A DFT investigation on aromatic nucleophilic substitution (S N Ar) reaction between 4-fluoro-1-naphthaldehyde/4-fluoro-2
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ORIGINAL RESEARCH
A DFT investigation on aromatic nucleophilic substitution (SNAr) reaction between 4-fluoro-1-naphthaldehyde/4-fluoro-2naphthaldehyde/1-fluoro-2-naphthaldehyde/1-fluoronaphthalene and methylthiolate ion in gas phase and in protic/aprotic solvents Harjinder Singh 1 Received: 7 May 2020 / Accepted: 26 June 2020 # Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract We have investigated the mechanism of aromatic nucleophilic substitution (SNAr) reaction between 4-fluoro-1-naphthaldehyde/ 4-fluoro-2-naphthaldehyde/1-fluoro-2-naphthaldehyde/1-fluoronaphthalene with methylthiolate by DFT calculations at B3LYP/ 6–31+G (d,p) level of theory in gas phase and also in polar protic/aprotic solvents. The results show that aromatic nucleophilic substitution reaction proceeds by concerted mechanism via formation of single transition state as it has lower activation energy barrier as compared with stepwise mechanism. The introduction of solvent increases the activation energy barrier indicating significant effect of solvents on transition state. The activation energy barrier was found to be a minimum in gas phase then in DMSO which is followed by protic polar solvents methanol and water. Thus, the polar aprotic solvent DMSO works best for aromatic nucleophilic substitution of fluorine. The presence of a formyl group was found to be essential for reaction to take place, and in the gas phase as well as in solution phase, the order of reactivity was found to be 4-fluoro-1-naphthaldehyde > 1-fluoro-2naphthaldehyde > 4-fluoro-2-naphthaldehyde > 1-fluoronaphthalene. Keywords Aromatic nucleophilic substitution . Reaction mechanism . DFT, solvent effect . Concerted mechanism
Introduction Aromatic nucleophilic substitution (SNAr) reactions are one of the most important reactions in organic chemistry from the view point of both mechanisms and practical applications as they result in formation of chemically important C-C, C-S, and C-O bonds [1–3]. These reactions have found enormous synthetic utility especially in the synthesis of reaction intermediates, pharmaceuticals, natural products, and herbicides [4–7]. The major drawback of this reaction is the lower reactivity of aromatic ring towards nucleophilic substitution. However, the presence of electron-withdrawing groups on aromatic ring Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11224-020-01581-1) contains supplementary material, which is available to authorized users. * Harjinder Singh [email protected] 1
Department of Chemistry, M.M. Modi College, Patiala, Punjab 147001, India
increases the reactivity as they stabilize negatively charged intermediate/transition state formed by addition of nucleophile on an aromatic ring [8, 9]. One of the fundamental questions in the study of the mechanism of any chemical reaction is whether it proceeds through a stepwise or concerted pathway as this has direct implications for the stereospecificity, rate of reaction, yield of product formed, and reacti
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