Model reactions, Grignard calculations

Calculations for Grignard reagent formation from 5-hexenyl bromide and product distribution in other solvents such as THF, DBE, DPE, using similar assumptions and neglecting the reactions of radicals with solvents were also compared in each case with the experimental results of Bodewitz et al. [76] to support the argument for the greater predictive competence of the D-model. 

The solutions lor the concentration profiles are complex (see the Appendix). We illustrate them with the parameter values used above for the T case plus L/ =4.4 x I 0 s s the value for the cycli/ation of the 5-hexcnyl radical. Thus, the calculations presented here for the R ease are I) model predictions for the Grignard reaction of 5 he.xonyl bromide in DKE at 40 ( ... 

The observed value of H is 1.3. but 2.0 is the value that would be expected if the pseudo-steady-slate imputations of R and Q- were spread homogeneously in space or over a surface. The D-model calculation gives // = 1.1. Values of// nearer 1.0 than 2.0 result from gradients in the pseudo-steady state concentration profiles of R- and Q- during the Grignard reaction (Section 7.2.7). 

In fact, such calculations fail for phenyl and 2 (3-butcnyl)phenyl halides I12. Cyclopropyl and 5-hexcnyl halides are not good models for the corresponding aryl halides. Grignard reactions of the aryl halides give far fewer by-products than predicted from their aliphatic models in D model calculations (equation 2.(i.7) using Tr 3x It) s. a value that describes reactions in DHK and Tl IF of cyclopropyl and 5-hexcnyl bromides (Sections 7.2.8 -7.2.9). 

The inlinite-dilution D model allows us to iddress some fundamental questions by comparing and contrasting the standard ZB and AB. The reader should recall that the standard ZB has llie Dmodel properties of the lMg/ R- intermediate in a Grignard reaction of a primary alkyl bromide in DEE and that the standard AB has properties similar to those of a typical pair of small alkyl radicals (e g., methyl). Thus, the calculations for the standard ZB are really those for the Grignard-reaction intermediate [Mgz R-]. 

The values of fi and tr for D-model descriptions of reactions of cyclopropyl bromide are indistinguishable from those for typical alkyl halides. For Tr = lO" s and ky = lO" s", l% retention of configuration is calculated. Thus, the ideal D model does not account for the observed degrees of retention of configuration in Grignard reactions of l-halo-l-methyl-2,2-diphenylcyclopropanes. Agreement would require S-values higher by factors of 10-30. 

In contrast, in Grignard reactions R- can approach 10 M near Mg/, making c/s tens to thousands of times larger thaji in homogeneous-solution reactions (Figure 7.33), D-Model calculations predict correctly that c will be signilicanl, and s not. in Grignttrd reactions of typical alkyl halides 174,84). 

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