Steric hindrance definition

In chain reactions involving three termination steps (two uncrossed and one crossed) the quantity = 1c /(1c 1c )1/2 is frequently used to interrelate the cross-termination constant with the two uncrossed termination constants. For many different types of radical < is found to be about 1 (or alternatively, if the statistical factor of 2 favoring the crossed termination process is ignored in the definition of the rate constants, < 2). In the present reaction system —3-6, in agreement with the value obtained by Russell at 90°C. (26). The crossed termination constant itself is somewhat less than half the value found for kt. This seems reasonable since only one hydrogen atom will be available for transfer in the crossed termination, compared with the two that are available in the self-reaction of two tetralylperoxy radicals. In addition, steric hindrance to reaction should be greater for the crossed termination than for Reaction 8. The products are presumably cumyl alcohol, a-tetralone [3,4-dihydro-l(2H)naphthalenone], and oxygen (28). 

The Grant-Cheney steric hindrance model74> to estimate the C-13 splittings were applied. The evidence was said to be ambiguous but they felt that there definitely are two forms of the monomer present. 

At present, the precise reason for low reactivity of alkylbenzothiophenes and alkyldibenzothiophenes is not definitely known. It is clear that steric factors are indeed important, and it has been proposed that steric hindrance lowers the adsorption constant for these species (5, 17, 25, 26). Molecular... 

A common probe of reaction mechanisms used to infer charge distribution in the transition state involves variation of substituent groups near the reaction center. From the variation in reaction rate produced by electron-donating and electron-withdrawing groups or by the steric hindrance of various sized groups, transition state characteristics can be inferred. Two empirical correlations have been proposed and refined which provide a common framework for this process. The Hammett equation is applied to aromatic systems [45]. The Taft correlation is applied to aliphatic systems [45], Definitions of terms, collections of substituent constants (steric and electronic effects for various substituents), and listings of observed reaction response parameters (for typical reaction types) have been collected [45]. 

Scheme 9 shows in fulgide 39, replacement of the isopropylidene group (IPD) by a dicyclopropylidene group (DCP) caused a bathochromic shift in the absorption band of the 7,7a-DHBF, owing to the partial double-bond character of the DCP group. It is obvious that the steric hindrance of R1 and R2 also affected the max of the colored form. There was not a definite role for the substituent effect on the quantum yield of photoreactions. Table 4.8 shows the absorption spectroscopic data and quantum yields of photoreactions of fulgides with different R R2 groups in toluene. 

Mathematics and statistics, graph theory, computational chemistry and molecular modelling techniques enable the definition of a large number of theoretical descriptors characterizing physico-chemical and biological properties, reactivity, shape, steric hindrance, etc. of the whole molecule, molecular fragments and substituents. 

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