Polar reaction constant

Roberts269 has studied the hydrolysis of series of ethyl esters271 272 and alkyl benzoates273 in a limited range of water-dimethyl sulphoxide mixtures in some detail, and finds that the data for the hydrolysis of aliphatic ethyl esters (at 35°C in 85% dimethyl sulphoxide-water) fit the modified Taft equation (eqn. 5, p. 132). The values of p = 1.88 and 8 = 0.88 can be compared with p = 2.39 and 8 = 1.04 for 85% EtOH-water at the same temperature. The polar reaction constant is reduced in the dipolar aprotic solvent, consistent with a reduced degree of bond formation in the transition state, expected if the activity of the hydroxide ion is increased. However, Roberts considers that the sensitivity to steric effects, as measured by 8, would be reduced more substantially if bond formation were less advanced. It is difficult to accept this argument, since we... 

The symmetrical cis- and trans-olefins RCH=CHR (Table III) are particularly suited to this evaluation by Equation 10 because the polar reaction constant p and the steric correlation coefficient 8 reflect, in this series of olefins, the effect of varying two substituents. Hence, the expected effects (both on p and 8) are more pronounced than those expected for olefins in which only one substituent is varied. 

The bacteriostatic activity of a series of substituted trans-3-benzoylacrylic acids has been successfully correlated by Hansch linear free energy relations involving polar and partition substituent constants. The activity-lipophilicity relations for this series closely parallel those found previously for other antibacterial agents, with an ideal lipophilic character for gram-positive cells of 6.1 and, for linear dependence, a slope of 0.7. A polar reaction constant, p, of about —0.6 to —0.7 is given. A possible mode of action for these acids and their related substituted cis- and trans-3-benzoyl acrylic acids and esters is discussed as an enzyme-inhibitor interaction. 

The constants oi were taken equal to a scaled value of the aliphatic polar substituent constants a (which are defined in Section 7.3), and a was set at 3 (or a = in for substituents capable of through resonance). The resulting plots of Eq. (7-32) gave good LEER, which was interpreted to justify the approach. Refinements, - of this treatment showed that a depends upon the reaction, although most values fell ... 

Other considerations aside, the use of dilute reagents minimizes effects of nonideality. This allows the use of concentrations in place of activities. Of course, the time scale, the sensitivity of the analytical method at different concentrations, and the use of other reaction components introduce additional considerations. Tied closely to this decision is the choice of solvent. Reaction rates may (or may not) be affected by such variables as polarity, dielectric constant, hydrogen-bonding ability, donor capacity, and viscosity. A change in solvent may change not only the rate but also the mechanism and possibly even the products. One cannot even assume that the net reaction is the... 

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 k° or K° denotes the statistical quantity (intercept term) approximating to k or K for the parent or unsubstituted compound. The substituent constant a 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... 

When the reaction times for Step 1 are 5 min or longer, the samples severely crack, curl, or dissolve. These results suggest that substantial reaction is occurring in the bulk of the polymer. Significant hydrophilization can occur with reaction times as short as 5 s with RTD concentrations of 0.2-0.5 M. However, 0.002-0.02 M solutions of MeTD or PhTD do not allow sufficient reaction rates for surface hydrophilization at the shorter reaction times. Thus, diffusion of MeTD and PhTD into the polymer must occur readily from the acetonitrile solutions. Acetonitrile was used because it does not swell or dissolve the polymer or RTD-polymer adduct, and the RTDs are soluble and stable in it. This solvent is quite polar (dielectric constant, 38) (25), and this is probably a major factor in the partitioning of the relatively nonpolar RTDs between the polydiene film and the solvent. As noted below, more polar RTDs show less tendency to diffuse into the polymer. 

In the nineteen sixties, there was some confusion about the value of p, the reaction constant for polar effects of alkyl groups on bromination rates, as calculated according to Taft s equation, log (k/k0) = p a. For 22 alkenes substituted by one, two or three linear alkyl groups, there is a satisfactory relation (16) between their reactivities, log k, in methanol and the sum of their... 

The reaction constant p depends on nature of reaction, solvent and temperature. It is a measure of the susceptibility of the reaction to polar effect. A positive value of p for a reaction shows that the reaction is accelerated by electron withdrawing substituents (o = + 1.0). Thus a positive value of p indicates the reaction center has higher electron density in the transition state than in the starting material. While negative value of p indicates that the reaction center has a lower electron density in the transition state than in the starting material and the reaction is accelerated by electron donating... 

The polar effect was at first invoked to explain various directive effects observed in aliphatic systems. Methyl radicals attack propionic acid preferentially at the a-position, ka/kp = 7.8 (per hydrogen), whereas chlorine " prefers to attack at the /3-position, ka/kp = 0.03 (per hydrogen). In an investigation of f-butyl derivatives, a semiquanti-tative relationship was observed between the relative reactivity and the polar effect of the substituents, as evidenced by the pK, of the corresponding acid. In the case of meta- and / ara-substituted toluenes, it has been observed that a very small directive effect exists for some atoms or radicals. When treated by the Hammett relation it is observed that p = —0.1 for H , CeHs , P-CH3C6H4 and CHs . On the contrary, numerous radicals with an appreciable electron affinity show a pronounced polar effect in the reaction with the toluenes. Compilation of Hammett reaction constants and the type of substituent... 

The effects of polar substituents on the alkaline hydrolysis of esters are well-established. Since the rate of the reaction is determined largely by the rate of addition of hydroxide ion to the carbonyl group of the ester, any substituent which withdraws electrons from the carbonyl group will increase the reactivity of the ester. The most accessible quantitative measure of the effect is the Hammett or Taft reaction constant, and a large number of measurements are available. Taft et al.2i0 found p = 2.53 for the base-catalyzed methanolysis of meta- and para-substituted (/)-menthyl benzoates, closely similar to the known value of p = 2.37 for the alkaline hydrolysis of substituted ethyl benzoates. Jones and Sloane s value239, obtained with five esters, of p = 2.41 for the methoxyl exchange reaction of substituted methyl benzoates in methanol, is almost identical. 

In this equation AEl 2 is the shift in half-wave potential as a result of the introduction of the substituent X in place of hydrogen, p is the reaction constant, and polar substituent constant. For pyrrole systems, where it is only the substituent that is reduced, the ring being nonreducible, the equation is modified275,276 to... 

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