Corrrelation of the Specific Rates of Solvolysis of Ethyl Fluoroformate Using the Extended Grunwald-Winstein Equation

The specific rates of solvolysis of ethyl fluoroformate have been measured at 24.2 °C in 21 pure and binary solvents. These give a satisfactory correlation over the full range of solvents when the extended Grunwald-Winstein equation is applied. The sensitivities to changes in the NT solvent nucleophilicity scale and the YCl solvent ionizing power scale, and the kF/kCl values are very similar to those for solvolyses of n-octyl fluoroformate, consistent with the addition step of an addition-elimination pathway being rate-determining. For methanolysis, a solvent deuterium isotope effect of 3.10 is compatible with the incorporation of general-base catalysis into the substitution process. For five representative solvents, studies were made at several temperatures and activation parameters determined. The results are also compared with those reported earlier for ethyl chloroformate and mechanistic conclusions are drawn

Extended Grunwald-Winstein Analysis - LFER Used to Gauge Solvent Effects in p-Nitrophenyl Chloroformate Solvolysis

Specific rates of solvolysis at 25 °C for p-nitrophenyl chloroformate (1) are analyzed using the extended (two-term) Grunwald-Winstein equation. For 39 solvents, the sensitivities (l = 1.68±0.06 and m = 0.46±0.04) towards changes in solvent nucleophilicity (l) and solvent ionizing power (m) obtained, are similar to those previously observed for phenyl chloroformate (2) and p-methoxyphenyl chloroformate (3). The observations incorporating new kinetic data in several fluoroalcohol-containing mixtures, are rationalized in terms of the reaction being sensitive to substituent effects and the mechanism of reaction involving the addition (association) step of an addition-elimination (association-dissociation) pathway being rate-determining. The l/m ratios obtained for 1, 2, and 3, are also compared to the previously published l/m ratios for benzyl chloroformate (4) and p-nitrobenzyl chloroformate (5)

Use of Empirical Correlations to Determine Solvent Effects in the Solvolysis of S-Methyl Chlorothioformate

The specific rates of solvolysis of S-methyl chlorothioformate (MeSCOCl) are analyzed in 20 solvents of widely varying nucleophilicity and ionizing power at 25.0 °C using the extended Grunwald-Winstein Equation. A stepwise SN1 (DN + AN) mechanism is proposed in the more ionizing solvents including six aqueous fluoroalcohols. In these solvents, a large sensitivity value of 0.79 towards changes in solvent nucleophilicity (l) is indicative of profound rearside nucleophilic solvation of the developing carbocation. In twelve of the more nucleophilic pure alchohols and aqueous solutions, the sensitivities obtained for solvent nucleophilicity (l) and solvent ionizing power (m) are similar to those found in acyl chlorides where an association-dissociation (AN + DN) mechanism is believed to be operative

Grunwald-Winstein Analysis - Isopropyl Chloroformate Solvolysis Revisited

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 Grunwald-Winstein 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 CO2 molecule. In nine of the more nucleophilic pure alchohols and aqueous solutions an association-dissociation mechanism is believed to be operative. Deficiencies in the acid production indicate 2-33% isopropyl chloride formation, with the higher values in less nucleophilic solvents

Analysis of the Nucleophilic Solvation Effects in Isopropyl Chlorothioformate Solvolysis

Correlation of the solvent effects through application of the extended Grunwald-Winstein equation to the solvolysis of isopropyl chlorothioformate results in a sensitivity value of 0.38 towards changes in solvent nucleophilicity (l) and a sensitivity value of 0.72 towards changes in solvent ionizing power (m). This tangible l value coupled with the negative entropies of activation observed indicates a favorable predisposition towards a modest rear-side nucleophilic solvation of a developing carbocation. Only in 100% ethanol was the bimolecular pathway dominant. These observations are very different from those obtained for the solvolysis of isopropyl chloroformate, where dual reaction channels were proposed, with the addition-elimination reaction favored in the more nucleophilic solvents and a unimolecular fragmentation-ionization mechanism favored in the highly ionizing solvents

Correlation of the Rates of Solvolysis of Neopentyl Chloroformate—A Recommended Protecting Agent

