Kinetics and Mechanism of the Reactions of Alkyllithium Reagents With Ketones and Esters in Hydrocarbon Solvents
Al-Aseer, Munther Adil
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https://hdl.handle.net/2142/70215
Description
Title
Kinetics and Mechanism of the Reactions of Alkyllithium Reagents With Ketones and Esters in Hydrocarbon Solvents
Author(s)
Al-Aseer, Munther Adil
Issue Date
1983
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Organic
Abstract
The kinetics of the reactions of alkyllithium reagents with ketones and esters in cyclohexane and benzene have been investigated by stopped-flow infrared spectroscopy at 25.0(DEGREES)C. A series of substituted phenyl-sec-butyl ketones (10a-i) and substituted ethyl benzoates (13a-i) along with several aliphatic ketones (14-17) and esters (18,19) were utilized in the study. These compounds were found to form coordination complexes with alkyllithium aggregates. Evidence for this was demonstrated by the appearance of a new carbonyl band in the infrared spectrum of a reacting solution of ester or ketone and alkyllithium reagent immediately after mixing. The new band, which is shifted to lower wave-number relative to the carbonyl band of the carbonyl compound, has been attributed to the formation of a substrate-alkyllithium aggregate complex in rapid equilibrium with the free substrate. Equilibrium constants (K(,c)) for complexation with sec-butyllithium in cyclohexane were determined for several compounds. A trend in the K(,c) values for the substituted phenyl-sec-butyl ketones and ethyl benzoates was observed: the more electron donating the substituent the larger the equilibrium constant of complexation. Further, larger K(,c) values were obtained for the esters, in their reactions with sec-butyllithium, than for the corresponding ketones.
The rate of disappearance of ketones 10a-i and esters 13a-i in the presence of excess sec-butyllithium exhibited good pseudo first order behavior. The dependence of the observed pseudo first order rate constant (k(,obs)) on sec-butyllithium concentration was complex indicating the lack of a simple order with respect to alkyllithium in these reactions. Within each class of carbonyl compounds a substituent effect on the rate data was noted. Electron-withdrawing substituents on the phenyl ring of the ketone or ester led to the common behavior of increasing k(,obs) with rising reagent concentration; whereas, electron-donating substituents brought about a maximum k(,obs) followed by a decrease in k(,obs) at higher lithium reagent concenrations.
A mechanism (Scheme 5) that is consistent with the kinetic and equilibrium data has been proposed in which product is formed by two pathways: decomposition of the ketone/ester-alkyllithium complex and reaction of uncomplexed ketone/ester with monomeric alkyllithium.
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