Effective molarities of intramolecular reactions

Effective charge and transition-state structure in solution, 27, 1 Effective molarities of intramolecular reactions, 17,183 Electrical conduction in organic solids, 16,159 Electrochemical methods, study of reactive intermediates by, 19, 131 Electrochemical recognition of charged and neutral guest species by redox-active receptor molecules, 31, 1... 

Effective charge and transition-state structure in solution, 27, 1 Effective molarities of intramolecular reactions, 17, 183... 

In the last two decades, there has been a large accumulation of experimental evidence as well as of theoretical interpretations of intramolecular reactions. One notes, however, that attention has been focused on the phenomena of immediate interest to the various specialists. As a consequence of the fact that specialisation implies intensification of knowledge on the one hand but limitation on the other, there has still been insufficient communication and cross-fertilisation between the different schools. This situation is well exemplified by the two most extensive reviews on intramolecular phenomena, namely, that of Kirby (1980), entitled Effective Molarities for Intramolecular Reactions , and that of Winnik (1981a), entitled Cyclisation and the Conformation of Hydrocarbon Chains , which present different approaches and apparently unrelated facts and theories. 

The effectiveness of intramolecular catalysis is quantitated by the effective molarity of the reaction, the ratio of the first-order rate constant for the intramolecular reaction to the second-order rate constant for a sterically and electronically analogous reaction. Effective molarities have a theoretical maximum of 10 M, but in small molecule systems rarely come anywhere near this. 

Pascal, R. Induced intramolecularity in the reference reaction can be responsible for the low effective molarity of intramolecular general acid-base catalysis. J. Phys. Org. Chem. 2002, 15, 566-569. 

The stability of a trivial assembly is simply determined by the thermodynamic properties of the discrete intermolecular binding interactions involved. Cooperative assembly processes involve an intramolecular cyclization, and this leads to an enhanced thermodynamic stability compared with the trivial analogs. The increase in stability is quantified by the parameter EM, the effective molarity of the intramolecular process, as first introduced in the study of intramolecular covalent cyclization reactions (6,7). EM is defined as the ratio of the binding constant of the intramolecular interaction to the binding constant of the corresponding intermolecular interaction (Scheme 2). The former can be determined by measuring the stability of the self-assembled structure, and the latter value is determined using simple monofunctional reference compounds. 

The paper first considers the factors affecting intramolecular reaction, the importance of intramolecular reaction in non-linear random polymerisations, and the effects of intramolecular reaction on the gel point. The correlation of gel points through approximate theories of gelation is discussed, and reference is made to the determination of effective functionalities from gel-point data. Results are then presented showing that a close correlation exists between the amount of pre-gel intramolecular reaction that has occurred and the shear modulus of the network formed at complete reaction. Similarly, the Tg of a network is shown to be related to amount of pre-gel intramolecular reaction. In addition, materials formed from bulk reaction systems are compared to illustrate the inherent influences of molar masses, functionalities and chain structures of reactants on network properties. Finally, the non-Gaussian behaviour of networks in compression is discussed. 

Figure 9. Molar mass between elastically effective junction points (Mc) relative to that for the perfect network (Mc°) versus extent of intramolecular reaction at gelatin (pr,c) for polyurethane networks (29).

The values of 0(ASD) /2.3O3 R listed in Table 5 are the entropic components of log EM. These are the log EM- alues for ideal strainless cyclisation reactions, i.e. reactions where 0AH° = 0. It is of interest to note that, as far as the entropic component is concerned, symmetry corrected effective molarities on the order of 102 106M are found. This observation leads to the important conclusion that cyclisation reactions of chains up to about 7 skeletal bonds are entropically favoured over reactions between non-connected 1 M end-groups. The intercept of 33 e.u. corresponds to an effective molarity of exp(33/R) or 107 2M, which may be taken as a representative value for the maximum advantage due to proximity of end-groups in intramolecular equilibrium reactions. It compares well with the maximum EM of about 108M estimated by Page and Jencks (1971). 

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). 

Intramolecular hydrogen abstraction by primary alkyl radicals from the Si—H moiety has been reported as a key step in several unimolecular chain transfer reactions [11]. In particular, the 1,5-hydrogen transfer of radicals 8-11 (Reaction 3.4), generated from the corresponding iodides, was studied in competition with the addition of primary alkyl radicals to the allyltributylstannane and approximate rate constants for the hydrogen transfer have been obtained. Values at 80 °C are in the range of (0.4-2) x 10" s, which correspond to effective molarities of about 12 M. 

What is the effective molarity of imidazole as an intramolecular general base in intramolecular alcoholysis and hydrolysis This is perhaps the most important single piece of information that is currently missing for analysis of the rates of a-chymotrypsin reactions in terms of specific catalytic effects. 

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