Alkali-metal plus halide reactions

The angular reactive scattering results when plotted in the CM system exhibit some interesting features for the reactions M + RX MX + R, and these features correlate with data such as total reaction cross-section. The following summarizes the more important experimental findings [8]. 

The electron-jump or harpooning mechanism [233], which has been discussed in Section 2.4 (see refs. 53—55), involves an electron exchange reaction 

This electron transfer occurs at relatively large distances, i.e. at ca. 5—7 A for stripping reactions and 3 A for rebound reactions [234]. At these large distances Coulomb attraction is dominant and the distance at which the covalent and ionic potential energy surfaces cross is given by 

The occurrence of rebound reactions for the alkyl iodides is due to the shorter range ( ca. 3 A) at which the electron exchange takes place compared with the ca. 7 A for stripping reactions. This is because the vertical electron affinity of alkyl iodides tends to be smaller [6] (i.e. F (CH3I) = —20 20 kcal mole ) than, for example, the halogen molecules [240] ( (I2) = +37 10 and (Br2) = +25 kcal mole ). The critical distance will be smaller for the alkyl iodides, since (as above) 

There is one point which should be noted when changing from LAB to CM coordinates for angular distribution measurements. The laboratory differential cross-section /lab (angle 0, velocity y) is related to the centre-of-mass differential cross-section /cm (angle 6, velocity u) by the Jacobian (v/u), i.e. 

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Slurries of finely divided Mg are prepared in TIIF at -1 IO C by v tpori/ing Mg in a rotary metal atom reactor. The resulting pyrophoric Mg clusters are "dean, alkali halide free, and extremely reactive 11311. In their Grignard reactions, there is immediate initiation. Reactions tend to be complete in short times (in the case of isopentyl bromide in THF at 2() C. a minimum time for the addition of RBr to the Mg slurry plus <1 min) and they give 100% yields of RH l rom isopenlyl bromide or f )tr/o-5-(2 -haIoeihyl)-2-norbornene]. alter quenching with 10% aqueous HCI. in reactions at temperatures from -80 to 20 C I3I. In no case have products of c (e g.. RR) been found. 

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