been studied by Brown and Cahn and by Clarke and Rothwell. Unfortunately, the only data available are for the parent compound and for alkyl derivatives, and, since the a-values for the various alkyl groups in a given position are substantially constant, this leaves a correlation of only three independent points. However, the rates of A-alkylation of the j8- and y-alkyl derivatives are so nearly equal that it appears as if no correlation existed. Clarke and Rothwell have also studied the alkylation with allyl bromide in nitromethane at various temperatures, and in this case a more extensive series is available. The authors state that no overall Hammett correlation is obtained however, the j8-substituted derivatives fall on one straight line and the y-derivatives on another one with a different slope. The data are shown in Fig. 2. The line for the j8-compounds, p = — 2.53 0.31, r = 0.95, is seen not to be very good the line for the y-derivatives, p = — 1.42 0.06, r = 0.99, is much more satisfactory. It does not seem likely that the discrepancy is due to the intervention of resonance effects, since in this case one would expect the correlation for the y-derivatives to be poorer than that for the j8-analogs. More extensive studies with a wider variety of substituents would seem very desirable. 

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. 

Aryl substitution on germanium, whether single or multiple, has only a small effect on the rate constants for hydrogen atom transfer, whereas the rate constant increases substantially with substitution of an alkyl group on Ge by a silyl group, much as observed with the silanes. A strong substituent effect also was observed for germane 19. 

Nieuwstad, Klapwijk, and van Bekkum (105) have added to the knowledge of aromatic hydrogenation by their study of the influence of alkyl substituents in the 1 and 2 positions of naphthalene on the rate. Tetrahydro-naphthalenes were the products of hydrogenation over palladium at 80°C. The selectivity of the reaction was also followed and expressed as the ratio of the rate constants for the saturation of the unsubstituted and substituted rings, respectively. Steric effects play an important role, and, beside steric hindrance by the bulky substituents, steric acceleration also has been observed, the latter being caused by a release of the strain between the 1-alkyl group and hydrogen in position 8. 

The problem with the expanded branching method is that it requires a large number of parameters. Data sets large enough to permit its use are seldom seen. It has been applied to a number of studies in which only alkyl groups are substituents that are varying. In this case electrical effects are constant, thus only steric effects need be considered. 

These Es parameters estimated by Eq. 4 for hetero atom substituents can be combined with those originally developed for various alkyl groups as a set of steric constants for QSAR studies of aromatic systems 6). Thus, apart from the original definition for the intramolecular steric effect, the combined set of Es parameters is able to represent intermolecular steric effects as well. The original Taft E, values for unsymmetrical alkyl groups seem to represent effective steric dimension of the groups which is scaled on the same standard as those for symmetric monoatomic substituents where the effective dimension coincides with the van der Waals radius. 

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