Electronic effects susceptibility

Electronic effects within the acid portion of the precursor have also been utilized for enhanced reactivity. The 4-hydroxybenzenesulfonate ester of octanoyloxyacetic acid, (15), undergoes efficient perhydrolysis at lower pHs because of the activation of the susceptible carbonyl by the beta-oxygen of the hydrophobic tail (100). 

Some authors use O] instead of cr as the substituent constant in such correlations.) An example is provided by the aminolysis of phenyl esters in dioxane the substrates RCOOPh were reacted with -butylamine, and the observed first-order rate constants were related to amine concentration by = k2 [amine] kj [amine]. The rate constants kz and k could be correlated by means of Eq. (7-54), the reaction constants being p = +2.14, b = + 1.03 (for A 2) and p = -1-3.03,8 = -1-1.08 (for ks). Thus, the two reactions are about equally sensitive to steric effects, whereas the amine-catalyzed reaction is more susceptible to electronic effects than is the uncatalyzed reaction. 

Introduction of first one, and then two, CH2 groups between the benzene ring and C02H progressively reduces the susceptibility of the acid s ionisation to the polar effect of the substituent X in the benzene ring. The susceptibility, as revealed by the value of p, rises again for (16), however, as CH=CH is a markedly better transmitter of electronic effects than is CH2—CH2. 

The symbol k or K is the rate or equilibrium constant, respectively, for a side-chain reaction of a meta- or para-substituted benzene derivative, and or Kf> denotes the statistical quantity (intercept term) approximating to k or K for the parent or unsubstituted compound. The substituent constant o measures the polar (electronic) effect of replacing H by a given substituent (in the meta- or para-position) and is, in principle, independent of the nature of the reaction. The reaction constant p depends on the nature of the reaction (including conditions such as solvent and temperature) and measures the susceptibility of the reaction to polar effects. Hammett chose the ionization of benzoic... 

The electronic properties of n-conjugated polymers reflect well the basic electron-withdrawing or electron-donating properties of the components of the Ti-conjugated polymer [62]. In view of the electrochemical reduction potential, the thiophene unit and tetrathiafulvalene unit (Nos. 8 and 9 in Table 1) have a similar electronic effect in PAEs. It is reported that poly(arylenevinylene)s are also susceptible to electrochemical reduction [63, 64]. Due to the electron-accepting properties, PAEs are usually inert in electrochemical and chemical (e.g.,by I2 [54]) oxidation. 

The next Table investigates these questions more fully. The second column shows the diamagnetic susceptibility, and the third the inner-electron effect calculated as before (75). Subtracting this effect, the ms electron-cloud radii follow immediately from Eq. (1). 

The positional reactivity order in thiazole is again the expected one. However, the magnitudes of the different from those in Scheme 7.12. The most probable cause for this is that the azoles are very susceptible to demands for resonance stabilization of the transition state for a particular reaction. This is not unexpected, because the reactivity of the azoles is the product of two opposing electronic effects from the heteroatoms, which are each large. A small alteration in the demand for resonance in a particular reaction may dramatically upset this balance. 

The regioselectivity of monohydroboration is governed both by steric and electronic effects, as illustrated in Figure 8. Once again, 9-BBN-H is more susceptible to electronic factors and less to steric factors than is disiamylborane. ... 

In general, the regioselectivities and stereoselectivities of reactions with disiamylborane are more susceptible to steric effects and less to electronic effects, but are otherwise similar to those of 9-BBN-H (Section 3.10.4.2). ... 

Azides and tetrazoles usually equilibrate slowly at room temperature and the position of the equilibrium, which varies with temperature and solvent, is also susceptible to steric and electronic effects, electron-withdrawing substituents favouring the azide tautomer. The azinyl-azide/tetrazole tautomeric equilibrium, which has been extensively investigated and reviewed [9, 10], also arises when the C = N double bond comprises part of a heterocyclic ring, as exemplified by 2-azidoadenosine (5a, R = J3-d-ribofuranosyl) synthesized for use, as the diphosphate, as a photoaffinity reagent for the ADP receptor [11]. The azide was found to equilibrate with one or both of the two possible tetrazoles (5b and 5c) with an azide. tetrazole ratio of 1 1 at pH 7.0, and in contrast to the azide, which was readily photolyzed at 270nm, the tetrazole tautomer was resistant to photolysis. Azides that exist predominantly as tetrazoles may thus exhibit different... 

Finally, it is of interest that electron-withdrawing groups attached to the aromatic nucleus of zirenediazonium compounds favour reaction through the acyclic intermediate (Table 13). Since the ratio Qcn/r is proportional to kch.flK.r it be inferred that is more susceptible to electronic effects than k f. 

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