Diffusion-reaction kinetics related to Grignard reagent formation. Analytic steady-state solution of a simplified D model, a mechanism with first-order surface and first- and second-order solution steps

The Grignard Reagent Formation Reaction of 2‐Chloro‐1,1,1‐triphenylethane Revisited

“…An exception is the cyclopropyl radical [ k s (THF) = 7 × 10 6 s –1 at 25 °C] 15. Hence, for radical 8 , a k S in the order of about 10 3 s –1 at ambient temperature seems reasonable which would imply that solvent attack by 8 is a minor pathway for the production of reduced product 2 , which is in agreement with the extended diffusion model when solvent attack can be neglected ( G 2 – 1 > 100) 2d,12…”

“…In the extended diffusion model, it is further assumed that radical R · and the rearranged radical Q · are converted into their respective organomagnesium compounds at the same rate. Garst et al2d,12 showed that this model is in excellent agreement with the experimentally determined yields of all products accounted for in the reaction of 6‐bromo‐1‐hexene with magnesium 13. In the case of chloride 1 , however, the rearranged radical is a relatively stable diphenylalkyl radical, which can react mainly with radical 8 to give products 2 and 4 due to a “persistent radical effect” (a disproportionation reaction discussed in detail later).…”

“…Garst et al also provided an extended diffusion model, which takes all usual side reactions into account 2d,12. This model is characterised by three (scaled) independent parameters V , Δ and G 2 – 1: V = [4 k C /3( k S 3 D ) 0.5 ] ν , Δ = ( Dk S ) –0.5 δ , G 2 – 1 = k Q / k S .…”

“…According to Garst et al,2d,12 the boundary condition for neglecting solvent attack is G 2 – 1 > 100, which condition is fulfilled in the reaction of 1 . Thus, solvent trapping of radical 8 cannot produce significant amounts of the reduction product 2 .…”

“…When radical isomerisation occurs in the Grignard reaction but solvent attack is negligible, the product distribution is governed by:2d,12 …”