Effect on chemical reactivity

Hassel shared the 1969 Nobel Prize in chemistry with Sir Derek Barton of Imperial College (London) Barton demonstrated how Hassel s structural results could be extended to an analysis of conformational effects on chemical reactivity... 

Secondary steric effects on chemical reactivity can result from the shielding of an active site from the attack of a reagent, from solvation, or both. They may also be due to a steric effect on the reacting conformation of a chemical species that determines its concentration. 

A triparametric model of the electrical effect has been introduced17 that can account for the complete range of electrical effects on chemical reactivities of closed shell species (carbenium and carbanions), that is, reactions which do not involve radical intermediates. The basis of this model was the observation that the electronic demand. On the assumption that they are generally separated by an order of magnitude in this variable it is possible to assign to each oq type a corresponding value of the electronic... 

Primary steric effects are due to repulsions between electrons in valence orbitals on atoms which are not bonded to each other. They are believed to result from the interpenetration of occupied orbitals on one atom by electrons on the other resulting in a violation of the Pauli exclusion principle. All steric interactions raise the energy of the system in which they occur. In terms of their effect on chemical reactivity, they may either decrease or increase a rate or equilibrium constant depending on whether steric interactions are greater in the reactant or in the product (equilibria) or transition state (rate). 

Stereoelectronic effects in chemical reactivity The bond-lengthening and -weakening influence of an antiperiplanar lone pair leads to strong stereoelectronic effects on chemical reactivity.97 In molecule 28a with lone-pair-bearing atom D adjacent to an A—B bond, a vicinal nD—s-cab hyperconjugative interaction can be associated (cf. Example 1.4 and Section 3.3.1) with a partial admixture of the alternative resonance structure 28b,... 

The several theoretical and/or simulation methods developed for modelling the solvation phenomena can be applied to the treatment of solvent effects on chemical reactivity. A variety of systems - ranging from small molecules to very large ones, such as biomolecules [236-238], biological membranes [239] and polymers [240] -and problems - mechanism of organic reactions [25, 79, 223, 241-247], chemical reactions in supercritical fluids [216, 248-250], ultrafast spectroscopy [251-255], electrochemical processes [256, 257], proton transfer [74, 75, 231], electron transfer [76, 77, 104, 258-261], charge transfer reactions and complexes [262-264], molecular and ionic spectra and excited states [24, 265-268], solvent-induced polarizability [221, 269], reaction dynamics [28, 78, 270-276], isomerization [110, 277-279], tautomeric equilibrium [280-282], conformational changes [283], dissociation reactions [199, 200, 227], stability [284] - have been treated by these techniques. Some of these... 

A steric effect on chemical reactivity resulting from a crowding of substituents around an otherwise reactive center. 

Quantitative relationships " between magnitudes of deuterium and tritium primary kinetic isotope effects on chemical reactivity. 

Secondary steric effects on chemical reactivity include ... 

Tapia, O. and Bertran, J. 1996. Solvent Effects on Chemical Reactivity, Kluwer Dordrecht. 

Some cyclic conjugated systems are much more stable than open-chain analogues while others are not compounds of the former type are termed aromatic. This extra stabilization naturally has a profound effect on chemical reactivity and it is therefore very important that we should have some way of predicting the degree of aromatic stabilization in given systems. 

The ability of micellar solutions and mlcroemulslons to dissolve and compartmentalize both polar and non-polar reactants has a significant effect on chemical reactivity. An Idealized representation of a typical micelle catalyzed reaction is depicted In Figure 2. Here the non-polar reactant is solubilized within the micelle while the ionic reactant is at the surface. The polar head groups of the surfactants generate a charge at the micelle surface which serves to attract an oppositely charged water soluble reactant increasing the concentration of that reactant near the micelle. The result Is an enhanced reaction rate. 

For many physical organic chemists, the Menschutkin reaction was a kind of guinea pig , which has been extensively used for the study of solvent effects on chemical reactivity. A comprehensive review of this reaction has been given by Abboud el al. [786], More recent theoretical treatments of the solvent influence on Menschutkin reactions can be found in references [787-789]. 

Thus, whenever a chemist wishes to carry out a chemical reaction he not only has to take into consideration the right reaction partners, the proper reaction vessels, and the appropriate reaction temperature. One of the most important features for the success of the planned reaction is the selection of a suitable solvent. Since solvent effects on chemical reactivity have been known for more than a century, most chemists are now familiar with the fact that solvents may have a strong influence on reaction rates and equilibria. Today, there are about three hundred common solvents available, nothing to say of the infinite number of solvent mixtures. Hence the chemist needs, in addition to his intuition, some general rules and guiding-principles for this often difficult choice. 

In this chapter we have stressed the importance of being able to predict the three-dimensional structure of a molecule. Molecular structure is important because of its effect on chemical reactivity. This is especially true in biological systems, where reactions must be efficient and highly specific. Among the hundreds of types of molecules in the fluids of a typical biological system, the appropriate reactants must find and react only with each other—they must be very discriminating. This specificity depends primarily on structure. The molecules are constructed so that only the appropriate partners can approach each other in a way that allows reaction. 

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