Electronic effects solvolysis rate

Electronic effects upon rates of solvolysis of benzoyl chlorides suggest that the mechanism, i.e. the extents of rehybridization of chloride and carbonyl oxygen atoms, depends upon the solvent. In aqueous acetone of low water content, a plot of log k against a is linear and p 2 for a limited range of substituents, whereas in moist formic acid the corresponding plot also is... 

The allyl cation thus formed may stabilize itself either by readdition of the leaving group — leading to a 2,3-dihalopropene — or by the addition of a nucleophile. The influences of steric and electronic effects on the stereochemistry and on the solvolysis rates of various alkyl-substituted chlorocyclopropanes have been investigated by Parham and co-workers [165, 166], who could show for example that os-2,3-dipropyl-l,l-dichlorocyclopropane solvolyzes 24 times faster than its trans-isomer, in accordance with predictions based on orbital symmetry arguments. When one propyl substituent of the trans-isomer is replaced by an ethoxy group the rate of solvolysis increases 200 fold. 

A quantitative scale for the structural effect of various silyl groups is established, as shown in entry 57 of Table 1, by the rates of solvolysis of 40 triorganosilyl chlorides in aqueous dioxane under neutral conditions69. The structural effect involves the steric effect and, in some examples, the electronic effect. Because little difference exists in the electronic effect among alkyl groups, their steric effect at silicon follows the order primary < secondary < tertiary substituents. 

Spinner, 1954 Vernon, 1954). The opposing destabilization is probably due to the greater (electron-withdrawing) inductive effect of the triple bond than that of the double bond in allyl cations. As a matter of fact, it has been evaluated from the solvolysis rate of derivatives 174 and 175 that the resonance contributions of the limiting forms (172a) and (173a) to their respective hybrids are similar (Richey and Richey, 1970). 

Rates of solvolysis of 17-substituted 5a-androstan-3-yl tosylates show greater sensitivity to the electronic effects of C-17 substitution than can be attributed to simple dipole-dipole or charge-dipole interactions between the centres of substitution and reaction. Moreover the solvolysis rates observed in formic, acetic, and trifluoroacetic acids do not reflect the pronounced differences between solvent... 

In an endeavor to study the transmission of substituent effects in imidazoles Noyce (73JOC3762) examined the solvolysis rates of a series of l-(l-methylimidazolyl)ethyl p-nitrobenzoates (75-77 OPNB = p-nitrobenzoate). The relative rates (75 76 77 = 1 13 15) parallel the relative electron densities in 1-methylimidazole as deduced from chemical shift data. By comparison with other heteroarylethyl p-nitrobenzoates the effective replacement constants, crXr, were determined as a-2-im = —0.82, o-J-im = —1.01 and <7-5.1 = —1.02. The effects on the 2-substituted compound of alkyl, aryl and halogen substituents at the 4- and 5-positions were examined, but though the rates for the 5-substituents could be represented satisfactorily by cTp, substituted compounds. It is not surprising that the distorting effects of annular heteroatoms make it difficult to superimpose the substitution behavior of benzenoid compounds into this series. 

A second CF3 group had an equally strong effect. In this case, two CF3 groups decrease the electron density of the C=C bond to the point that the solvolysis rate for 44 (R = = CF3) was about the same as (acmally 17 times slower... 

The relative rates of solvolysis for RCMe20PNB of 1 (Me), Ph (969), and c-Pr (503 X 10 ) was taken as evidence by Brown and Peters " that the net electron-donating ability of these groups increased in this order. Other examples of the c-Pr > Ph order of substituent effects on solvolysis rates are found in systems 112 and 136 which give k(c-Pr)//c(Ph) rate ratios of 200 in the former (Section V.F), and 34 in the latter (equations 21, 22). ... 

Introduction to of electron-releasing substituents sharply decreases the exo endo rate ratio there is no such effect if two geminal methyl groups are introduced to C. The value 3 for the exo endo rate ratio resembles greatly the usual epimeric rate ratio for secondary substrates without anchimeric assistance (see above). Introduction of an electron-releasing substituent to levels the o-participation in the solvolysis of the secondary 2-exo-epimer, but it does not affect the rate ratio of the tertiary analogues for which the solvolysis rate is predominantly determined by steric factors, for example ... 

In contrast to Brown s assertions and in accord with Winstein s and Trifan s assumption, the solvolysis of these secondary systems proceeds with anchimeric acceleration. This is concluded from the following facts a) the exo endo rate ratio for 2-norbomyl systems is 10 -10 as the reaction rate of the endo isomer is not anomalous (see above), hende the exo isomer reacts at an elevated rate b) the rate of solvolysis of exo isomers is 10 to 10 times as high as that calculated according to the semiempirical scheme from only steric effects c) the ratio of the reaction rate of secondary 2-exo-norbomyl systems to the solvolysis rate of secondary cyclopentyl analogues is 100 times as great as that of tert-2-exo-norbomyl derivatives and tert-cyclopentyl analogues since tert-2-norbomyl derivatives are solvolyzed without anchimeric assistance, the factor of 100 characterizes tentatively the amount of anchimeric assistance in the secondary 2-exo-norbornyl systems d) exo- and endo-6-substituents decrease the solvolysis rate of 2-exo-norbomyl tosylate this cannot be accounted for without participation of the electrons of the 1,6 bond in the transition state their participation increases the non-bonded interaction due to a decrease in the C -C distance. 

Gassman s results on the solvolysis of compounds 77 and 125 are of great interest the high exo endo rate ratio, the formation of the same products, and the absence of endo products show that the norboraanes in which a cationic centre on C is suppressed by electronic effects behave like unsubstituted 2-norbomyl tosylates. 

The solvolysis rates of the C-5-substituted epimeric 2-norbomyl p-bromobenzene-sulphonates (92) and (93) and their 2-methyl homologues have been investigated in 60% aqueous ethanol and in 97% hexafluoropropan-2-ol at various temperatures. The rate ratio exo-(92) endo-(92) is solvent-dependent, rising to the value 1746in hexafluoro-propan-2-ol at 25 °C, indicating the absence of internal return for exo-(92) and nucleophilic solvent assistance for endo- 92). The rates for the epimeric brosylates (93) are lower, the exo-brosylate being affected more, even in hexafluoropropan-2-ol. The rate ratio reductions are much less pronounced in the tertiary series. The results are as expected for the stabilization of the transition state from exo-(92) by 0-delocalization or some other electronic effect. 

---