Substituent effects solvolysis

The more extensive problem of correlating substituent effects in electrophilic substitution by a two-parameter equation has been examined by Brown and his co-workers. In order to define a new set of substituent constants. Brown chose as a model reaction the solvolysis of substituted dimethylphenylcarbinyl chlorides in 90% aq. acetone. In the case ofp-substituted compounds, the transition state, represented by the following resonance structures, is stabilized by direct resonance interaction between the substituent and the site of reaction. 

In the second, which belongs to a systematic study of the transmission of substituent effects in heterocyclic systems, Noyce and Forsyth (384-386) showed that for thiazole, as for other simple heterocyclic systems, the rate of solvolysis of substituted hetero-arylethyl chlorides in 80% ethanol could be correlated with a constants of the substituent X only when there is mutual conjugation between X and the reaction center. In the case of thiazole this situation corresponds to l-(2-X-5-thiazolyl)ethyl chlorides (262) and l-(5-X-2-thiazolyl)ethyl chlorides (263). 

Another line of evidence that bridging is important in the transition state for solvolysis has to do with substituent effects for groups placed at C-4, C-5, C-6, and C-7 on the norbomyl system. The solvolysis rate is most strongly affected by C-6 substituents, and the exo isomer is more sensitive to these substituents than is the endo isomer. This implies that the transition state for solvolysis is especially sensitive to C-6 substituents, as would be ejqiected if the C(l)—C(6) bond participates in solvolysis. ... 

These results demonstrate that, within experimental error, the corresponding reaction constants for the two reactions, solvolysis and rearrangement, are the same. In other words, the two reactions have the same dependence on substituent effects, which is consistent with Scheme 8-10 because the transition state for rearrangement is identical to the first transition state in the mechanism of solvolytic dediazoniation. 

The kinetic studies on the solvolysis of 1 -aryl-l-chloro-3-methylbuta- 1,2-dienes (15, Ar = Ph, p-MeOCgH4) showed that the p values (p+ = —2.8 in 80% aqueous ethanol and —2.9 in aqueous acetone)21 are much lower than those of the correspondingly substituted vinyl derivative 16 (p+ = —4.3 in aqueous ethanol)22. This result indicates that the u-substituent effect in cations 17a is much smaller than in the vinyl cations, suggesting... 

In addition to these probes, theoretical calculations are being made in efforts to decide upon the inherently most stable stmcture of the ion. These will be discussed along with the conclusions which have been derived from the study of isotope effects in solvolysis and substituent effects in stabilizing media. 

Quantitative data are available for the solvolysis of l-(2-aryl)ethyl acetates (where 2-aryl = 2-furyl, 2-thienyl, 2-selenienyl and 2-tellurienyl) in 30% ethanol (77AHC(21)119). The rates at 60 °C relative to the thienyl derivative (1) are selenienyl (1.67), furyl (3.03) and tellurienyl (5.63). These data provide a measure of the effectiveness of the four ring systems in delocalizing the positive charge of the intermediate carbonium ions (80). The rates of the 5-methyl substituted compounds relative to the unsubstituted compounds at 25 °C are thienyl (70), selenienyl (23.3), furyl (160) and tellurienyl (11.8). Thus the furyl derivative is most sensitive to the substituent effect and the tellurienyl the least. The effect of benzo fusion on solvolysis rates has also been studied the solvolysis rate of 2-benzo[6]-thienyl-, -selenienyl- and -tellurienyl-ethyl acetates is less than that of the corresponding monocyclic esters by a factor of about 100. 

The Hammett cr—p correlation has proved useful in studying substituent effects of aryl-substituted systems.55 In Section 6.1 we consider applications of this linear free-energy relationship to solvolysis with rearrangement. 

Fuchs, R. Carlton, D. M. Multiple substituent effects in the solvolysis and thiosulfate reactions of 4-substituted a-chloro-3-nitrotoluenes. /. Org. Chem. 1962, 27,1520-1523. 

Acid-catalysed solvolysis of N-aryl phosphoramidates is characterised by the negative value of the reaction constant (p = -1.2).These inversed substituent effects illustrate two points discussed before. First, if the N-protonated form repre -sents the reactive intermediate in solvolysis of (2), much stronger dependence of the protonation preequilibrium on the effect of N-substitution is expected. Secondly, if the resonance effects are poorly transmitted to the P atom through the -NH- bridge, structural variation in the N-aryl substituent should have weak effect upon the ability of phosphorus to accept a nucleophile. 

However, the substituent effect is opposite to that for the SN1 route as discussed above, but fits an addition-elimination path. The kinetics at 65° may be explained by a fast base-catalysed addition forming the a J8-adduct (240), followed by its rate-determining solvolysis. Decreasing the temperature or substitution by electron-donating substituents... 

Simpler benzylic systems have been studied only later. The resurgence of organic photochemistry in the sixties touched this field, in particular with a seminal paper by Zimmerman and Sandel in 1963 [13], where it was shown that photoinduced solvolysis in substituted benzylic derivatives underwent a different substituent effect with respect to the ground state reaction. Activation by a m-methoxy group was found and rationalized in terms of LCAO MO electron distribution in the first electronic excited state. 

The use of the Hammett p constant as a measure of selectivity (and hence of transition state structure) in solvolysis reactions also appears to be unreliable. This is because p reflects not only the extent of bond making or breaking in the transition state but also the ability of a system to transmit the substituent effect to the reaction centre. This ability is variable and may in some cases even be dominant. Nishida (1967a, b), for example, noted that while a plot... 

The activation by a 5-methyl group on the 2-position was calculated as 170-fold (77G339) from unpublished data of Noyce et al. for solvolysis of I-arylethyl p-nitrobenzoates. This indicates a sensitivity to substituent effects greater than either those of furan or thiophene, despite the far higher reactivity of pyrrole. 

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