Protic acids, common

The most common procedure previously employed to effect the elimination of thiols from thioacetals has been heating in the presence of a protic acid. For example, propionaldehyde diethyl thioacetal is converted to 1-ethylthio-l-propene on heating at 175° in the presence of phosphoric acid. The relatively high temperature and acidic conditions of such procedures are, however, distinct disadvantages of this method. Another approach consists of oxidation of a thioacetal to the mono S-oxide and thermal elimination of a sulfenic acid at 140-150°. ... 

A number of "Hf OH " equivalents are used Oxvmercuration conditions or formic acid and protic acid, followed by hydrolysis of the formate ester are two common approaches. 

Percarboxylic acids have been used widely in Baeyer-Villiger oxidation. Peracetic acid is one of the most commonly used peracids distilled peracetic acid is employed commercially by Solvay Interox for the production of e-caprolactone.246 The use of distilled peracetic acid is essential, as it contains no strong protic acids which can catalyse polymerization of the resulting lactone and cause other side reactions. Figure 3.53 illustrates the use of pre-formed peracetic acid for the Baeyer-Villiger oxidation of ketones.247,248... 

Aqueous systems where one chemical species (say, L) is exchanged are quite common, and the study of the equilibria involved is crucial to understanding their behavior. For example, when a mono-protic acid is dissolved in water, the following equilibrium exists ... 

The commonly accepted mechanism for addition of halogens to alkenes has two steps, and is quite analogous to the mechanism for addition of hydrogen-containing acids (protic acids). In step (1) halogen adds as a positive halogen ion... 

As with HX, H2O, and ROH (see above), the metals also react with other protic acids such as NH3, PH3, H2S, HCN, mineral acids, and organic acids, with release of H2 The same reaction is observed with a variety of acidic C-H compounds (e.g., R —CH2—R R"CH— ) to give organic derivatives, MR2. In the cases above intermediates, HMR, are not observed. Cleavage of C—H bonds is also common with saturated and unsaturated hydrocarbons, along with C—C fission at high temperatures, metal-carbon bonds being formed . 

The acylation of alkanes has also been known for a long time, but for synthetic purposes is limited to simple substrates. The initial step is hydride abstraction by an acylium ion, a process well established in the presence of a powerful Lewis acid, most commonly an aluminum halide, or strong protic acid. The carbocation so formed can then undergo elimination, possibly after hydride or alkyl migration, to give an alkene which is then acylated. In the presence of excess alkane, saturated ketones are formed by a further intermolecular hydride transfer, whereas with an excess of acyl halide, the product is the (conjugated) unsaturated ketone. -" The synthetic potential is obviously likely to be limited to simple substrates. 

Tremillon introduced the common oxoacidity function, ft, as a measure of the relative acidity (basicity) of solutions of various substances in molten media. This parameter is similar to the well-known protic acidity functions, H0 (Hammett function) and pA (Izmailov s acidity function) considered in Part 1. In terms of the primary medium effects, the function ft may be represented as the sum of the primary medium effect of O2- for the given medium (—logy0O2 ) and the instrumentally measured pO (pOl) in this medium ... 

Because aromatic compounds are subject to electrophilic attack, D+ provides a ready entry of deuterium into the aromatic molecule. The reaction of various anhydrides with D2O is one of the most common methods for providing protic acids. The acidity of protic acids can be enhanced by co-catalysts , which are usually Lewis acids. Hence electrophilic substitutions by protic acids and Lewis acids are of primary interest in hydrogen isotope exchange. 

Various other 2,4,6-trisubstituted-l,3,5-trioxanes obtained by cyclotrimerization of the corresponding aldehydes have been reported (cf. Scheme 70 Table 17) common catalysts employed were, besides protic acids, zeolites, bentonitic earth, Lewis acids, heteropoly acids, organic metal oxides, and ion-exchange resins. When X-ray structures were reported, the chair conformer with the substituents in equatorial positions was always found (e.g., <19988417>). Also, the trimerization of acetaldehyde and propionaldehyde in the presence of SO2 has been reported <1983ZOB1787> wherein the aldehydes and SO2 initially form CT complexes. In the case of acetaldehyde, the corresponding enthalpy and entropy of complexation were determined to be —ISkJmoL and —32JmoL respectively. 

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