Simonne Pace

Les équilibres de rédistribution dans les dérives fluorés des éléments du IVeme Groupe : II. Affinité relative du fluor et des substituants Cl, Br, H, OCH3, N(CH3)2 et SCH3 pour des centres diversement alkylés du silicium

A set of constants, which describe the exchange equilibria of fluorine with a substituent Z, where Z = Br, H, OCH3, N(CH3)2 and SCH3, between the variously alkylated silicon-based central moieties of the series (CH3)nSi, with n = 1—, are predicted on the basis of previously published data. Their values correspond to significant variations in the SiF vs. SiZ bond energies as a function of n. The validity of such predictions was experimentally verified by a detailed quantitative study of the redistribution equilibria of fluorine atoms vs. methoxy groups between methyl-, dimethyl- and trimethylsilicon moieties, as well as by the study of the exchange of fluorine vs. thiomethyl and dimethylamino groups between the dimethyl- and trimethylsilicon moieties. The common substituents Z could thus be ordered in the following series with respect to their preferential affinity for the least alkylated silicon atom: OCH3>N(CH3)2⪞H>F⪞SCH3>Cl>Br.

Les equilibres de redistribution dans les derives fluores des elements du IVème groupe : III. Affinite relative du fluor et du chlore pour des centres bases sur les elements carbone, silicium, germanium, etain, phosphore et arsenic

Abstract Equilibrium distributions have been investigated for the scrambling of fluorine vs. chlorine between pairs of the following centres: Me 2 C, Me 2 Si, Me 3 Si, Me 2 Ge, Me 3 Ge, MeP(O), Me 2 As. It was found that in Group IV there is a strong preference for fluorine to be bound to silicon rather than to comparable carbon or germanium centres. There is only a slight preference for fluorine to be bound to Me 2 P(O) rather than to Me 2 Si, and this tendency can be inverted by decreasing the number of alkyl groups on silicon or by decreasing the electronegativity of the atom which is bound to phosphorus in the “isolated position”. On the other hand, fluorine definitely prefers Me 2 As to Me 3 Ge. It is further confirmed that the mixed fluorochloro species are less abundant at equilibrium than would be expected from random sorting. The preferential affinities of fluorine vs. chlorine for a range of common silicon and phosphorus-based centres are predicted to be: MeSi ⪢ MeP(O) ⪢ P(O) ⪢ Me 2 Si ⪢ Me 2 P(O) ⪢ MeP(S) ⪢ Me 3 Si ⋍ MeP(Se) –⪢ MeP.

Prevision des enthalpies libres de redistribution sur des elements du ivème groupe par le biais d'un schema de decomposition des energies de liaison

Abstract A method is described which permits the emprical prediction of redistribution free energies on, and between, central groups based on Group IV elements. It is based on a bond-energy decomposition scheme analogous to that of Zahn, using additive second-order parameters assigned to each pair of substituents of the central element. The most useful implications of this treatment are: (1) The possibility of writing linear relationships between redistribution free energies. Thus for example the 27 equilibria linking the 15 molecular species which result from the redistribution of 3 distinct substituents on a tetrafunctional centre can be expressed as a function of 3 redistribution free energies only, for which reliable experimental data are available. The Δ G dev have been calculated for 90 equilibria concerning the exchange of the most usual substituents. When compared with 42 available experimental data, these predictions were in 90% of the cases within experimental error. (2) The Δ G dev corresponding to the exchange of 2 substituents X and Y on a given element are independent of the number of exchangeable sites of the central atom. It therefore suffices to know one single experimental result per couple of substituents and per central element in order to predict the molecular distributions resulting from the exchange of these substituents on any centre that can be derived from this element by the presence of non-exchangeable substituents. A set of 134 experimental data have been examined and have confirmed this prediction. (3) The exchange of 2 substituents between distinct centres is also given by a same quantity, but taken a variable number of times, depending on the relative functionalities of the centres involved. Again, a single experimental vlaue allows the prediction of numerous equilibria. The Δ G dev have been calculated for 85 equilibria of this type, and were found to be in good agreement with the experimental values where known. The analysis of the body of calculated Δ G dev brought to light several general trends concerning the redistribution equilibria on the variously alkylated Group IV elements.