Effect of alkylated

Fig. IV-24. Effect of alkyl chain length of n-alcohols on the resistance of water evaporation at 25°C. (From Ref. 275.)...

The steric effects of alkyl substituents (R= methyl, ethyl, i-propyl, f-butyl) on the nitrogen have been related to the steric factors of these same groups as measured in kinetic studies (152). 

The effect of alkyl groups in the 5-position on the reactivity of the thiazole nitrogen is analogous to that found for 3-alkylpyridines, in other words, a simple inductive effect. In passing from the unsubstituted heterocycle to the methyl derivative, the rate constant doubles a further increase in substitution produces a much less pronounced variation. 

Consider first the electronic effect of alkyl groups versus hydrogen atoms attached to C=0 Recall from Section 17 2 that alkyl substituents stabilize C=0 making a ketone carbonyl more stable than an aldehyde carbonyl As with all equilibria factors... 

A comparison of the relative basicities of pyrrole, furan and thiophene may be made by comparing the pK values of their 2,5-di-t-butyl derivatives, which were found to be -1.01, —10.01 and —10.16, respectively. In each case protonation was shown by NMR to occur at position 2. The base-strengthening effect of alkyl substitution is clearly apparent by comparison of pyrrole and its alkyl derivatives, e.g. A-methylpyrrole has a pKa. for a-protonation of -2.9 and 2,3,4,5-tetramethylpyrrole has a pK of 4-3.7. In general, protonation of a-alkylpyrroles occurs at the a -position whereas /3-alkylpyrroles are protonated at the adjacent a-position. As expected, electron-withdrawing groups are base-weakening thus A-phenylpyrrole is reported to have a p/sTa of -5.8. The IR spectrum of the hydrochloride of 2-formylpyrrole indicates that protonation occurs mainly at the carbonyl oxygen atom and only to a limited extent at C-5. 

The reactivity of a series of hydrocarbons toward oxygen measured under a standard set of conditions can give some indication of the susceptibility of various structural units to autoxidation. Table 12.10 gives the results for a series of hydrocarbons. These data indicate the activating effect of alkyl, vinyl, and phenyl substituents. 

The effects of alkyl substituents in positions 6 and 7 on the rates of hydration and dehydration of 2-hydroxypteridine are not very great. The observed decrease in the equilibrium ratio of [HY]/[HX] has been... 

The structural effect of alkyl groups such as methyl, ethyl, and -butyl on the Rp is small. Alkyl 4-methyl-phenylcarbamate can be chosen as a model compound for the hard segment of poly(ether-urethane) (PEU). This group can initiate grafting reaction with Ce(IV) ion and the grafting site was proposed at the hard segment of PEU [3,15] as shown in Scheme (1). 

What conclusion can you draw about the effect of alkyl substitution on UV absorption maxima Approximately what effect does each added alkyl group have ... 

The effect of alkyl substitution on alcohol acidity is due primarily to solvation of the alkoxide ion that results from dissociation. The more readily the alkoxide ion is solvated by water, the more stable it is, the more its formation is energetically favored, and the greater the acidity of the parent alcohol. For example, the oxygen atom of an unhindered alkoxide ion, such as that from methanol, is stericallv accessible and is easily solvated by water. The oxygen... 

Another significant feature found recently is that the effect of the chain length on the field-effect mobility is much less pronounced than indicated in earlier reports [68, 74]. The increase from 4T to 6T corresponds to about a factor of ten, while that from 6T to 8T is only two (the low mobility measured for the dihexyl-substituted 8T must be ascribed to the difficulty in synthesizing and purifying this compound 75 J). Representative data arc gathered in Table 14-1. Also note that the effect of alkyl end substitution is reduced by a factor of two to three (as compared to up to 1000 in earlier reports 68 ). 

Figure 3.82 (a) The effect of or/Ao-substituents on substitution reactions of cis-Pt(PR3)2ArCl complexes (b) the effect of alkyl substituents on substitution reactions of dien complexes. 

TABLE 29 Influence of Chain Length on the Effect of Alkyl Sulfates on Rabbit Eye Mucosa... 

The effect of surfactant structures and properties on emulsion polymerization have been investigated by numerous authors [82-89]. Efforts were made to study the effects of surfactants with different molecular weights on the rate of polymerization [82], swelling and solubilization effects [83], effects of alkyl chain length of homologous series on the rate of polymerization, particle size... 

For a review of the field effects of alkyl groups, see Levitt, L.S. Widing, H.F. Prog. Phys. 

The only groups in Table 9.5 with negative values of CT/ are the alkyl groups methyl and tert-butyl. There has been some controversy on this point. One opinion is that CT/ values decrease in the series methyl, ethyl, isopropyl, /ert-butyl (respectively, — 0.046, —0.057, —0.065, —0.074). Other evidence, however, has led to the belief that all alkyl groups have approximately the same field effect and that the a/ values are invalid as a measure of the intrinsic field effects of alkyl groups. Another attempt to divide ct values into resonance and field contributions is that of Swain and Lupton, who have shown that the large number of sets of ct values (cTm, <3p, a, a+, CT/, csp etc., as well as others we have not mentioned) are not entirely independent and that linear combinations of two sets of new values F (which expresses the field-effect contribution) and R (the resonance contribution) satisfactorily express 43 sets of values. Each set is expressed as... 

Prasad, B. L. V. Stoeva, S. 1. Sorensen, C. M. and Klabimde, K J. (2002). Digestive Ripening of Thiolated Gold Nanoparticles The Effect of Alkyl Chain Length. Langmuir, 28, 7515-7520. 

Markovnikov s rule describes a specific case of regioselectivity that is based on the stabilizing effect of alkyl and aryl substituents on carbocations. 

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