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Electronic and solvent effects on kinetics of SNAr substitution reactions of substituted anilines with 2,6-bis(trifluoromethanesulfonyl)-4-nitroanisole in MeOH-Me2SO mixtures of varying composition: one reaction with two mechanistic pathways.

El Guesmi N, Berionni G, Asghar BH - Monatsh. Chem. (2013)

Bottom Line: These results indicate a change in mechanism from the polar (SNAr) for less basic nucleophiles (X = 4-Cl, 4-I, 4-F, and H) to the single electron transfer (SET) for more basic nucleophiles (X = 4-OH, 4-OMe and 4-Me).The changes of the structure of the transitions states with substituents and solvent are in accordance with the results of kinetics studies.These results provide an ideal framework for understanding the paramount importance of the specific molecular structure of solvent molecules in determining chemical reactivity versus solvent effects.

View Article: PubMed Central - PubMed

Affiliation: Département de chimie, Faculté des Sciences de Monastir, 5019, Avenue de l'Environnement, Monastir, Tunisia.

ABSTRACT

Abstract: The kinetics and mechanism of the aromatic nucleophilic substitution reactions of 2,6-bis(trifluoromethanesulfonyl)-4-nitroanisole with para-X-substituted anilines (X = OH, OMe, Me, H, F, I, Cl) were studied in MeOH-Me2SO mixtures and pure Me2SO at 25.0 °C. The second-order rate coefficients depend on the substitutent in aniline and give good Hammett and Brønsted correlations; a polar SNAr reaction is proposed for the reaction in different MeOH-Me2SO mixtures. The measured rate coefficients of the reaction demonstrated dramatic variations for aniline donor with the increasing dimethyl sulfoxide composition in MeOH-Me2SO mixtures. In this case, the Hammett and Brønsted plots are biphasic and concave upwards with a break point at 4-methylaniline. These results indicate a change in mechanism from the polar (SNAr) for less basic nucleophiles (X = 4-Cl, 4-I, 4-F, and H) to the single electron transfer (SET) for more basic nucleophiles (X = 4-OH, 4-OMe and 4-Me). The changes of the structure of the transitions states with substituents and solvent are in accordance with the results of kinetics studies. The solvation model described is well supported by the solvatochromism exhibited by aniline in the solvent mixture under investigation. These results provide an ideal framework for understanding the paramount importance of the specific molecular structure of solvent molecules in determining chemical reactivity versus solvent effects.

No MeSH data available.


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Mentions: Aromatic nucleophilic susbstitution reactions involving primary amines are an important class of organic synthetic reactions and continue to inspire studies of kinetics and mechanisms [1–5]. Studies have revealed that the displacement of the substituent at the 1-position is faster when the aromatic ring contains electron-withdrawing substituents such as –NO2, –CN, –CF3, or –SO2CF3 at ortho and para positions [1, 2, 6–10]. It is believed that this reaction generally proceeds through an addition–elimination mechanism. In the first step the nucleophile preferably attacks the position ipso to the leaving group of the electron-deficient aromatic ring to yield a zwitterionic intermediate. Typically, this intermediate with a tetrahedral (sp3) carbon is unstable, and the reaction could proceed forward by rearomatization to generate the substituted product (Scheme 1).


Electronic and solvent effects on kinetics of SNAr substitution reactions of substituted anilines with 2,6-bis(trifluoromethanesulfonyl)-4-nitroanisole in MeOH-Me2SO mixtures of varying composition: one reaction with two mechanistic pathways.

El Guesmi N, Berionni G, Asghar BH - Monatsh. Chem. (2013)

© Copyright Policy - OpenAccess
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC4495024&req=5

Mentions: Aromatic nucleophilic susbstitution reactions involving primary amines are an important class of organic synthetic reactions and continue to inspire studies of kinetics and mechanisms [1–5]. Studies have revealed that the displacement of the substituent at the 1-position is faster when the aromatic ring contains electron-withdrawing substituents such as –NO2, –CN, –CF3, or –SO2CF3 at ortho and para positions [1, 2, 6–10]. It is believed that this reaction generally proceeds through an addition–elimination mechanism. In the first step the nucleophile preferably attacks the position ipso to the leaving group of the electron-deficient aromatic ring to yield a zwitterionic intermediate. Typically, this intermediate with a tetrahedral (sp3) carbon is unstable, and the reaction could proceed forward by rearomatization to generate the substituted product (Scheme 1).

Bottom Line: These results indicate a change in mechanism from the polar (SNAr) for less basic nucleophiles (X = 4-Cl, 4-I, 4-F, and H) to the single electron transfer (SET) for more basic nucleophiles (X = 4-OH, 4-OMe and 4-Me).The changes of the structure of the transitions states with substituents and solvent are in accordance with the results of kinetics studies.These results provide an ideal framework for understanding the paramount importance of the specific molecular structure of solvent molecules in determining chemical reactivity versus solvent effects.

View Article: PubMed Central - PubMed

Affiliation: Département de chimie, Faculté des Sciences de Monastir, 5019, Avenue de l'Environnement, Monastir, Tunisia.

ABSTRACT

Abstract: The kinetics and mechanism of the aromatic nucleophilic substitution reactions of 2,6-bis(trifluoromethanesulfonyl)-4-nitroanisole with para-X-substituted anilines (X = OH, OMe, Me, H, F, I, Cl) were studied in MeOH-Me2SO mixtures and pure Me2SO at 25.0 °C. The second-order rate coefficients depend on the substitutent in aniline and give good Hammett and Brønsted correlations; a polar SNAr reaction is proposed for the reaction in different MeOH-Me2SO mixtures. The measured rate coefficients of the reaction demonstrated dramatic variations for aniline donor with the increasing dimethyl sulfoxide composition in MeOH-Me2SO mixtures. In this case, the Hammett and Brønsted plots are biphasic and concave upwards with a break point at 4-methylaniline. These results indicate a change in mechanism from the polar (SNAr) for less basic nucleophiles (X = 4-Cl, 4-I, 4-F, and H) to the single electron transfer (SET) for more basic nucleophiles (X = 4-OH, 4-OMe and 4-Me). The changes of the structure of the transitions states with substituents and solvent are in accordance with the results of kinetics studies. The solvation model described is well supported by the solvatochromism exhibited by aniline in the solvent mixture under investigation. These results provide an ideal framework for understanding the paramount importance of the specific molecular structure of solvent molecules in determining chemical reactivity versus solvent effects.

No MeSH data available.