Centre of Reactions

Oxidation of the heterocycles with common reagents such as MCPBA, sodium periodate or hydrogen peroxide cleanly affords the sulfoxides and sulfones, and it is clear that the sulfur atom is the principal centre of reaction for electrophiles. While the sulfone is a quite inert functionality, the sulfoxides may be reduced to the sulfides with phosphorus pen-tasulfide as for the tetrahydro systems (78CJC1423). Positive halogen sources likewise react at sulfur, and the intermediate sulfonium halide rearranges, usually by 1,2-shift to the a-halo product. 

Upon contacting 1.3 Torr propene enzene (1/6) mixture with HP at 453 K, fast formation of cumene and the disappearance of the OH bands (the active centres of reaction) were observed. After the first 20 minutes the development of cumene bands stopped, while the parallel formation of alkenyl carbenium ions increased slightly and continuously with reaction time. 

It would appear from the foregoing that there is a class of gas-phase reactions for which the transition state is best represented as having an essentially carbonium-ion pair character. In this way the effect of substitution at or near the centre of reaction can be interpreted, and the vast body of theory in the literature of physical organic chemistry used for the purpose of predicting rates of gas-phase reactions. In addition, the known properties of carbonium ions, as determined by the mass-spectrometer, can be invoked—as indeed they were in discussions of the SN1 and El reactions in polar solvents (Evans, 1946)—to correlate the effects of substituents in gas-phase eliminations. The advantage of studies in the gas-phase lies in the fact that the behaviour of a single molecule can be observed, without the added complication of the cooperative effect of the solvent. But gas-phase studies may, in turn,... 

The metaphor of a construction project can be taken further. Just as a partially completed component of the project might need to be protected while building goes on around it, so a partially constructed molecule might have tender regions that, without protection, would be centres of reaction and result in unwanted products. Thus, chemists sometimes might attach a... 

In the viscous flow regime an irregularly shaped particle wiU settle in such a way that the line joining the centres of reaction and mass is parallel to the direction of gravity, giving a preferred orientation and thus a definitive settling viscosity. The use of Stokes law (Equation 3.15) to obtain an equivalent spherical diameter Xse for an irregular... 

A bimoleciilar reaction can be regarded as a reactive collision with a reaction cross section a that depends on the relative translational energy of the reactant molecules A and B (masses and m ). The specific rate constant k(E ) can thus fonnally be written in tenns of an effective reaction cross section o, multiplied by the relative centre of mass velocity... 

Several reactivity trends are worth noting. Reactions that are rapid frequently stay rapid as the temperature or centre-of-mass kinetic energy of the reactants is varied. Slow exothenuic reactions almost always show behaviour such tliat... 

Onsager s original reaction field method imposes some serious lunitations the description of the solute as a point dipole located at the centre of a cavity, the spherical fonn of the cavity and the assumption that cavity size and solute dipole moment are independent of the solvent dielectric constant. 

Figure B2.3.16. Velocity diagram for die reaction of a photolytically generated reagent with an assumed stationary co-reagent. In this case, the relative velocity of the reagents is parallel to the velocity c of the centre of mass.

Alcohols can be synthesized by the addition of carbanions to carbonyl compounds (W.C. Still, 1976) or epoxides. Both types of reactions often produce chiral centres, and stereoselectivity is an important aspect of these reactions. 

Using the (— )-Aowocincholoipon produced as described, Rabe and Schultze, by the same sequence of reactions, have produced (—)-dihydro-quininone (m.p. 98-9[a]f, ° — 70-0° (final value EtOH)), which on hydrogenation in presence of palladium gave a mixture of bases, of which (—)-dihydroquinidine and (-j-)-dihydroquinine were isolated. The characters of these mirror-image isomerides of dihydroquinidine and dihydroquinine respectively have been given already with the directions of rotation at the centres of asymmetry C , C , C , C (see table, p. 446). 

Monomeric thiazyl halides can be stabilized by coordination to transition metals and a large number of such complexes are known (Section 7.5). In addition, NSX monomers undergo several types of reactions that can be classified as follows (a) reactions involving the n-system of the N=S bond (b) reactions at the nitrogen centre (c) nucleophilic substitution reactions (d) halide abstraction, and (e) halide addition. Examples of each type of behaviour are illustrated below. 

We may also speak of the pressure at a point in the interior of a mass of liquid or gas, because if a very small plane area removed from the immediate vicinity of one side, a definite force P must be applied to keep the area in position. From the principle of reaction we see that each of the two portions of fluid divided by an im a(f in ary plane opposite forces is called a stress. 

K2C03 3 H202 contains hydrogen peroxide of crystallization and the solid phase decomposition involves the production of the free radicals OH and HOi, detected by EPR measurements [661]. a—Time curves were sigmoid and E = 138 kJ mole-1 for reactions in the range 333—348 K. The reaction rate was more rapid in vacuum than in nitrogen, possibly through an effect on rate of escape of product water, and was also determined by particle size. From microscopic observations, it was concluded that centres of decomposition were related to the distribution of dislocations in the reactant particles. 

Dynamics of intramolecular metal-centred rearrangement reactions of tris-chelate complexes. L. H. Pignolet, Top. Curr. Chem., 1975,56,93-137 (85). 

The centre of experimental and theoretical investigation on cationic polymerization is the propagation reaction, Eq. (1), and the influence on it. 

The steric environment of the atoms in the vicinity of the reaction centre will change in the course of a chemical reaction, and consequently the potential energy due to non-bonded interactions will in general also change and contribute to the free energy of activation. The effect is mainly on the vibrational energy levels, and since they are usually widely spaced, the contribution is to the enthalpy rather than the entropy. When low vibrational frequencies or internal rotations are involved, however, effects on entropy might of course also be expected. In any case, the rather universal non-bonded effects will affect the rates of essentially all chemical reactions, and not only the rates of reactions that are subject to obvious steric effects in the classical sense. 

As already mentioned, complexes of chromium(iii), cobalt(iii), rhodium(iii) and iridium(iii) are particularly inert, with substitution reactions often taking many hours or days under relatively forcing conditions. The majority of kinetic studies on the reactions of transition-metal complexes have been performed on complexes of these metal ions. This is for two reasons. Firstly, the rates of reactions are comparable to those in organic chemistry, and the techniques which have been developed for the investigation of such reactions are readily available and appropriate. The time scales of minutes to days are compatible with relatively slow spectroscopic techniques. The second reason is associated with the kinetic inertness of the products. If the products are non-labile, valuable stereochemical information about the course of the substitution reaction may be obtained. Much is known about the stereochemistry of ligand substitution reactions of cobalt(iii) complexes, from which certain inferences about the nature of the intermediates or transition states involved may be drawn. This is also the case for substitution reactions of square-planar complexes of platinum(ii), where study has led to the development of rules to predict the stereochemical course of reactions at this centre. 

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