Dissociation Behavior of Benzylalkali Compounds in Tetrahydrofuran: Effect of Countercation, Aromatic Methoxy Substitution, and α-Alkyl Substitution

Article abstract of DOI:10.1021/j100276a024

Conductometric measurements are carried out on different benzylalkali compounds (benzylcesium, -potassium, -sodium, and -lithium, and their o-CH3O and p-CH3O derivatives) in tetrahydrofuran at various temperatures, in order to get information on their dissociative behavior and on the influence of the alkali cation, aromatic methoxy substitution, and α-alkyl substitution on the carbon-metal bond strength.The dissociation constants of the organometallic compounds are determined by using the Kraus and Bray equation, the Fuoss equation, the Wooster equation, or by curve fitting depending on which species are involved in the solution conductance.Temperature variation of K_d is used to determine the enthalpy and entropy of dissociation.The information previously acquired by quantum chemical calculations and NMR measurements on the electronic structure of benzyl-type carbanions and the corresponding organometallic compounds appears to be of great value in interpreting the dissociation behavior of the benzyl- and styrylalkali compounds (which are also included in the study).The resonance saturation phenomenon encountered in the quantum chemical and NMR study turns out to be an important factor in the dissociative behavior of p-CH3O compounds, as compared to the unsubstituted cases.The smaller dissociation costants of the o-CH3O compounds also parallel the results of the quantum chemical calculations, showing an additional interaction between the cation and the CH3O group.The weakening of the carbon-metal bond upon α-alkyl substitution parallels the increase of dissociation capability when alkyl or polymer chain substituents are present on the α-carbon atom.