The specific rates of solvolysis of neopentyl chloroformate (1) have been determined in 21 pure and binary solvents at 45.0 °C. In most solvents the values are essentially identical to those for ethyl and n-propyl chloroformates. However, in aqueous-1,1,1,3,3,3-hexafluoro-2-propanol mixtures (HFIP) rich in fluoroalcohol, 1 solvolyses appreciably faster than the other two substrates. Linear free energy relationship (LFER) comparison of the specific rates of solvolysis of 1 with those for phenyl chloroformate and those for n-propyl chloroformate are helpful in the mechanistic considerations, as is also the treatment in terms of the Extended Grunwald-Winstein equation. It is proposed that the faster reaction for 1 in HFIP rich solvents is due to the influence of a 1,2-methyl shift, leading to a tertiary alkyl cation, outweighing the only weak nucleophilic solvation of the cation possible in these low nucleophilicity solvents

Concerted Solvent Processes for Common Sulfonyl Chloride Precursors used in the Synthesis of Sulfonamide-based Drugs

Specific rates of solvolysis in hydroxylic solvents available for the solvolysis of 2-thiophenesulfonyl chloride and phenylmethanesulfonyl chloride are supplemented by determining the values in fluoroalcohol-containing solvents. The data sets are then correlated using the extended Grunwald-Winstein equation. For both substrates, it is found that a single correlation controls the influence of solvent over the full range of solvent composition. The sensitivities to solvent nucleophilicity and solvent ionizing power are compared to values available for other substrates. Three of these previous studies are upgraded by the incorporation of additional specific rate values from the recent literature. With a methyl, isopropyl, benzyl, aromatic or heteroaromatic group as the R group of RSO2Cl, a concerted SN2 mechanism is proposed for the solvolysis

The Influence of Carbon-Carbon Multiple Bonds on the Solvolyses of Tertiary Alkyl Halides: a Grunwald-Winstein Analysis

The influence of carbon-carbon multiple bonds on the solvolyses of 3-chloro-3-methylbutyne (1), 2-chloro-2-phenylpropane (2), 2-bromo-2-methyl-1-phenylpropane (3), 4-chloro-4-methyl-2-pentyne (4) and 2-chloro-2-methylbutane (5) is critically evaluated through the extended Grunwald-Winstein equation. Substrates 1, 3 and 5 lead to correlations with unexpected negative sensitivity, h, to changes in the aromatic ring parameter, I. It is claimed that I is not a pure parameter, reflecting also some solvent nucleophilicity, NOTs, character. In substrates 2 and 4 the possibility of rearside solvation is reduced due to steric hindrance and/or cation stabilization and the best found correlations involve only the solvent ionizing power, Y, and I

Heading to Helsinki

Sarah Brain, Michelle Bond and Sarah Kevill share their experience of visiting the University of Helsinki during International Staff Exchange Week 2015

The adaptive reuse of grain elevators into housing: how policy and perspectives affect the conversion process and impact downtown revitalization

This study seeks to examine how the conversion of grain elevators into housing is an effective method of adaptive reuse. It uses theories and concepts on heritage preservation, downtown revitalization, place theory and environmental sustainability. Based on the literature review, there is a need for change in planning policy and there are both advantages and disadvantages to adaptive reuse. The methodology and data sources include examining and analyzing planning documents, surveys for the public and professionals, and demographic data. Case studies included converted grain elevators located in Australia and Norway and also a case study in Canada for the purpose of future recommendations. These methods answer the research question of how do planning policies and the perspectives of planning professionals and the public affect the process of the adaptive reuse of grain elevators into housing? Subsequent questions include topics such as whether adaptive reuse is an effective approach to downtown revitalization, which policies impede or facilitate the process, how perspectives influence decisions, and how demographics are linked to housing availability. The significance of this study on planning practice is that it helps form policy recommendations to address the needs of the public and help improve the efficiency of adaptive reuse in the planning process. In conclusion, the public and professionals were generally in favour of this type of adaptive reuse but many had concerns about cost and gentrification. Also, more policies need to be created that address adaptive reuse specifically. For the future use of the Toronto case study I recommended that converting the grain elevator into housing is the optimal choice. The limitations of this study include data availability, non-responses for surveys, language barriers, case study locations, and time constraints