Menshutkin complexes

Grechishkin V.S., Yusupov M.Z. The nature of shifts of nuclear quadrupole resonance frequency in Menshutkin complexes // Zh. strukt. khimii. - 1973. -N14. - P. 1028-1032. 

Finally here, it is useful to note that arsenic tribromide forms a 2 1 complex with hexaethylbenzene , with a structure similar to those of the Menshutkin complexes, obtained from antimony halides and arenes (see Section III.A.6). Complex formation between AsClj and both 15-crown-5 and [2.2.2]paracyclophane has also been investigated. 

There is evidence, both experimental and theoretical, that there are intermediates in at least some Sn2 reactions in the gas phase, in charge type I reactions, where a negative ion nucleophile attacks a neutral substrate. Two energy minima, one before and one after the transition state, appear in the reaction coordinate (Fig. 10.1). The energy surface for the Sn2 Menshutkin reaction (p. 499) has been examined and it was shown that charge separation was promoted by the solvent.An ab initio study of the Sn2 reaction at primary and secondary carbon centers has looked at the energy barrier (at the transition state) to the reaction. These minima correspond to unsymmetrical ion-dipole complexes. Theoretical calculations also show such minima in certain solvents, (e.g., DMF), but not in water. "... 

After in the foregoing chapter thermodynamic properties at high pressure were considered, in this chapter other fundamental problems, namely the influence of pressure on the kinetic of chemical reactions and on transport properties, is discussed. For this purpose first the molecular theory of the reaction rate constant is considered. The key parameter is the activation volume Av which describes the influence of the pressure on the rate constant. The evaluation of Av from measurement of reaction rates is therefor outlined in detail together with theoretical prediction. Typical value of the activation volume of different single reactions, like unimolecular dissociation, Diels-Alder-, rearrangement-, polymerization- and Menshutkin-reactions but also on complex homogeneous and heterogeneous catalytic reactions are presented and discussed. 

A1CI3.C6H5 N02 It yel ndls(Ref 1) grn-yel crysts(Ref 2) mp 90°(Refs 1 2). Was first prepd in 1910 by Menshutkin(Ref 1) and recently by Grossman (Ref 2), by adding dry purified nitrobenzene to dry AlClg. This complex has been used as a catalyst in some organic reactions. 

Bismuth trichloride forms a 1 1 Menshutkin-like complex with mesitylene " and a 2 1 complex with hexamethylbenzene , in which the arene is bonded to bismuth at distances to the ring centre of ca 3.1 A. As with the related antimony complexes, there are also a number of intermolecular chlorine bridges, raising the bismuth coordination number and leading to either sheets or tetrameric bismuth chloride networks. These compounds differ from the antimony halide analogues where the arene is usually acentrically bonded. 

The comparison of results obtained with different methods could be extended to other classes of reactions. There is a considerable wealth of results for several classes of reactions with simple mechanisms, like Sjv2, S l, ET reactions, which are the favourite examples selected by theoreticians to test new models. There is a rapid increase in the number of reactions for which a comparison among different methods is possible, and there is also an increase in the complexity of the studied mechanisms. We quote, as examples, Menshutkin and Friedel-Craft s reactions, Claisen s rearrangement, Diels-Alder s and other pericyclic reactions. 

A full quantum-mechanical description of the Menshutkin reaction has been obtained for gas phase and solution by using density functional theory (DFT) and the self-consistent isodensity polarizable continuum model (SCI-PCM). Ammonia and pyridine were the nucleophiles and methyl chloride and methyl bromide, the electrophiles. In the gas phase an initial dipole complex intermediate is followed by a transition state leading to an ion pair. In the solvent-effect calculations, the dipole complex disappears with both cyclohexane and DMSO. The transition state is stabilized compared with the gas phase. The ion-pair product is strongly stabilized and in DMSO it is dissociated into free ions. 

Figure 8.4 shows the optimized TS structure for the Menshutkin reaction in the isolated state and that in aqueous solution obtained by the FEG method. The optimized values of / n-c and l c-ci for TS in solution are 1.97 and 2.07 A, respectively. In comparison with those lengths in the isolated state, it is found that the TS in solution shifts toward the reactant side. Moreover, the distance between NH3 and Cl group of TS in solution (1 n-ci = 4.04 A) becomes larger than that in the isolated state (/ n i = 3.90 A). Such stmctural deformation should enhance the charge separation of the solute complex, and consequently brings about the FE stability of TS. In fact, from the radial distribution functions (RDFs) with respect to... 

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