Grunwald-Winstein Analysis - Isopropyl Chloroformate Solvolysis Revisited

Abstract

Specific rates of solvolysis at 25 °C for isopropyl chloroformate (1) in 24 solvents of widely varying nucleophilicity and ionizing power, plus literature values for studies in water and formic acid, are reported. Previously published solvolytic rate constants at 40.0 °C are supplemented with two additional values in the highly ionizing fluoroalcohols. These rates are now are analyzed using the one and two-term GrunwaldWinstein Equations. In the more ionizing solvents including ten fluoroalcohols negligible sensitivities towards changes in solvent nucleophilicity (l) and very low sensitivities towards changes in solvent ionizing power (m) values are obtained, evocative to those previously observed for 1-adamantyl and 2-adamantyl chloroformates 2 and 3. These observations are rationalized in terms of a dominant solvolysis-decomposition with loss of the CO 2 molecule. In nine of the more nucleophilic pure alchohols and aqueous solutions an association-dissociation mechanism is believed to be operative. Deficiencies OPEN ACCESSInt. J. Mol. Sci. 2009, 10 863 in the acid production indicate 2-33% isopropyl chloride formation, with the higher values in less nucleophilic solvents.

Kinetic Studies that Evaluate the Solvolytic Mechanisms of Allyl and Vinyl Chloroformate Esters

“…On the other hand for secondary chloroformates, an evaluation of the kinetic rate data obtained for the bulk of the solutions studied resulted in a proposal that isopropyl chloroformate [53] and 2-adamantyl chloroformate [54] show dominant solvolysis-decomposition with loss of the CO 2 molecule. This mechanism parallels the behavior observed for tertiary 1-adamantyl chloroformate in all of the solvents studied where the major products are the decomposition product, 1-adamantyl chloride, and an ether and/or the alcohol (depending on the solvent components) [55].…”

Detailed analysis for the solvolysis of isopropenyl chloroformate

“…On the other hand dissecting the data obtained from extensive correlation analysis [17,27] results using equation 1, for 1 ’s alkyl analog isopropyl chloroformate ( i -PrOCOCl, 2 ), the observed [15,27,61] k 2 / k i-PrOCOF rate ratio, and a consideration of a previously reported KSIE value [51] of 1.25 in water, it was shown [27] that 2 solvolyzes by dual channels; with an addition-elimination pathway being dominant in the more nucleophilic solvents and a fragmentation-ionization mechanism (Scheme 2) proceeding in the strongly hydrogen-bonding (ionizing) fluoroalcohols.…”

“…Furthermore, with equation 2 an h value of 0.42 ± 0.15 signified that there was minimal charge dispersion into the aromatic ring during the formation of the thioacylium transition state that was stabilized by intense rear-side nucleophilic solvation (as indicated by the large l value). These sensitivity values are now considered typical [2,7,23,27,29,30,33,34,49,56] for substrates believed to solvolyze with the formation of an acylium or a thioacylium ion in the transition state.…”

“…Also, it was demonstrated [2,12,14,15,18,20,21,23–27] that dual reaction channels occurring simultaneously in a side-by-side fashion are possible in some alkyl and aryl chloroformate esters, and that the highly ionizing aqueous 2,2,2,-trifluoethanol (TFE) and 1,1,1,3,3,3,-hexafluoro-2-propanol (HFIP) mixtures are shown [2,3,6–49,55,56,71–74] to be extremely important for meaningful treatments leading to analyses using the Grunwald-Winstein equations. To probe the possibility of an ionization pathway for 1 , we have raised the temperatures (so that the kinetic runs could be followed within a reasonable time frame) and in Table 1 have included 16 specific rates of solvolysis in six solvents with strong hydrogen bonding (highly ionizing) fluoroalcohol components.…”

Rate and Product Studies of 1-Adamantylmethyl Haloformates Under Solvolytic Conditions

“…In methanol, the k MeOH /k MeOD ratio was in the range of 2.00 to 3.98 for solvolyses of alkyl and aryl haloformates, which have been reported to proceed through a bimolecular mechanism. [1][2][3][4][8][9][10][11]30,31 The k ROH /k ROD values for i-propyl chloroformate and t-butyl fluoroformate in the range of an ionization mechanism were 1.25 5 in pure water (S=H) and 1.26 12 in methanol (S=Me), respectively. Activation Parameters.…”