Steric effects chemical reactivity

Electrical effects are the major factor in chemical reactivities and physical properties. Intermolecular forces are usually the major factor in bioactivities. Either electrical effects or intermolecular forces may be the predominant factor in chemical properties. Steric effects only occur when the substituent and the active site are in close proximity to each other and even then rarely account for more than twenty-five percent of the overall substituent effect. 

The chemical structures of the monomers also determine their reactivity toward cationic polymerizations. Electron-donating groups enhance the electron densities of the double bonds. Because the monomers must act as nucleophiles or as electron donors in the course of propagation, increased electron densities at the double bonds increase the reaction rates. It follows, therefore, that electron-withdrawing substituents on olefins will hinder cationic polymerizations. They will, instead, enhance the ability for anionic polymerization. The polarity of the substituents, however, is not the only determining factor in monomer reactivity. Steric effects can also exert considerable controls over the rates of propagation and the modes of addition to the active centers. Polymerizations... 

The work by Hammett and Taft in the 1950s had been dedicated to the separation and quantification of steric and electronic influences on chemical reactivity. Building on this, from 1964 onwards Hansch started to quantify the steric, electrostatic, and hydrophobic effects and their influences on a variety of properties, not least on the biological activity of drugs. In 1964, the Free-Wilson analysis was introduced to relate biological activity to the presence or absence of certain substructures in a molecule. 

The effects of fluonnation on bond strengths, reactive intermediates, and steric interactions that directly relate to chemical reactivity are summanzed in this section... 

In order to smdy the effect of perturbation arising from spiro-conjugation on the chemical reactivities, in particular the facial selectivities, sterically unbiased dienes (96 and 97) based on fluorenes in spiro geometry have been synthesized [165]. These dienes react as Diels-Alder dienes with several dienophiles (maleic anhydride (MA), A-phenylmaleimide (PMI), A-phenyl-l,3,5-triazoUne-2,4-dione (PTD) and iV-methyl-l,3,5-triazoline-2,4-dione (MTD)). 

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. 

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). 

Chemical reactivity of unfunctionalized organosilicon compounds, the tetraalkylsilanes, are generally very low. There has been virtually no method for the selective transformation of unfunctionalized tetraalkylsilanes into other compounds under mild conditions. The electrochemical reactivity of tetraalkylsilanes is also very low. Kochi et al. have reported the oxidation potentials of tetraalkyl group-14-metal compounds determined by cyclic voltammetry [2]. The oxidation potential (Ep) increases in the order of Pb < Sn < Ge < Si as shown in Table 1. The order of the oxidation potential is the same as that of the ionization potentials and the steric effect of the alkyl group is very small. Therefore, the electron transfer is suggested as proceeding by an outer-sphere process. However, it seems to be difficult to oxidize tetraalkylsilanes electro-chemically in a practical sense because the oxidation potentials are outside the electrochemical windows of the usual supporting electrolyte/solvent systems (>2.5 V). 

Assuming that all B groups have the same reactivity, the chemical reaction giving rise to a branched molecule is identical to the reaction resulting in a linear polymer. Statistically this will eventually result in a hyperbranched polymer. However, dependent on the chemical structure of the monomer, steric effects might favor the growth of linear polymers. Computer simulations of of ABX-monomer condensation and AB -monomers co-condensed with B-functional... 

Now we turn to a discussion of the influence of a-substitution at C(6) or C(7) on the chemical reactivity of the lactam ring (Table 5.4,B). This substitution has been introduced mainly to improve lactamase stability (see Sect. 5.2.2.2). The insertion of an additional a-substituent at C(6) or C(7) of penicillins or cephalosporins, respectively, has a relatively small effect on the rate of base hydrolysis [82] [83], 6a-Methoxypenicillin is hydrolyzed at a rate that is approximately half that observed for the unsubstituted parent penicillin. This decrease is due mainly to unfavorable steric interaction between the... 

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

Set 0X14, l-oximino-3-X-5-methyl-l,2-benzoquinones, is of interest because the substituents in this data set do not differ significantly from each other in electrical effects. As chemical reactivities in water normally do not depend on polarizability, it would seem that the variations in pXa in this data set should be a function of steric effects. Correlation with the two parameter segmental steric effect equation gave best fit with the v/ parameters. The correlation is significant at the 99.0% confidence level and is good. It seems probable that the pXa values of these compounds are dependent on steric effects. 

Second, molecular mechanics calculations reveal nothing about bonding or, more generally, about electron distributions in molecules. As will become evident later, information about electron distributions is key to modeling chemical reactivity and selectivity. There are, however, important situations where purely steric effects are responsible for trends in reactivity and selectivity, and here molecular mechanics would be expected to be of some value. 

Chemoselectivity and regioselectivity are enzymatic properties of significant synthetic interest [1]. Their common feature is the fact that the enzymatic preference toward one of the several functional groups present on a substrate molecule is dictated by its accessibility to the protein active site (steric effect) and not necessarily by its chemical reactivity. 

The characteristics of synthetic polymer-metal complexes having uniform structure were illustrated. The chemical reactivity of a metal complex is often affected by the addition of a polymer ligand that exists outside the coordination sphere and air-rounds the metal complex. The effects of polymer ligands have been summarized under two heads steric effects, and environmental effects. 

In a parallel development, structural effects on the chemical reactivity and physical properties of organic compounds were modelled quantitatively by the Hammett equation 8). The topic is well reviewed by Shorter 9>. Hansen 10) attempted to apply the Hammett equation to biological activities, while Zahradnik U) suggested an analogous equation applicable to biological activities. The major step forward is due to the work of Hansch and Fujita12), who showed that a correlation equation which accounted for both electrical and hydrophobic effects could successfully model bioactivities. In later work, steric parameters were included 13). 

In the third step, when it occurs, chemical bonds are made and/or broken. Steric effects in this case should resemble those generally observed for chemical reactivity... 

A major method of modeling the effect of structural variation on chemical reactivity, physical properties or biological activity of a set of substrates is the use of correlation analysis. In this method it is assumed that the effect of structural variation of a substituent X upon some chemical, physical or biological property of interest is a linear function of parameters which constitute a measure of electrical, steric, and transport effects. 

Steric effects may arise in a number of ways. We consider primary steric effects to result from repulsions between nonbonded atoms. Such repulsions can only result in an increase in the energy of a group of atoms. In the case of chemical reactivities, if steric repulsions are greater in the transition state (rate data) or product. (equilibrium data) than in the reactant, the latter is more stable than either of the former and compared to a system free of steric effects the rate constant or equilibrium constant will show steric diminution. If the reverse situation obtains, and steric repulsions are greater in the reactant than in the transition state or product, the former is less stable than the latter, and compared to a system free of steric effects the rate or equilibrium constant will show steric augmentation. 

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