Mechanisms esters

Both schemes accommodate the kinetics, the primary isotope effect and the induction factor, which indicates that Cr(IV) is the initial stage of reduction of the oxidant. RoCek s mechanism does not accord with the solvent isotope effect of 2.5 per proton, which has just the value to be expected for acid-catalysis, for the O-H bond cleavage should be subject to a primary isotope effect of about 7. The ester mechanism is not open to this criticim. 

The Aq dependence of the oxidation in strongly acidic media was taken by Rodek as implying that no water molecule is involved in the transition state and that consequently the ester mechanism as portrayed above cannot hold. However, the Zucker-Hammett hypothesis upon which this argument is based, i.e. that a reaction forming a transition state containing a water molecule will follow a [H30 ] dependence, but that otherwise an dependence will be followed, may not be valid, and in any case the ester can be depicted as breaking down as fol-lows - ... 

JohnsonMK. 1975. The delayed neuropathy caused by some organophosphorus esters Mechanism and challenge. Crit Rev Toxicol 3 289-316. 

Nucleophilic substitution, in phosphate esters, mechanism and catalysis of, 25,99 Nucleophilic substitution, single electron transfer and, 26, 1 Nucleophilic vinylic substitution, 7,1... 

Partitioning of carbocations between addition of nucleophiles and deprotonation, 35, 67 Perchloro-organic chemistry structure, spectroscopy and reaction pathways, 25, 267 Permutational isomerization of pentavalent phosphorus compounds, 9, 25 Phase-transfer catalysis by quaternary ammonium salts, 15, 267 Phosphate esters, mechanism and catalysis of nucleophilic substitution in, 25, 99 Phosphorus compounds, pentavalent, turnstile rearrangement and pseudoration in permutational isomerization, 9, 25... 

It appeared to us that the only reasonable non-ionic reaction product of an acid and an olefin would be an ester, and for this reason we put forward the idea that this is the active species in the pseudo-cationic polymerizations. Of course, the idea of an ester in this role has a respectable ancestry which has been discussed in this new context [6]. The ester mechanism of polymerization will be discussed in sub-section 3.3. It must be understood that our conclusion concerning the non-ionic nature of the chain-carriers in the pseudocationic polymerizations is quite independent of our view that the chain-carriers are esters this is at present merely an hypothesis to explain our factual conclusion. 

Although the ester mechanism is not yet generally accepted, the evidence accumulating since it was first put forward is in its favour, and the evidence which is alleged to be against it, or which has been interpreted in terms of ion-pairs in place of the ester, is certainly compatible with the ester theory [13, 14,15]. We note in passing an interesting application of the polymerisation of styrene by perchloric acid it was used as an indicator-reaction in the enthalpy titration of weak bases [16]. 

The ester mechanism also explains well Gandini s and Plesch s observations on the dehydration of aromatic carbinols by acids [60], and it is actually an old item in the stock-cupboard of organic reaction mechanisms which has proved useful in explaining numerous organic reactions. This respectable antiquity and usefulness seem to have done nothing to make it more acceptable to many polymer chemists. 

The impact which was made by the writer s revival of the old ester mechanism in the context of polymerisations is attested by the number of polymer chemists who set about examining the validity of the theory experimentally. For example, Bywater in Canada confirmed that during the progress of a polymerisation of styrene by perchloric acid the acid could not be distilled out of the reaction mixture, but after exhaustion of the monomer it could be. This regeneration of the initiating acid after the consumption of the monomer is an often attested characteristic of pseudocationic polymerisations with many different protonic acids it is most simply explained by the decomposition of the ester to an alkene and the acid, i.e., a reversal of the initiation, when the monomer has been consumed. Enikolopian in the USSR found that the effect of pressure on the rate of polymerisation in the same system was not compatible with the propagation step involving an ion, and... 

It should be remembered, of course, that in the carbonium theory the word ion has a different connotation than it does in inorganic chemistry the degree to which the ester is dissociated may actually lie somewhere between the undissociated ester as in the ester mechanism and the free ions as usually written (although actually believed to be very short-lived) in the carbonium ion mechanism. ... 

In accordance with the two ester mechanism, the first step in the polymerization of ethylene in the presence of phosphoric acid is the formation of an ethyl phosphate (Ipatieff and Pines, 70). It was found that rapid absorption of ethylene took place when orthophosphoric acid was treated with etyhlene at 180° under an initial pressure of 50 atmos-... 

The presence of alkoxide ion would enhance the rate of ammonolysis, and the formation of bis(amido) derivatives by an ortho-ester mechanism (see Section VI, p. 110) would be partially suppressed in the competitive set of reactions. Thus, ammonolysis of penta-O-benzoyl-D-glucose in the presence of 5 mmolar proportions of sodium meth-oxide showed a decrease of 11% in the yield of the bis(benzamido)-glucitol derivative as compared with the same reaction conducted without added methoxide ion.47... 

In the homogeneous mechanism, the reaction is assumed to start by protonation of one of the reactants, either ester (mechanisms denoted as Aac1 and Aac2 [397,398]) or, less frequently, alcohol (mechanism Aal1). It seems likely that protonation of reactants is an important step in esterification catalysed by ion exchangers, too. This follows from all that has been said above about the effect of the acidic properties of ion exchangers on their catalytic activity and is further supported by the effect of the dielectric constant of solvents (Fig. 18), which indicates that the reaction mechanism involves a positive ion and a dipolar molecule [454]. 

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