Steric effects on reactivity

Reactions such as catalytic hydrogenation that take place at the less hindered side of a reactant are common m organic chemistry and are examples of steric effects on reactivity Previously we saw steric effects on structure and stability m the case of CIS and trans stereoisomers and m the preference for equatorial substituents on cyclo hexane rings... 

The steric constant Es and related quantities do not constitute the only approach to the study of steric effects on reactivity. Steric strain energy calculations and topological indices are more recent approaches. Qualitative concepts have been... 

In conclusion, the chemistry of atropisomers holds a promising future in providing reactivity data for rotamers and information on molecular interactions that are otherwise not detectable. It is especially promising in that a variety of functional groups can be introduced into the 9-arylfluorene and 9-substituted triptycene systems and thus the electronic as well as steric effects of substituents can be examined. In the past, steric effects on reactivity were almost die sole... 

Many examples of the influence of steric effects on reactivity and regioselectivity in free radical additions are known. The anti-Markownikoff regioselectivity apparently is smalier than originally assumed and frequently dependent on... 

By changing the reactivity of the alkylating agent, it is possible to vary the magnitude of an ortho steric effect. Thus, the reactivity of 2-substituted pyridines toward Mel in acetone is linearly related on a logarithmic scale to the reactivity of the same substrates toward methyl fluorosulfonate in benzene. The fluorosulfonate is about 104 times more reactive than the iodide, and so the transition state for quaternization occurs earlier. The earlier transition state gives rise to a smaller steric effect the slope of the plot demonstrating the dependence of the steric effect on reactivity is 0.69.76... 

The profoundly complex influence of the steric effect on reactivity shows the need for caution in using steric substituent constants to correlate chemical activity with structure because the reaction model from which these constants were obtained may not apply to the interactions that occur in biological systems. 

WA. Pavelich and R.W. Taft, The Evaluation of Inductive and Steric Effects on Reactivity. The Methoxide Ion-catalysed Rates of Meth-anolysis of 7-Menthyl Esters in Methanol, J. Am. Chem. Soc., 1957, 79, 4935. 

Pavelich, W. A. and R. W. Taft. 1957. The evaluation of inductive and steric effects on reactivity. The methoxide ion-catalyzed rates of methanolysis of /-menthyl esters in methanol, J. Am. Chem. Soc., 79 4935-4940. 

The results mentioned are best demonstrated on secondary nitramines [7], which in their molecular structure are relatively simple polynitro compounds, and the mechanism of primary homolysis of their molecules is well understood [11,14] (Scheme 1). Polynitro arenes, on the other hand, have a more complex structure and intramolecular effects in their molecules here the mesomeric, inductive and steric effects on reactivity operate simultaneously. This fact makes the problem of their primary fragmentation somewhat complicated too [6,9] (Schemes 2, 3a, 3b, 7 and 8). If a molecule of these compounds contains several types of substituents, it can contain several potential reaction centres (e.g., the PYX and TMPM molecules, see Schemes 7 and 8). The initiation proper can then be realized by the molecule simultaneously participating by several centres or always by a single centre in a given type of initiation (the initiation of PYX by impact or shock versus its initiation by electric spark, see Figs. 12 and 26) [6]. 

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