A comprehensive N₊ scale of nucleophilicity in an equation including a Swain‐Scott response/selectivity parameter

Abstract

A set of N + nucleophilicity parameters (N + 00 ) are reported for amines, carbanions and various other nucleophiles in methanol at 20 C. N + 00 = N 00 +2.63, where N 00 refers to logarithms of second-order rate constants for reactions of the dimethylamino-benzhydrylium cation (dma) 2 CH + with various nucleophiles in any solvent at 20 C; as for other N + parameters, N + 00 = 4.75 for hydroxide in water. Logarithms of second-order rate constants (log k) are correlated by a hybrid of Swain-Scott and Ritchie (SSR) equations: log k = s E Â N + 00 + c, where s E is the response of the electrophile to changes in N + 00 and c is a residual intercept term. Satisfactory results are obtained for some nucleophilic reactions at sp 3 carbon, including S-methyldibenzothiophenium triflate (reference substrate for the N T scale of solvent nucleophilicity), a methoxymethyl derivative and methyl p-nitrobenzenesulfonate. Less satisfactory results are obtained for acetyl chloride. The results extend the scope of the hybrid SSR equation to nucleophilic substitutions and provide additional insights into the factors influencing s E . A previously published equation containing two response (s) parameters is shown to be less reliable, and an alternative is investigated. † Dedicated to the memory of Rory More O'Ferrall, who made key contributions to the development of Physical Organic Chemistry within Europe, and whose recent review provides the background to this project (see Reference [18] ).

Positioning solvolyses within the SN2‐SN1 and SN3‐SN2 spectrum of reaction mechanisms

“…For 13 solvents out of 19 available in Reference Bentley; exclusions due to insufficient input data for MeOTs (95% acetone, 90% and 80% TFE–water, 90% HFIP and 90% TFE‐EtOH); also excluded data for 97% HFIP (observed is sevenfold faster than calculated).…”

“…As is usual when establishing reactivity scales, in both cases, the slope ( l ) was defined as unity (1.00) for each substrate. However, according to recent Swain–Scott correlations 13 is more responsive than methyl tosylate to changes in the nucleophilicity of the attacking nucleophile. Because both substrates have been defined with the same l = 1.00, published l values based on N T should be almost the same as those based on N OTs ; in contrast, the two scales do lead to different m values in Eqn .…”

“…5 For the same eight solvents as above, but replacing Y Cl with Y OTs . [13] ; exclusions due to insufficient input data for MeOTs (95% acetone, 90% and 80% TFE-water, 90% HFIP and 90% TFE-EtOH); also excluded data for 97% HFIP (observed is sevenfold faster than calculated). 3 For 30 solvents, including 97% HFIP; data from Reference Kevill et al [25] 4 For 28 solvents from Reference Kevill et al [26] ; addition of a 0.20Y Cl term lowers the standard error to 0.16 and R increases to 0.988.…”

“…Initial objectives of the research reported here were as follows: (i) to test Eqn , using published data for alkyl, benzyl and benzoyl tosylates; (ii) to supplement recent Swain–Scott correlations relevant to the definition of N OTs (Eqn ), using new experimental data for solvolyses in higher alcohols. The correlations were then applied to the more complex solvolyses of carboxylic acid chlorides and sulfonates, taking advantage of insights obtained from recent data on solvolyses of mixed anhydrides and of acid chlorides in weakly nucleophilic media .…”

Calculated Third Order Rate Constants for Interpreting the Mechanisms of Hydrolyses of Chloroformates, Carboxylic Acid Halides, Sulfonyl Chlorides and Phosphorochloridates

“…Consequently, l values and/or l / m ratios help to position substrates within a spectrum of mechanisms (e.g., for sulfonyl chlorides [ 40 ]). Values of m depend on how the N scale is defined [ 84 ], and published values are usually based on the N T scale [ 85 , 86 ].…”