Homogenous solvolysis

From the above one might be tempted to attribute ultrasonically enhanced chemical reactivity mainly to the mechanical effects of sonication. However this cannot be the whole reason for the effect of ultrasound on reactivity because there are a variety of homogeneous reactions which are also affected by ultrasonic irradiation. How, for example, can we explain the way in which power ultrasound can cause the emission of light from sonicated water (sonoluminescence), the fragmentation of liquid alkanes, the liberation of iodine from aqueous potassium iodide or the acceleration of homogeneous solvolysis reactions ... 

The homogeneous solvolysis of this substrate in aqueous ethanolic solvents can be monitored by the change in conductance as HCl is produced. Initial studies of the reaction in aqueous ethanol as solvent at 25 °C using a cleaning bath (45 kHz) revealed modest rate enhancements (up to about 2-fold) with the larger values being obtained in the more alcoholic media [37]. Similar results were found for the solvo-lyses in aqueous propan-2-ol and 2-methylpropan-2-ol. More substantial rate enhancements were obtained in the more ethanolic media and at lower temperature [38,39]. Detailed studies of the aqueous ethanol system led to the following main conclusions ... 

Another example of the effects of ultrasound on a solvolysis reaction can be found in the homogeneous hydrolysis of 2-chloro-2-methylpropane in aqueous alcoholic media (Eq. 3.3). This system has been the subject of numerous kinetic studies since it is one of the classic examples of a unrmolecular nucleophilic displacement reaction (termed... 

SOLVOLYSIS. A generalized conception of the relation between a solvent and a solute (i.e., a relation between two components of a single-phase homogeneous system) whereby new compounds arc produced, In most instances, the solvent molecule donates a proton to. or accepts a proton from a molecule of solute, or both, forming one or more different molecules. A particular case of special interest occurs when water is used as solvent, m which case die interaction between solute and solvent is called hydrolysis. 

The product elimination step proceeds with cleavage of the catalyst-substrate bonds. This may occur by dissociation, solvolysis, or a coupling of substrate moieties to form the product. The last of these involves covalent bond formation within the product, and corresponds to the microscopic reverse of oxidative addition. Upon reductive elimination both the coordination number and formal oxidation state of the metal complex decrease. In most homogeneous catalytic processes, the product elimination step, while essential, is usually not rate determining. The larger kinetic barriers are more frequently encountered in substrate activation and/or transformation. 

Since this paper ended with a claim of rather universal validity of the ECE mechanism in anodic addition and substitution reactions, cf. the mechanism proposed for the solvolysis of Th + in homogeneous medium, namely, disproportionation, it started a controversy. We shall not go into detail with this story (Hammerich and Parker, 1972 Marcoux, 1971, 1972 Parker, 1972 Parker and Eberson, 1970b), since it encompasses a lot of rather involved argumentation which... 

Table 19 shows results from the a-acetoxylation of two 2-alkyl-indans under different conditions analogous homogeneous reactions are included for comparison. Table 19 demonstrates that, at most, a weak effect is noticeable in the case of 2-t-butylindan, in that anodic oxidation on platinum, a strongly adsorbing metal, gives a cis/trans ratio that is about 10 times that observed from a 2-t-butyl-1-indanyl cation generated by solvolysis. Ratios on the weaker adsorbing electrode materials, carbon and lead dioxide, fall in between. For a smaller substituent (R = methyl) no effect is observable. 

It has been argued that insight into catalytic mechanisms can most readily be obtained by choosing reactions whose rate-determining step is unimolecular. The reactions best known to be unimolecular in homogeneous media are undoubtedly the solvolyses of tertiary butyl halides [161-163]. Their SN1 mechanisms are typified by the solvolysis of f-butyl bromide in an ethanol + water medium... 

Reaction of this W04 - with H2O2 produces peroxocomplexes, which in an aqueous methanohc medium epoxidize allyHc alcohols. The reactivity of our system agrees well with that of tungstate salts, dissolved in a single polar Liquid phase. The alkaline nature of the LDH support seems however to prevent solvolysis reactions. In the epoxidation of (homo)allyhc alcohols, selectivities are therefore better with the W04 -LDH A than with the homogeneous W salts [2,31. However, for some of the simple olefins, allyhc oxidation is not neghgible. 

Homogeneous process involves chemolysis or solvolysis. Here, plastics are treated with some chemicals or solvents and are depolymerized to monomers. Based on the chemicals used, it is classified as hydrolysis, methanolysis, alcoholysis, glycolysis, etc. [26]. 

The discussion and experiments presented below illustrate methods of studying chemical kinetics and determining the effects of structure on reactivity, as exemplified by the solvolysis of tertiary alkyl halides. The term "solvolysis" describes a substitution reaction in which the solvent, HOS, functions as the nucleophile (Eq. 14.30). In principle, solvolyses may be performed in any nucleophilic solvent such as water (hydrolysis), alcohols (alcoholysis), and carboxylic acids (for example, acetolysis with acetic acid). However, a practical limitation in choosing a solvent is the solubility of the substrate in the solvent because the reaction mixture must be homogeneous if it is not, surface effects at the interface of the phases will make the kinetic results difficult to interpret and probably nonreproducible as well. In the experiment described here, you will explore solvolyses in mixtures of 2-propanol and water. 

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