Persistent-Intermediate Mechanisms

Perhaps the chief contribution of beam studies so far is to provide a rather clear distinction between direct and persistent-intermediate mechanism types. " A persistent intermediate is defined as a collision complex in which the constituent particles stay within normal chemical bonding distances of each other for at least a few periods of rotation (> 10" sec). This provides optimum conditions for internal energy redistribution. Hence the break-up of the complex that follows can be regarded as a unimolecular decay which may be considered separately from the initial collision. A direct mechanism, on the other hand, involves an intermediate which breaks up in a time that is less than one molecular rotation. 

Biodegradation, hydrolysis, and sorption influence the environmental fate of LAS, AS, and AES. Primary degradation of surfactants is important because this process usually results in loss of surfactancy and reduced toxicity (5, 6). Complete mineralization ensures that persistent intermediates will not be formed and that biodegradation will be an effective mass-removal mechanism in the environment. Sorption and association of surfactants with particles or dissolved organic substances are processes that decrease bioavailability and can be correlated with decreased surfactant toxicity (7). 

The reaction scattering in polyatomic ion-molecule systems has been studied in a number of cases and both direct and persistent complex mechanisms have been observed. Although these reactions are too complex to be amenable to the detailed theoretical analysis in the manner that three atom ion-molecule systems are, they can provide us with valuable information on internal energy equilibrium, uni-molecular reaction rates, life times of shortlived intermediates, and so on. 

The criteria which decide whether a given reaction goes by a direct mechanism or via a persistent intermediate have been examined in some detail in a recent review by one of the authors. The reader is referred to this, as the matter will be only briefly discussed here. 

The rearrangement of 2-phenyl-3-butyn-2-ol to 3-phenybut-2-en-l-al was examined computationally. The carbocation PhMeC+-C=CH is considered an intermediate or, under some conditions, a transition state. The stepwise Diels-Alder reaction of 1-trimethylsiloxy-1,3-butadiene and 4,6-dinitrobenzofuroxan was studied by computational methods. A stepwise mechanism with a persistent intermediate is supported. However, this intermediate is suggested to be a heterocyclic adduct and not the zwitterionic allyl cation/furoxan anion adduct proposed in the previous studies. 

Addition of HCl to an alkene generally proceeds by a stepwise mechanism. The HCl bond breaks as the CH bond forms and this gives two reaction intermediates. The intermediates are not observed, but they persist until a second collision brings Cl close to the other carbon of the alkene. 

If Scheme 2 accurately represented the PhCH2CCl chemistry, curvature in the addn/rearr vs. [alkene] correlation would persist when the carbene was generated from 37. The absence of curvature in this case counts against Scheme 2 (and the CAC mechanism), but accords with the RIES mechanism, Scheme 3. Elimination of the diazirine precursor eliminates the diazirine excited state. From 37, both cyclopropane formation and 1,2-H rearrangement proceed from a single (carbene) intermediate, and addn/rearr vs. [alkene] is linear.25... 

The other point concerns the persistent controversy about the detailed mechanisms of general reactions of this type—i.e., the deoxymercuration reactions, not only of methoxycarbonyl compounds but also of the corresponding olefin adducts. These are clearly concerted processes that are aided in some way by coordination of X- on the mercury and coordination of H+ on the oxygen. There has been controversy, in which Wright and others have participated, as to whether the detailed mechanism involves a cyclic intermediate, in other words whether undissociated HX participates in the reaction or whether the assistance at the metal and oxygen centers are independent processes. Have you any comments on this ... 

In view of the present calculated results, the SET mechanism would be described as follows. Basically, the polar four-center reaction in Scheme 14 leads to C—C bond formation. However, when the alkyl group is bulky, only the two-center (Mg—O) reaction takes place. The aUcyl-Mg bond is cleaved homolytically owing to the persistent Mg tetravalency and the stability of the resultant radical species. Hence, biradical intermediates are formed not by a single electron transfer but by the C—Mg homolytic scission. 

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