Rate constants solvolysis, ethyl

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). 

As a result of the inductive and hyperconjugative effects it is to be expected that tertiary carbonium ions will be more stable than secondary carbonium ions, which in turn will be more stable than primary ions. The stabilization of the corresponding transition states for ionization should be in the same order, since the transition state will somewhat resemble the ion. Thus the first order rate constant for the solvolysis of tert-buty bromide in alkaline 80% aqueous ethanol at 55° is about 4000 times that of isopropyl bromide, while for ethyl and methyl bromides the first order contribution to the hydrolysis rate is imperceptible against the contribution from the bimolecular hydrolysis.217 Formic acid is such a good ionizing solvent that even primary alkyl bromides hydrolyze at a rate nearly independent of water concentration. The relative rates at 100° are tertiary butyl, 108 isopropyl, 44.7 ethyl, 1.71 and methyl, 1.00.218>212 One a-phenyl substituent is about as effective in accelerating the ionization as two a-alkyl groups.212 Thus the reactions of benzyl compounds, like those of secondary alkyl compounds, are of borderline mechanism, while benzhydryl compounds react by the unimolecular ionization mechanism. 

We have examined the competing isomerization and solvolysis reactions of 1-4-(methylphenyl)ethyl pentafluorobenzoate with two goals in mind (1) We wanted to use the increased sensitivity of modern analytical methods to extend oxygen-18 scrambling studies to mostly aqueous solutions, where we have obtained extensive data for nucleophilic substitution reactions of 1-phenylethyl derivatives. (2) We were interested in comparing the first-order rate constant for internal return of a carbocation-carboxylate anion pair with the corresponding second-order rate constant for the bimolecular combination of the same carbocation with a carboxylate anion, in order to examine the effect of aqueous solvation of free carboxylate anions on their reactivity toward addition to carbocations. 

The solvolysis rates of 2-(dimethylphenylsilyl)-l-(Y-phenyl)ethyl 3,5-dinitroben-zoates (62) in 60% aqueous ethanol were analysed using using the Yukawa-Tsuno equation.103 The p value of -2.95 with r 1.04 found by changing the a-aryl substituent was much smaller than the p value of -5.45 found for the corresponding non-silylated system. This, the fact that the rate constant for the silylated substrate was > 105-fold faster than the rate of the non-silylated compound, and the small a value of 0.52 found from the log( Y/ H)si = a log( Y/ H)non-si plot was taken as evidence that the reaction of the silyl compound occurred via a tight transition state with significant neighbouring silyl participation (Scheme 25). 

The Yukawa-Tsuno equation continues to find considerable application. 1-Arylethyl bromides react with pyridine in acetonitrile by unimolecular and bimolecular processes.These processes are distinct there is no intermediate mechanism. The SnI rate constants, k, for meta or j ara-substituted 1-arylethyl bromides conform well to the Yukawa-Tsuno equation, with p = — 5.0 and r = 1.15, but the correlation analysis of the 5 n2 rate constants k2 is more complicated. This is attributed to a change in the balance between bond formation and cleavage in the 5 n2 transition state as the substituent is varied. The rate constants of solvolysis in 1 1 (v/v) aqueous ethanol of a-t-butyl-a-neopentylbenzyl and a-t-butyl-a-isopropylbenzyl p-nitrobenzoates at 75 °C follow the Yukawa-Tsuno equation well, with p = —3.37, r = 0.78 and p = —3.09, r — 0.68, respectively. The considerable reduction in r from the value of 1.00 in the defining system for the scale is ascribed to steric inhibition of coplanarity in the transition state. Rates of solvolysis (80% aqueous ethanol, 25 °C) have been measured for 1-(substituted phenyl)-l-phenyl-2,2,2-trifluoroethyl and l,l-bis(substi-tuted phenyl)-2,2,2-trifluoroethyl tosylates. The former substrate shows a bilinear Yukawa-Tsuno plot the latter shows excellent conformity to the Yukawa-Tsuno equation over the whole range of substituents, with p =—8.3/2 and r— 1.19. Substituent effects on solvolysis of 2-aryl-2-(trifluoromethyl)ethyl m-nitrobenzene-sulfonates in acetic acid or in 80% aqueous TFE have been analyzed by the Yukawa-Tsuno equation to give p =—3.12, r = 0.77 (130 °C) and p = —4.22, r — 0.63 (100 °C), respectively. The r values are considered to indicate an enhanced resonance effect, compared with the standard aryl-assisted solvolysis, and this is attributed to the destabilization of the transition state by the electron-withdrawing CF3 group. 

