Relative Reactivities and Mechanistic Aspects of the Reactions of Organic Halides with Alkali Metals in Alcohol Environments

Article abstract of DOI:10.1021/ja00260a014

The relative reactivities of organic halides over wide concentration ranges have been determined with limited amounts of lithium, sodium, and potassium in 2-ethoxyethanol (1) at 0 deg C.Under these conditions the organometallics formed protonate to their hydrocarbons rather than undergo exchange, elimination, and simple or crossed coupling.In dilute solution in 1 the relative reactivities (r₁/r₂) of varied halides with lithium are essentially structure independent.However, as the concentrations of the halides increase, their relative reactivities become significantly different and depend on the total concentrations T (M) = 1X> + 2X>> of the organic halides.With lithium at increased halide concentrations (1) the reactivities are iodides > bromides > chlorides, (2) halides of lower molecular weight react more rapidly than their higher homologues, and (3) the reactivity orders of chlorides are (a) allyl > primary > secondary > tertiary > neopentyl, (b) 2-buten-1-yl > 1-buten-3-yl, (c) benzyl > phenyl, and (d) p-chlorotolyl > o-chlorotolyl > m-chlorotolyl.As examples, the relative reactivities of 1-chlorobutane/2-chloro-2-methylpropane (C_T = 5.83 M), 3-chloropropene/1-bromobutane (C_T = 4.60 M), bromobenzene/p-chlorotoluene (C_T = 4.37 M), and benzyl chloride/chlorobenzene (C_T = 4.02 M) are 6.71, 5.43, 24.1, and 22.1, respectively.Additions of aprotic solvents to 1-chlorobutane and 2-chloro-2-methylpropane in 1 decrease the relative reactivities of the halides.The effectiveness of cosolvents in lowering the relative reactivities of lithium with 1-chlorobutane and 2-chloro-2-methylbutane is tetrahydrofuran > dioxane ca. 2-ethoxyethanol (1) > cyclohexene ca. benzene.The relative reactivities of halides with sodium and with potassium in 1 at 0 deg C are also total halide concentration (C_T) dependent.Under comparable concentrations the relative reactivity differences of halides are greater with lithium than sodium than potassium.The reactivities of halides under conditions of chemical control can be correlated with the ionization potentials of the alkali metals, and the kinetically controlling features of these systems are different from those with magnesium.The behavior of the alkali metals, the effects of concentration, and the roles of solvents on the reactivities of halides are discussed on the basis of (1) the active sites on the metal surfaces as modified by induction and (2) steric and electronic factors in the organic substrates.The kinetically controlled reactions of lithium with sp³ halides may be interpreted to invole formation of lithio organohalide radical anions (R^.-X(1-), Li(1+)), electron transfer to the lithio radical anions on the metal surface, or unsymmetrical four-center carbanionic processes on the metal.In addition to incorporating an electron into the lowest unoccupied ? level of its C-X bond, an sp² halide offers the possibility for kinetically controlling electron transfer into the ? system of its carbon-carbon double bond(s).