Subject relative reactivity towards

The absolute rate constants for a variety of cyclizafions have been measured. In particular, the rates of decarbonylafion of a variety of alkoxycarbonyl radicals have been obtained by LFP studies on PTOC oxalates." From these data, rate constants for the reduction of alkoxycarbonyl radicals with BusSnH and their 5-exo cyclizafions were determined. Whereas cyclizations were slightly faster than the analogous alkyl radical 5-exo cyclizations, their reactions with BusSnH were 10 times slower, indicating that cyclization processes should be synthetically useful. The rate constants for the cyclization of a number of variously substituted a-amide radicals have been determined together with their relative reactivities towards reduction using BusSnH (Scheme 16). Cyclizations of secondary-based radicals were found to be similar to the corresponding alkyl-substituted radicals. In addition, the rate constants were subject to minor electronic... 

This subject has been thoroughly reviewedand only the main features will be briefly summarized here in order to set the following sections in context. Nucleophiles generally exhibit a smaller range of reactivities toward arynes than toward other substrates. This is because of the extreme reactivity and electrophilicity of arynes. Even so, selectivities are evident with benzyne in liquid ammonia, for example, the nucleophilicity order is PhS > PhO" > RO" and with benzyne in alcohol the order is F > Br > CF EtOH. In water at 318 °C the relative reactivities toward 4-methylbenzyne are PhS (46), F (6.2), piperidine (3.0), Br (1.7), PhNH2 (1.3), PhO (1.0), CF (1.0), NH3 (0.5), F (0.2)229 this compressed... 

It is widely accepted that the carbonyl reactivity toward nucleophiles increases in the order aldehyde>ketone>ester>amide [6]. This reactivity order is simply based on the extent to which each carbonyl carbon is sterically and electronically activated. However, reactivities might change when these carbonyl substrates are subjected to a Lewis acid. It is generally assumed that the coordination capability of the carbonyl oxygen to Lewis acids is the means by which Lewis acids activate carbonyl substrates. Thus, in some re.spects, the reaction rate parallels the Lewis basicity of the carbonyls. Furthermore, the reactivity of a carbonyl substrate depends on the reaction type as well as the Lewis acid employed. Special care must be taken in assessing the relationship between the relative reaction rate, the relative Lewis basicity, and the inherent carbonyl reactivity of each substrate. It is instructive to take a look at the following example (Schemes 2-2 and 2-3 Fig. 2-1). 

At this point we want to consider the relative reactivity of carboxylic acid derivatives and other carbonyl compounds in general terms. We return to the subject in more detail in Chapter 7. Let us first examine some of the salient structural features of the carbonyl compounds. The strong polarity of the C=0 bond is the origin of its reactivity toward nucleophiles. The bond dipole of the C-X bond would be expected increase carbonyl reactivity as the group X becomes more electronegative. There is another powerful effect exerted by the group X, which is resonance electron donation. 

CHEMICAL reactivity presents one of the great unsolved problems of organic chemistry. We know a great deal about how to approach the problem but are usually stymied by the fact that we always seem to have more parameters to fix than we have results to calculate. In this chapter we shall consider contributions of the LCAO method toward predicting relative reactivities of organic molecules. We shall be illustrative rather than comprehensive, and many excellent treatments will necessarily have to be omitted to keep the discussion within reasonable bounds. Fortunately, a number of comprehensive reviews on the subject are available. 

With 77 % aqueous acetic acid, the rates were found to be more affected by added perchloric acid than by sodium perchlorate (but only at higher concentrations than those used by Stanley and Shorter207, which accounts for the failure of these workers to observe acid catalysis, but their observation of kinetic orders in hypochlorous acid of less than one remains unaccounted for). The difference in the effect of the added electrolyte increased with concentration, and the rates of the acid-catalysed reaction reached a maximum in ca. 50 % aqueous acetic acid, passed through a minimum at ca. 90 % aqueous acetic acid and rose very rapidly thereafter. The faster chlorination in 50% acid than in water was, therefore, considered consistent with chlorination by AcOHCl+, which is subject to an increasing solvent effect in the direction of less aqueous media (hence the minimum in 90 % acid), and a third factor operates, viz. that in pure acetic acid the bulk source of chlorine ischlorineacetate rather than HOC1 and causes the rapid rise in rate towards the anhydrous medium. The relative rates of the acid-catalysed (acidity > 0.49 M) chlorination of some aromatics in 76 % aqueous acetic acid at 25 °C were found to be toluene, 69 benzene, 1 chlorobenzene, 0.097 benzoic acid, 0.004. Some of these kinetic observations were confirmed in a study of the chlorination of diphenylmethane in the presence of 0.030 M perchloric acid, second-order rate coefficients were obtained at 25 °C as follows209 0.161 (98 vol. % aqueous acetic acid) ca. 0.078 (75 vol. % acid), and, in the latter solvent in the presence of 0.50 M perchloric acid, diphenylmethane was approximately 30 times more reactive than benzene. 

While high isotope conversion factors are desirable in many instances, in other cases simplicity of construction and care of handling the materials involved can be more important. The present application is therefore primarily directed toward the provision of a simply constructed 60 isotope converter wherein the active materials are in solid form, can be easily cooled in place, and easily re-mov for processing, and replaced with fresh material. It will be understood that the selection of fissionable materials, moderator materials and fertile materials, the 65 relative amounts thereof and the critical size of the reactive composition required to produce a self-sustaining chain reaction, are not in themselves the subject of the present invention. These criteria are now familiar to persons skilled in the art The invention is concerned 70 with a novel construction which may be employed with any of the various combinations of materials which are already well known. 

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