Okamoto, Y., Inukai, T. and Brown, H.C. (1958c). Rates of Solvolysis of Phenyldimethylcarbinyl Chlorides in Methyl, Ethyl and Isopropyl Alcohols. Influence of the Solvent on the Value of the Electrophilic Substituent Constant. J.Am.Chem.Soc., 80,4972-4976. 

Quantitative data are available for the reaction of solvolysis of 1(2-tellurienyl)ethyl acetates in 30% ethanol.38,59 The first-order rate constants for the reaction at 60° of all the four congener systems, the activation parameters, the rates relative to the thiophene derivative (k/kTb), and the rates of the 3-methyl-substituted compounds relative to the corresponding unsubstituted derivatives (k Jkf,) are summarized in Table XIX. 

Rate Constants, Activation Parameters, and Relative Rates for the Solvolysis of I(2-Aryl)Ethyl Acetates in 30% Ethanol... 

The relative rate constants for solvolysis in buffered acidic solution of compounds with the structure H2NCH2CR2CH2CH2Br vary with the structure of the R group as follows R = hydrogen, 1.0 R = methyl, 158 R = ethyl, 594 R = isopropyl, 9190. Explain this variation in reactivity. 

CMAIUNGE] [(l-Bromo-l-methyl)ethyl]benzene, shown below, undergoes solvolysis in a unimolecular, strictly first-order process. The reaction rate for [RBr] = 0.1 M RBr in 9 1 acetone water is measured to be 2 X 10 mol L s . (a) Calculate the rate constant k from these data. What is the product of this reactiou (b) In tlie presence of 0.1 M LiCl, the rate is foimd to increase to 4 X 10 mol L s , although the reaction stiU remains strictly first order. Calculate the new rate constant and suggest an explanation, (c) When 0.1 M LiBr is present instead of LiCl, the measured rate to 1.6 X 10 mol L s . Explain this observation, and write the appropriate chemical equations to describe the reactions. 

The rate constants for the solvolysis of chloromethyl ethyl ether, chloromethyl octyl ether, and chloromethyl methyl sulfide have been determined in several pure and binary solvents. Application of the extended Grunwald-Winstein equation, logffe/fe, ) = /Nj + mY + c, gave appreciable T values (0.55-0.71) for the three substrates indicating that there is significant nucleophilic solvation of the developing carbenium ion in the transition states of these reactions. The kQ lkp = 1.2 x 10 found for the hydrolysis of chloromethyl methyl ether in water is virtually identical to that observed for the uni-molecular solvolyses of t-butyl chloride and trityl halides confirming the unimolecular mechanism for these reactions. 

In contrast, neither electrophilic substituent constants nor constants provide a suitable basis for correlating the relative rates of solvolysis of l-(2-substituted 4-thiazolyl)ethyl chlorides, recalling similar poor correlations in several other heterocyclic systems. An alternative working model, based on a comparison of appropriately substituted pyridines and thiazoles, provides an adequate treatment of the substituent effects in these instances. ... 

The rate of solvolysis of l-(2-phenyl-5-thiazolyl)ethyl chloride is consistent with the substituent having an electrophilic substituent constant value of cTp = -0.34. This result is accounted for satisfactorily in terms of the coplanarity of the triazole and phenyl rings in this structure. ... 

Evaluation of solvent-sensitive properties requires well-defined referena i ran eis. A macroscopic parameter, dielectric constant, does not always give interpretable correlations of data. The first microscopic measure of solvent polarity, the Y-value, based on the solvolysis rate of t-butyl chloride, is particularly valuable for correlating solvolysis rates. Y-values are tedious to measure, somewhat complicated in physical basis, and characterizable for a limited number of solvents. The Z-value, based on the charge-transfer electronic transition of l-ethyl-4-carbomethoxy-pyridinium iodide , is easy to measure and had a readily understandable physical origin. However, non-polar solvent Z-values are difficult to obtain b use of low salt solubility. The Et(30)-value , is based on an intramolecular charge-transfer transition in a pyridinium phenol b ne which dissolves in almost all solvents. We have used the Er(30)-value in the studies of ANS derivatives as the measure of solvent polarity. Solvent polarity is what is measured by a particular technique and may refer to different summations of molecular properties in different cases. For this reason, only simple reference processes should be used to derive solvent parameters. 

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. 

---