Brpnsted plots nucleophilic substitution

The biphasic Hammett and Brpnsted plots found for the 5 2 reaction between 3-and 4-substituted pyridines and anilines and (2i ,4f ,55)-(+)-2-chloro-3,4-dimethyl-5-phenyl-l,3,2-oxazaphospholidine 2-sulfide in CH3CN at 5 C is attributed to a change from a backside to a frontside attack by the nucleophile. The Hammett p = -6.l5 and Brpnsted = 1.11 found for strongly basic pyridines was attributed to a frontside attack by the nucleophile, while the Hammett p = 4.73 and Brpnsted fl = -0.75 found with the weakly basic pyridines was thought to be for a backside attack by the nucleophile. The rate constants, AH and AS values, and the isokinetic temperature where 8 AH = T... 

The reactions of 4-(4 -nitrophenyl)-4-X-butan-2-one (X = Cl, OTs) with added bases/nucleophiles show second-order kinetics. It is concluded that these substitution and elimination reactions are of concerted 5ivr2 type and irreversible ElcB type, respectively. The discussion involves Brpnsted plots. 

The kinetics of the reactions of phthalic and maleic anhydrides with Z-substituted phenols (Z = H, m-Me, p-Me, m-Cl, p-C 1, and -CN) (Scheme 8) were studied in aqueous solution at pH 8.5. Two kinetic processes well separated in time were observed. The fast process was attributed to the formation of the aryl ester in equilibrium with the anhydride and allowed the determination of the rate of nucleophilic attack of the phenol on the anhydride. From the slow kinetic process, the equilibrium constant for this reaction was determined. The Brpnsted-type plots for the nucleophilic attack of substituted phenols on the anhydrides were linear with slopes /SNuc of 0.45 and 0.56 for phthalic and maleic anhydride, respectively. The results are consistent with a mechanism involving rate-determining nucleophilic attack and also with a concerted mechanism.27... 

The use of liquid ammonia as a solvent for Ar reactions has been explored. With 4-fluoronitrobenzene and chloropyrimidines, solvolysis occurs although substitution by added nucleophiles such as phenoxide or triazolate ions can compete successfully. Kinetic studies of the reactions of 2-chloro- and 2-ethoxy-3,5-dinitropyridine with substituted anilines in DMSO in the presence of DABCO indicate a base-catalysed pathway, which is likely to involve rate-limiting proton transfer from the zwitterionic intermediate to base." ° In the reaction of 2,6-bis(trifluoromethanesulfonyl)-4-nitroanisole, (26), with substituted anilines in DMSO/methanol mixtures, nucleophilic attack is rate-limiting. Rate constants have higher values in DMSO than in methanol, and in DMSO-rich mixtures, there is evidence, from changes in slope of Brpnsted and Hammett plots, that formation of the zwitterionic intermediate involves an SET process." ... 

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