Nucleophiles, effective molarities

Exocyclic reactions for aromatic carboxylic esters 174 Exocyclic reactions for aliphatic carboxylic esters 187 Endocyclic reactions for carboxylic esters 191 Carbon acid participation for carboxylic esters 195 Effective molarities 198 Ring size 199 Initiating nucleophile 200 Phosphate and sulphonate esters 200... 

Obviously, in such cases the CD is acting as a true catalyst in esterolysis. The basic cleavage of trifluoroethyl p-nitrobenzoate by a-CD occurs by both pathways approximately 20% by nucleophilic attack and approximately 80% by general base catalysis (GBC) (Komiyama and Inoue, 1980c). The two processes are discernible because only the former leads to the observable p-nitrobenzoyl-CD. For the ester, Ks = 3.4 mM and kjka = 4.4 for the GBC route (1.25 for the nucleophilic route), and so KTS = 0.77 mM. For reaction within the ester CD complex [28], it was estimated that the effective molarity of the CD hydroxyl anion was 21-210 m (for Br0nsted /3 = 0.4 to 0.6 for GBC). Such values are quite reasonable for intramolecular general base catalysis (Kirby, 1980). 

As we have seen (Section 4, p. 191) the range of effective molarities associated with ring-closure reactions is very much greater than that characteristic of intramolecular general acid-base catalysis the main classification is therefore in terms of mechanism. By far the largest section (I, Tables A-D) gives EM s for intramolecular nucleophilic reactions. These can be concerted displacements (mostly at tetrahedral carbon), stepwise displacements (mostly addition-elimination reactions at trigonal carbon), or additions, and they have been classified in terms of the nucleophilic and electrophilic centres. 

Intermolecular alcoholysis of carbamate esters will also take place, but the reactions are very slow. In comparison with intermolecular nucleophilic attack by a phenoxide ion of the same on the unsubstituted ester [28], the effective molarity of the neighbouring phenoxide ion of [27] is 3 x 10 M. Thus, a phenoxide ion is an... 

First-order and second-order rate constants have different dimensions and cannot be directly compared, so the following interpretation is made. The ratio ki k,ma has the units mole per liter and is the molar concentration of reagent Y in Eq. (7-72) that would be required for the intermolecular reaction to proceed (under pseudo-first-order conditions) as fast as the intramolecular reaction. This ratio is called the effective molarity EM), thus EM = kimnJk,Ma- An example is the nucleophilic catalysis of phenyl acetate hydrolysis by tertiary amines, which has been studied as both an intermolecular and an intramolecular process. ... 

Intramolecular reactions are faster because AS - the entropy of activation (the probability of the reactant groups meeting) - is high and fastest when the reaction is a cyclisation (corresponding to intramolecular nucleophilic catalysis), which may be particularly favorable enthalpically. The simple measure of efficiency is the effective molarity (EM), the (often hypothetical) concentration of the neighboring group needed to make the corresponding intermolecular process go at the same rate [36]. It is simply measured, as the ratio of the first order rate constant of the intramolecular reaction and the second order rate constant for the (as far as possible identical) intermolecular process. In some convenient cases both reactions can be observed simultaneously, (Scheme 2.15) [37], and EM = ki/k2 measured di-... 

Table 19 includes the results for the intermolecular reactions of HC=CCH20 with (CO)5M=C(OMe)Ph discussed earlier. Comparison with the intramolecular reactions of 102 yields the effective molarities (EMs) " given in the table. The EM values in Table 19 have been corrected for the difference in the p a values of the oxyanion nucleophiles. These EM values are at the low end of the range which is common for the formation of 5-membered rings in conformationally flexible systems (10" -10 153,154... 

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