Steric effects of a substituent

An example for such a parameter is the steric constant Sb which can be used as an alternative to E, or o. This constant has been derived on the assumption that the steric effect of a substituent is mainly due to branching24 . By regression... 

Now that the steric parameter can be evaluated, the inductive parameter is available. Taft noted that the transition-state structures for acid- and base-catalyzed hydrolysis of esters (15 and 16, respectively) differ from each other by only tiny protons. Therefore the steric effect of a substituent should be approx-... 

One of the simplest methods to evaluate the steric effect of a substituent, in a position ortho to a reactive center, is to compare results for the same group at a... 

Assuming that the relative distance contributions to the steric effect of a substituent are not significantly different, all the regression coefficients b were settled equal to one and a simplified expression was proposed as ... 

It essentially establishes and also determines the prevailing relationship amongst electronic, hydrophobic and steric effects of a substituent and a change in biological effect. 

Table 3.2 lists some examples of E values. Note that the E values of hydroxy and methoxy are Identical, thus it can be seen that E values describe the minimum steric effect of a substituent. 

The Ge(CH3)3 9> oup causes a large redshift of the n- 7c transition of the carbonyl group but a quite small redshift of the transition of the aromatic system. The data have been compared with values for C(CH3)3COCeH5, Si(CH3)3COCeH5, and various other ketones and with data obtained by Huckel MO calculations [16] for the electronic and steric effects of a substituent X in typical XCOCgHg compounds, see also [21]. 

A numba of later variants of Hammett s model exist and a review of these and a large listing of the respective constants has been written by Hansch a al For example, Taft applied a modified Hammett model to the reaaion rates of ester hydrolysis, in which the inclusion of a steric parameter allowed for a better isolation of the respective polar and steric effects of a substituent ... 

Steric effects of the substituents in positions 4 and 5 cannot shift the protomeric equilibrium sufficiently to permit spectroscopic observation of the thiol form (43b) ultraviolet spectra of 4-terr-butyl-5-methyl-A-4-thia2oline-2-thione (49a) in neutral solvents do not reveal any trace of the thiol protomer (49bi (Scheme 21) (70). 

The reactivity of the individual O—P insecticides is determined by the magnitude of the electrophilic character of the phosphoms atom, the strength of the bond P—X, and the steric effects of the substituents. The electrophilic nature of the central P atom is determined by the relative positions of the shared electron pairs, between atoms bonded to phosphoms, and is a function of the relative electronegativities of the two atoms in each bond (P, 2.1 O, 3.5 S, 2.5 N, 3.0 and C, 2.5). Therefore, it is clear that in phosphate esters (P=0) the phosphoms is much more electrophilic and these are more reactive than phosphorothioate esters (P=S). The latter generally are so stable as to be relatively unreactive with AChE. They owe their biological activity to m vivo oxidation by a microsomal oxidase, a reaction that takes place in insect gut and fat body tissues and in the mammalian Hver. A typical example is the oxidation of parathion (61) to paraoxon [311-45-5] (110). 

Numerous literature references104 attest to the fact that the naturally occurring spiroketals and many synthetic products adopt conformations in which the anomeric effects are maximized and the steric effects are minimized. However, in some such compounds, the steric effects of bulky substituents and diaxial interactions can result in a conformation in which the anomeric effect cannot operate. 

The chemoselectivity of the other alkenes of Table 1 is more variable. It appears that bulky substituents favour bromide over methanol attack of the bromonium ion, since dibromlde increases from 39 to 70 % on going from methyl to tert-butyl in the monosubstituted series. The same trend is observed in the disubstituted series with a contraction of the chemoselectivity span (37 to 43 % on going from methyl to teH-butyl) for the trans isomers. Since the solvated bromide ion can be viewed as a nucleophile larger than methanol, the influence of steric effects, important in determining the regioselectivity, does not seem very significant as regards the chemoselectivity. This result has been interpreted in terms of a different balance between polar and steric effects of the substituents on these two selectivities. 

According to the orbital phase theory, the Gloss s diradical 62 is predicted to have a triplet ground state due to the same orbital-phase topology as the TM (2). In derivatives of 62, electronic and steric effects of various substituents as well as ring strains in the cyclic diradicals have successfully been applied to modulate the... 

