Mechanism of formation of Grignard reagents. Kinetics of reaction of substituted aryl bromides with magnesium and with tri-n-butyltin hydride in ethereal solvents

Abstract

Whitesides et al. / Reaction of Substituted Aryl Bromides with Magnesium23 1 clohexane of s for CH3Br-. and 3 X lo-* s for CH3CI-. (P. P. lnfelta and R. H. Schuler, J. phys. Chem., 76,987 (1972)). If these data are Correct, alkyl bromides and alkyl chlorides might react by different initial steps.(14) Radical chain reactions are usually detected by rate inhibitions resulting from addition of free-radical scavengers. Any reactions involving freeradical chains in this system must be fast to be compatible with the observation that the overall reaction is mass transport limited. Although it is possible to find a limited number of radical scavengers which are compatible with a reacting mixture of alkyl halide and magnesium, and although these scavengers produce no obvious effect on the reaction rate, it is difficult to guarantee that these scavengers would be effective in trapping the very short-lived radicals implied by the high rates observed.(15) Clear derivations of equations relating current density to stirring rate and kinematic viscosity for a rotating disk electrode are given by J. Albery. Abstract:Hammett constants for reaction of substituted aryl bromides with magnesium in several solvents follow: diethyl ether ( p = I , I ) ; di-n-butyl ether ( p = I ,9); 3: 1 v:v di-n-butyl ether-cyclohexane ( p = 2.0); tetrahydrofuran ( p N 0 ) ; 3: 1 v:v tetrahydrofuran-hexamethylphosphoramide ( p N 0); diethyl ether containing 0.8 M cyclopentylmagnesium bromide ( p N 0). In the polar solvents, reaction appears to be mass transport limited. The rate of reaction of aryl iodides with magnesium in diethyl ether is probably also mass transport limited ( p = 0). The rate of reaction of chlorobenzene with magnesium ( p N 2.0) is ca. IO4 slower than that of bromobenzene in diethyl ether, and is not mass transport limited. The reaction of aryl bromides with tri-n-butyltin hydride (AIBN, hu) has p = 1.3. Analysis of these values of p narrows the choice for the rate-determining step in the reaction of bromobenzene with magnesium in diethyl ether to one of three limiting possibilities: electron transfer from the metal to the aryl bromide (3), aryl radical formation by bromine abstraction from the aryl bromide by the metal ( 6 ) . or, less probably, insertion of a magnesium atom into the carbon-bromine bond (7).

Physical‐Organic Chemistry: A Swiss Army Knife

“…A conceptually related set of studies [1,2,[36][37][38][39] , with entirely different mechanistic problems, concerned the mechanism of formation of Grignard reagents by the reaction of organic halides with metallic magnesium in ethereal solvents. Studies of stereochemistry and product revealed that this heterogeneous reaction proceeded predominantly by electron transfer [36][37][38][39] , followed by reaction of the resulting organic free radical with the surface of magnesium, and made it possible to define some of the characteristics of the corrosion reaction that removed magnesium atoms from the bulk metal. (It thus used vpc to determine important elements of a very rapid reaction occurring at a reactive metal surface.)…”

Kinetics Model for Designing Grignard Reactions in Batch or Flow Operations

“…Whitesides and coauthors − have carried out competitive kinetics studies (rates relative to a reference cyclopentyl bromide substrate) using different types of magnesium (turnings or rods or powder) and alkyl and aryl halides substrates. They reported the influence of different substituent and solvent on the relative rates of reactions of aryl halides with magnesium.…”

Quantum-chemical study of the Grignard reaction mechanism within the cluster model of reaction center

“…Kinetic studies of the reaction were conduct ed in many investigatio ns [8][9][10][11][12][13][14][15]. Products and intermedia tes of the reaction between magnesium and hydrocarb on halides were charac terized [6,[15][16][17][18], the morphol ogy of a metallic surface after the reaction was investigate d [19].…”