Dennis N. Kevill

Sixty years of the Grunwald–Winstein equation: development and recent applications

The development of scales of values for solvent nucleophilicity and for the aromatic-ring parameter are described. These are applied to solvolytic addition to carbocations and, together with improved solvent ionising power scales, to solvolyses proceeding with a 1,2-aryl shift and to solvolytic displacements at acyl carbon and at a heteroatom, such as phosphorus or sulfur.

Growth of crystals of several boron-carbon compositions by chemical vapor deposition

Abstract Boron-carbon compounds have been deposited by the flow of carbon tetrachloride and boron trichloride, in the presence of a large excess of hydrogen, over a graphite surface maintained at 1000–1300°C. Deposits were formed on either an RF-heated disc or a tube or insert heated by a resistance furnace. Crystalline materials ranging in composition from B2C to B17C have been obtained.

Solvent nucleophilicity. A scale based on triethyloxonium ion solvolysis

First-order rate coefficients have been obtained for the solvolysis of triethyloxonium hexafluorophosphate in a variety of organic and aqueous-organic solvents. These have been used to set up a scale of solvent nucleophilicities based upon the four-parameter (two-term) Grunwald-Winstein equation. The required corrections for variation in solvent influence upon the leaving group were estimated from data available for tert-butyldimethylsulfonium ion solvolysis. A previously established scale, based upon methyl p-toluenesulfonate solvolysis, can be brought into good agreement with this scale if a revised value is used for methyl p-toluenesulfonate sensitivity to the electrophilic influence of the solvent. The scale is applied to previously studied solvolyses of alkyl p-toluenesulfonates and chlorides. Determination of solvent nucleophilicities is complicated by concurrent solvent influence upon the leaving group. Previous attempts to establish a nucleophilicity scale for solvolytic reactions have used data from initially neutral substrate^;^,^ to a first approximation, the overall influence of the solvent could be considered as a nucleophilic push at the cy carbon and an electrophilic pull a t the leaving group. Two-term (fourparameter) equations have been proposed2t4 for the combination of these two effects within a linear free energy relationship. Subsequent ~ o r k ~ , ~ * has favored the earlier form ~ l a t i o n , ~ expressed in the equation log ( k / k o ) = IN + mY (1) I n eq 1, k represents the specific rate in a given solvent, ko the specific rate in 80% ethanol, and, for a given substrate, I and m represent the sensitivities of the solvolysis to N and Y , the 0002-?863/?9/1501-3916

CORRELATION OF THE RATES OF SOLVOLYSIS OF PHENYL CHLOROFORMATE

01.00/0 solvent nucleophilicity and the solvent ionizing power. The equation was developed as an extension of the more familiar one-term (two-parameter) GrunwaldWinstein equation9 (eq 2) from S N 1 to S N ~ solvolyses.

Grunwald-Winstein Analysis - Isopropyl Chloroformate Solvolysis Revisited

The specific rates of solvolysis of phenyl chloroformate in 21 solvents can be very well correlated using the extended Grunwald–Winstein equation, with incorporation of the NT solvent nucleophilicity scale and the YCl solvent ionizing scale, with sensitivities towards changes in the scale having values of 1.68 ± 0.10 and 0.57 ± 0.06, respectively. This is a solvolysis which, on the basis of several other types of evidence, is believed to follow an addition–elimination pathway with addition being rate-determining (or possibly an enforced concerted variant), and these sensitivities can be considered to be representative values for chloroformate esters following such a pathway.

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

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.

Rate and product studies with dimethyl phosphorochloridate and phosphorochloridothionate under solvolytic conditions

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.

Use of the Simple and Extended Grunwald-Winstein Equations in the Correlation of the Rates of Solvolysis of Highly Hindered Tertiary Alkyl Derivatives.

The specific rates of solvolysis of dimethyl phosphorochloridate and of dimethyl phosphorochloridothionate are very well correlated using the extended Grunwald-Winstein equation, with incorporation of the NT solvent nucleophilicity scale and the YCl solvent ionizing power scale. The sensitivity parameters (l and m) are similar to each other and also similar to previously recorded values for solvolyses of arenesulfonyl chlorides, which were proposed to follow a concerted displacement mechanism. For solvolyses in aqueous ethanol or aqueous methanol the product selectivities (S) are close to unity. For solvolyses in aqueous 2,2,2-trifluoroethanol, the values are too small to accurately measure, showing a very large preference for product formation involving nucleophilic attack by the water component. It is concluded that the chloride and chloridothionate solvolyses, in common with the solvolyses of arenesulfonyl chlorides, follow a concerted displacement mechanism.

Nucleophilic participation in the solvolysis of the t-butyldimethylsulfonium ion☆

The original Grunwald-Winstein equation (1948) involved the development of a scale of solvent ionizing power (Y). Subsequent work has refined this scale and involved the development of scales of solvent nucleophilicity (N) and a term to correct for deviations when aromatic rings are present, governed by the aromatic ring parameter (I). These three scales, and the sensitivities towards each, can be related to specific rates of solvolysis through linear free energy relationships (LFERs).One important area of application of LFERs has been to the solvolyses of tert-alkyl halides. It has been proposed that the solvolysis of tert-butyl chloride involves a nucleophilic component, although other workers have suggested that the effects observed are related to electrophilic not nucleophilic influences. Takeuchi (1997) studied a compound with two of the methyl groups of tert-butyl chloride replaced by neopentyl groups. For this highly-hindered substrate there was no evidence for nucleophilic participation. Liu (1998) and Takeuchi (2001) have reported concerning the solvolyses of additional significantly-hindered tertiary alkyl chlorides. Liu (2009) has presented a parallel study of bromides. Martins (2008) has considered hindered tertiary alkyl halides, mainly with carbon-carbon multiple bonds as substituents. It was proposed that the hI term was of importance, with the sensitivities (h) sometimes positive and sometimes negative. To explain negative values, it was suggested that the I scale might contain a nucleophilicity component. In this review, we bring together, with analysis and commentary, the work of Takeuchi, Liu, Martins and others concerning the solvolyses of tertiary alkyl halides, with emphasis on the relevance of the three scales that have been developed for use in Grunwald-Winstein correlations.

Kinetic evaluation of the solvolysis of isobutyl chloro- and chlorothioformate esters

Abstract Studies with the t -butyldimethylsulfonium ion give evidence for nucleophilic participation in the solvolyses of t -butyl compounds but electrophilic assistance would be the dominant factor for t -butyl chloride solvolysis.