Ester unimolecular processes

The first term, representing acid-"catalyzed" hydrolysis, is important in reactions of carboxylic acid esters but is relatively unimportant in loss of phosphate triesters and is totally absent for the halogenated alkanes and alkenes. Alkaline hydrolysis, the mechanism indicated by the third term in Equation (2), dominates degradation of pentachloroethane and 1,1,2,2-tetrachloroethane, even at pH 7. Carbon tetrachloride, TCA, 2,2-dichloropropane, and other "gem" haloalkanes hydrolyze only by the neutral mechanism (Fells and Molewyn-Hughes, 1958 Molewyn-Hughes, 1953). Monohaloalkanes show alkaline hydrolysis only in basic solutions as concentrated as 0.01-1.0 molar OH- (Mabey and Mill, 1978). In fact, the terms in Equation(2) can be even more complex both elimination and substitution pathways can operate, leading to different products, and a true unimolecular process can result from initial bond breaking in the reactant molecule. 

Physical Properties.—Further Z-ray studies of methyl thio- and dithio-esters have confirmed previous findings (see Vol. 5, p. 180) that the ester group is almost planar, with a Z conformation, Kinetic studies of the gas-phase pyrolysis of dithioacetates have shown that the reaction is a concerted unimolecular process, consistent with the mechanism shown in Scheme 10. °... 

Intramolecular general base catalysed reactions (Section II, Tables E-G) present less difficulty. A classification similar to that of Table I is used, but since the electrophilic centre of interest is always a proton substantial differences between different general bases are not expected. This section (unlike Section I, which contains exclusively unimolecular reactions) contains mostly bimolecular reactions (e.g. the hydrolysis of aspirin [4]). Where these are hydrolysis reactions, calculation of the EM still involves comparison of a first order with a second order rate constant, because the order with respect to solvent is not measurable. The intermolecular processes involved are in fact termolecular reactions (e.g. [5]), and in those cases where solvent is not involved directly in the reaction, as in the general base catalysed aminolysis of esters, the calculation of the EM requires the comparison of second and third order rate constants. 

The kinetics and mechanisms of gas-phase elimination of ethyl 1-piperidinecarboxyl-ate, ethyl pipecolinate, and ethyl 1-methylpipecolinate has been determined in a static reaction system.9 The reactions proved to be homogeneous, unimolecular, and obey a first-order rate law. The first step of decomposition of these esters is the formation of the corresponding carboxylic acids and ethylene. The acid intermediate undergoes a very fast decarboxylation process. The mechanism of these elimination reactions has been suggested on the basis of the kinetic and thermodynamic parameters. 

Scheme 11 summarizes the various unimolecular transformations of the [ct, ct]-, [ct, 7t]-, and [%, 7i]-type dioxyl diradicals, which have been disclosed by our qualitative analysis of the orbital orientations in these electronic isomers. The answer to the original question as to the reasons for the persistence of dioxiranes rests on the appreciable electronic barrier towards rearrangement into the ester for the thermally produced [ct, ct] diradical instead, the [ct, ct] dioxyl species recloses to the dioxirane Consequently, these most highly strained cyclic peroxides can be prepared, isolated, handled, and utilized for synthetic purposes even under ambient conditions. On photochemical activation (n, CT excitation), however, the [ct, it] diradical results, which prefers P-scission into a carboxy-alkyl radical pair rather than rearranging into the ester product. For the latter process,... 

Lewis acid (BF3 or EtAlC ) complexes of a,(3-unsaturated esters can shift the photoequilibrium (PSS) toward the thermodynamically less stable Z-isomer even more and may inhibit other competing unimolecular photochemical processes.563 Such enhanced isomerization results are explained by selective excitation of the ground-state Lewis acid ester carbonyl complex, which exhibits a red shift in the long-wavelength k,k absorption band (/lmax) and higher molar absorption coefficients ( 313) (Scheme 6.6). 

The acidity function criterion offers little prospect as a basis on which the reaction mechanism can be assigned as both 1-methylcyclohexanol and its acetate form olefins by unimolecular carbonium-ion processes which show a unit dependence on in acetic acid containing aqueous sulphuric acid . The ten-fold greater reactivity of the ester was explained in terms of the greater electronegativity of the protonated ester function than the oxonium ion, which more than counterbalances the lower basicity of the acetate than the alcohol. 

However, less is know about the quantitative details of these processes than is the case for the alkanes. In general, unimolecular decomposition of the alkoxy radicals [e.g., CH3CH20CH(0 )CH3 in the above reaction sequence] is more rapid than for an alkane-derived radical of similar structure. The major end product of ether oxidation is quite often an ester, the analog to the carbonyl compounds (aldehydes and ketones) generated from alkane oxidation. 

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