First, one of the strongest piece of evidence in support of the existence of a thianorcaradiene intermediate is the steric effect of the substituents at C-2 and C-7 of a thiepin. Substantial stability gained by 2,7-di-tert-butyl substitution on thiepin implies that these groups force the nonbonding interaction in the thianorcaradiene structure to be large and hence the thiepin structure will be favored (see Section 4-1, 4-3 and 4-4). 

Stewart33, who proposed a parallel between the rate of esterification of 2-substituted benzoic acids and the molecular weights of the substituents (the nitro group strongly deviating from this relationship) was the first to attempt to relate the steric effect of a... 

A high concentration of B will help to compensate for its lesser reactivity, since the growing chain has a greater chance of meeting B molecules more often than A molecules. The relative reactivity of monomers depends on polar and steric effects of the substituents on the monomers. 

The constants of ion pair formation of 33 amines with 2,4-dinitrophenol in benzene (Ab) have been compared with the pAa in water56. The effects of structural variations on basicity are larger in water than in benzene for primary and secondary cyclic amines, but similar for tertiary amines. The Taft and Hancock equation [where <7 has the usual meaning and E° (R ) is the steric effect of a component substituent]... 

To verify such a steric effect a quantitative structure-property relationship study (QSPR) on a series of distinct solute-selector pairs, namely various DNB-amino acid/quinine carbamate CSPpairs with different carbamate residues (Rso) and distinct amino acid residues (Rsa), has been set up [59], To provide a quantitative measure of the effect of the steric bulkiness on the separation factors within this solute-selector series, a-values were correlated by multiple linear and nonlinear regression analysis with the Taft s steric parameter Es that represents a quantitative estimation of the steric bulkiness of a substituent (Note s,sa indicates the independent variable describing the bulkiness of the amino acid residue and i s.so that of the carbamate residue). For example, the steric bulkiness increases in the order methyl < ethyl < n-propyl < n-butyl < i-propyl < cyclohexyl < -butyl < iec.-butyl < t-butyl < 1-adamantyl < phenyl < trityl and simultaneously, the s drops from -1.24 to -6.03. In other words, the smaller the Es, the more bulky is the substituent. The obtained QSPR equation reads as follows ... 

First, we examined the kinetic parameters (Kj and k(.jt) of some orf/jo-substitu-ted compounds, as well as a control substrate. The results are shown in Table 8. The Kj and k(.jt values of a standard substrate (X, R=H) are 13.9 mM and 353 s, respectively. Introduction of a chlorine atom on the ortho-position of the benzene ring (X=Cl,R=H) accelerates the rate of reaction obviously because of its electron-withdrawing property. The steric effect of this substituent is con-... 

The A-benzoylhydrazones of acylpinacolines 75 (R = alkyl R = H R = r-Bu R" = Ph) in solution form equilibria involving the participation of two hydrazone (75A- and Z) tautomers, one or two enhydrazine (75A -E and Z) tautomers, and one cyclic (75B) tautomers (84ZOR1371). Here, clear evidence is presented characterizing the influence of the steric effect of the substituent on the C = N bond an increase in the steric demands in the series R = H < Me < t-Pr < t-Bu shifts the equilibrium toward the cyclic tautomer 75B. When R = H, this tautomer is absent but when R = r-Bu, the equilibrium is shifted entirely in favor of 75B. 

Stewart" proposed a parallel between the rate of esterification of 2-substituted benzoic acids and the molecular weights of the substituents. The nitro group deviated strongly from this relationship. It is the first work to attempt to relate the steric effect of a group to some property that might at least in part be a measure of size. Kindler made the first attempt at defining a set of steric parameters. These parameters were later shown to be a function of electrical effects. The first successful parameterization of the steric effect is due to Taft, who defined the steric parameter Es for aliphatic systems by equation 27 ... 

To characterize the electronic and steric effects of the substituents at positions 4-6 on the stability of the 2-aryl-l,3-oxazine ring forms exhibiting ring-chain tautomerism, a relative ring stability parameter (c) has been introduced. The value of c is calculated as the difference between the value of log Xx = h for a 2-aryl-l,3-oxazine derivative bearing substituents at positions 4-6 and the intercept value (log Xo = —0.15) for the parent unsubstituted 2-arylperhydro-l,3-oxazine (c = log Xx = h log Xq). 

---