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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.


Related in: MedlinePlus

Plots showing dependence of log k1 on variation of solvatochromic π*, α, β, and  parameters with vol% of Me2SO in MeOH–Me2SO mixtures in reaction of 2,6-bis(trifluoromethanesulfonyl)-4-nitroanisole with para-substituted anilines at 25 °C
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Fig2: Plots showing dependence of log k1 on variation of solvatochromic π*, α, β, and parameters with vol% of Me2SO in MeOH–Me2SO mixtures in reaction of 2,6-bis(trifluoromethanesulfonyl)-4-nitroanisole with para-substituted anilines at 25 °C

Mentions: As can be seen, the reaction rate constant increases with π* and β parameters and decreases with α; this behavior is illustrated in Fig. 2. The intermediate of the reaction has zwitterionic character (Scheme 2), and the activated complex of the reaction, therefore, has higher polarity relative to those of the reactants. The activated complex with zwitterionic character is expected to be favored by the increase in the and π* of media, because zwitterionic molecules were more stabilized in higher polarity media than in lower polarity media; but, in this case the rate reaction decreases with the increase in the of the media. Hence, it is evident that the polarity of the solvent does not suffice to explain the experimental observations.Fig. 2


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)

Plots showing dependence of log k1 on variation of solvatochromic π*, α, β, and  parameters with vol% of Me2SO in MeOH–Me2SO mixtures in reaction of 2,6-bis(trifluoromethanesulfonyl)-4-nitroanisole with para-substituted anilines at 25 °C
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

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

Fig2: Plots showing dependence of log k1 on variation of solvatochromic π*, α, β, and parameters with vol% of Me2SO in MeOH–Me2SO mixtures in reaction of 2,6-bis(trifluoromethanesulfonyl)-4-nitroanisole with para-substituted anilines at 25 °C
Mentions: As can be seen, the reaction rate constant increases with π* and β parameters and decreases with α; this behavior is illustrated in Fig. 2. The intermediate of the reaction has zwitterionic character (Scheme 2), and the activated complex of the reaction, therefore, has higher polarity relative to those of the reactants. The activated complex with zwitterionic character is expected to be favored by the increase in the and π* of media, because zwitterionic molecules were more stabilized in higher polarity media than in lower polarity media; but, in this case the rate reaction decreases with the increase in the of the media. Hence, it is evident that the polarity of the solvent does not suffice to explain the experimental observations.Fig. 2

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.


Related in: MedlinePlus