Resolution of the Non-Steady-State Kinetics of the Two-Step Mechanism for the Diels−Alder Reaction between Anthracene and Tetracyanoethylene in Acetonitrile

• Published in: Journal of the American Chemical Society Vol. 125; no. 31; pp. 9381 - 9387
• Main Authors: Handoo, Kishan L, Lu, Yun, Parker, Vernon D
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 06.08.2003
• Subjects: Chemistry; Exact sciences and technology; Kinetics and mechanisms; Organic chemistry; Reactivity and mechanisms; Anthracene; Nitrile; Stepwise mechanism; Diels Alder addition; Tricyclic compound; Experimental study; Condensed benzenic compound; Organic solvent; Tetracyanoethylene; Acetonitrile; Reaction mechanism; Charge transfer compound; Kinetic parameter; Rate constant; Ethylenic compound; Ultraviolet visible spectrometry
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja0299396

The Diels−Alder reaction between anthracene and tetracyanoethylene in acetonitrile does not reach a steady-state during the first half-life. The reaction follows the reversible consecutive second-order mechanism accompanied by the formation of a kinetically significant intermediate. The experimental observations consistent with this mechanism include extent of reaction−time profiles which deviate markedly from those expected for the irreversible second-order mechanism and initial pseudo first-order rate constants which differ significantly from those measured at longer times. It is concluded that the reaction intermediate giving rise to these deviations cannot be the charge-transfer (CT) complex, which is formed during the time of mixing, but rather a more intimate complex with a geometry favorable to the formation of the Diels−Alder adduct. The kinetics of the reaction were resolved into the microscopic rate constants for the individual steps. The rate constants, as shown in equation 1, at 293 K were observed to be 5.46 M-1 s-1 (k f), 14.8 s-1 (k b), and 12.4 s-1 (k p). Concentration profiles calculated under all conditions show that intermediate concentrations increase to maximum values early in the reaction and then continually decay during the first half-life. It is concluded that the charge-transfer complex may be an intermediate preceding the formation of the reactant complex, but due to its rapid formation and dissociation it is not detected by the kinetic measurements.