Hydrogen exchange acid catalysis

Only a-hydrogens exchange. Acid catalysis is also used, especially with aldehydes. 

The reaction takes place extremely rapidly and if D2O is present in excess all the alcohol is con verted to ROD This hydrogen-deuterium exchange can be catalyzed by either acids or bases If D30 is the catalyst in acid solution and DO the catalyst in base wnte reasonable reaction mech anisms for the conversion of ROH to ROD under conditions of (a) acid catalysis and (b) base catalysis... 

The hydration reaction has been extensively studied because it is the mechanistic prototype for many reactions at carbonyl centers that involve more complex molecules. For acetaldehyde, the half-life of the exchange reaction is on the order of one minute under neutral conditions but is considerably faster in acidic or basic media. The second-order rate constant for acid-catalyzed hydration of acetaldehyde is on the order of 500 M s . Acid catalysis involves either protonation or hydrogen bonding at the carbonyl oxygen. 

For exchange of non-labile organic hydrogen atoms, acid-base catalysis (or some other catalytic hydrogen-transfer agent such as palladium or platinum) is required. The method routinely gives tritiated products having a specific activity almost 1000 times that obtained by the Wilzbach method shorter times are required (2-12h) and subsequent purification is easier. 

Larcher, A. V., Alexander, R., Rowland, S. J. and Kagi, R. I. (1986) Acid catalysis of alkyl hydrogen exchange and configurational isomerisation reactions acyclic isoprenoid acids. Organic Geochemistry 10, 1015. 

This involves rate-determining proton transfer, equation (37) in principle it should show general acid catalysis, but in practice this usually cannot be seen as the catalyzing acid is simply H30+ . A typical example would be aromatic hydrogen exchange, such as the detritiation of tritiated benzene shown in equation (38) 147... 

The study of cocatalysis in cationic polymerizations is extremely complicated by the fact that Lewis acids can participate in a variety of ill defined reactions (91). Satchell (91, 92) regards the hydrogen exchange reaction between Bronsted acids and aromatics catalyzed by Lewis acids as a prototype for Friedel-Crafts catalysis. He postulates that cocatalytic efficiency is determined by the stability of the complex anion B -f HX + SnCl4 - BH SnCl4Xe. A simple enhancement of conventional acidity by B + HX -> BH Xe as proposed by Plesch (93) and Russel (94) is considered to be unimportant. The stability of the complex and its catalytic activity are determined by the electron density... 

The hydrogen exchange of phosphine in aqueous solution was studied by deuterium labelling. The reaction shows little sensitivity to bases other than hydroxide ion, though it shows general acid catalysis, viz. 

Exchange of amine hydrogen atoms with deuterated hydroxylic solvents occurs rapidly, especially upon acid catalysis ... 

For diazoacetone, it was shown that a-hydrogen exchange occurs more rapidly than hydrolysis and that general acid catalysis could not be detected in chloroacetic-chloroacetate buffers. Measurements over a range of perchloric acid concentrations in dioxane water mixtures gave... 

The catalytic addition of H2S to olefins follows the Markovnikov rule, with the -SH moeity attaching itself to the carbon atom connected to the least number of hydrogen atoms. With linear olefins, having terminal or internal double bonds, acid catalysis using zeolites, cation-exchange resins, and silica-alumina yields secondary mercaptans. The preferred route for secondary mercaptans is the H2S addition to olefinic compounds. 

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