A-values table

Exercise 26-42 From appropriate p values (Table 26-7) and a- values (Table 26-6), calculate the rates of hydrolysis of 4-CH3-, 4-CH30, 4-N02-phenyl methyl chlorides relative to phenylmethyl chloride (a) in water at 30° in the presence of base, and (b) in 48% ethanol at 30°. Explain why there is a greater spread in the relative rates in (b) than in (a). 

Here we refer to the enumeration data of benzenoid isomers as complete if, for a given h, all the numbers of C HS isomers are given at least for the Kekulean and non-Kekulean systems separately. Such data are known for h values up to 14 cf. Tables 1 -4. In addition, we have specified the numbers of normal (n) and essentially disconnected (e) benzenoids among the Kekulean systems in the tables. The smallest essentially disconnected benzenoid, viz. perylene, occurs at h = 5. Furthermore, the non-Kekulean systems are classified according to the A values (Tables 1-4). 

Cobalt (III) complexes appear to have B and C values of about 500 and 3800 cm. , respectively. These typical values give a crossover point of 16,500 cm. , which has been used in Table III to estimate the spin-free activation energies of the other species, the spin-free activation energies being the amount by which 2A exceeds 5 B + 8 C. That the position of the spin-free crossover is not too low is indicated by the fact that the hypothetical spin-paired [Co Fe]" ion has a A value (Table IV) about equal to the crossover point estimated from the Racah parameters, B and C. The spin-paired activation energies, estimated at 0.4A, are also given in Table III for comparison purposes. 

Ortho substituents are omitted from the table of representative a values (Table 5) because the steric effect which may be felt by the reaction in question will be largely absent from a a value (the change in size and in solvation on ionisation of the carboxyl group is not very large). Nevertheless it is quite possible to have a linear correlation for a reaction with constant ortho substituents. 

In this equation, k/k is the relative rate of reaction of the substituted versus the unsubstituted (R = CH3) ester, and the subscripts B and A refer to basic and acidic conditions respectively. Equation (3.5) assumes that the reactivity in acid is independent of electronic effects, whereas in base the reactivity depends on both steric and electronic effects. The divisor of 2.48 is arbitrarily chosen to place (7 values on a scale similar to that of normal a values. Table 3.1 lists cr values of typical substituents. 

The river run-off (Tables 2.28 and 2.29) is about 0.46 10 g yr carbon and is much larger than the total wet deposited carbonate (0.13 10 g yr carbon). The global volcanic CO2 emission is uncertain and there is given a value (Table 2.41) of... 

These parameters show that the rate of the hydrolysis reaction is faster with hydrogen than methyl, and slows as the R group incrementally increases in size to f-butyl (that is, Es becomes more negative). Phenyl, however, comes out surprisingly large in this analysis (Es = -2.55). In Chapter 2 we noted that efforts to characterize the steric size of a group will be strongly context dependent, and this is an example. In an alternative measure of the steric size of substituents—the cyclohexane A value (Table 2.14)—phenyl is much smaller than f-butyl. 

Figure 5.13 Comparison of a, values (Table 5.11) with those predicted from the use of Eq. (5.20).

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