Lithium ammonia exchange

Fig. 11. Energy profile for ammonia exchange around the lithium cation (relative energies B3LYP/6-311+G ) (90).

Lithium citrate buffers (Li-A, Li-B, and Li-C) and a lithium cation exchange column (20 X 4.6 cm) were purchased from Beckman. The amino acid standard was prepared from the Beckman standard with the addition of Hyl/flZZo-Hyl (from Sigma). Optimization of the method with respect to temperature and buffer change times was required to adequately resolve Hyl from ammonia. Hyl is partially converted to allo-Hyl during hydrolysis. The peak areas for Hyl and allo-Hyl were combined for standards and samples. 

Kinetic studies of base-catalysed hydrogen exchange have been carried out by Roberts, by Shatenshtein, and by Streitweiser and their coworkers. In earlier work, potassium amide was used as base in liquid ammonia as solvent, whereas later workers used lithium and caesium cyclohexylamides in cyclohexylamine. The reaction can be represented by equilibria (239) and (240)... 

Yakushin and Shatenshtein (1960) and Yakushin et al. (1959) have determined the ratio of the rate constants for tritium and deuterium replacement by protium in ammonia solutions of fluorene and methyl 2-naphthyl ketone at 25°. Yakushin has obtained data for these reactions in anhydrous methylamine. Streitwieser et al. (1960, 1962b) have measured the ratio fcD/ T for the hydrogen exchange of ethylbenzene and toluene catalysed by a solution of lithium cyclohexylamide in cyclohexylamine at 49-9°. 

A systematic study of the reductive alkylation of acetophenones revealed that the desired transformation (Scheme 30) required a careful selection of reagents and conditions. The best results were obtained from reduction by potassium in ammonia at -78 °C, with t-butyl alcohol as the proton source. Exchange of the potassium counterion of the enolate (152 M = K) for lithium then ensured regioselective alkylation at C-1 to give (153) in 80-90% yields (Scheme 30). Metals other than potassium as the reductant led to undesirable side reactions with the carbonyl group, which included simple reduction to the methylcar-binol and ethylbenzene (lithium or sodium), while the absence of a proton source or presence of a strong... 

The solubility of potassium superoxide KOz in liquid ammonia has been measured from —75 to -40 °C. Equilibrium in the system was slow, taking 3.5—4 h. At —40 °C, 0.024 + 0.003 g of 92% pure KOz dissolves in 100 cm3 of ammonia.155 On heating mixtures of K02 with lithium perchlorate, cation exchange occurs between the components, accompanied by liberation of oxygen. At 250—300 °C the mixture melts with evolution of oxygen. At 360—500 °C, perchlorate is also decomposed. These melts are recommended for oxygen preparation.156... 

The industrial development of silica sol manufacturing methods is reviewed. Primary attention is focused on the preparation of monodispersed sols from water glass by the ion-exchange method. Details are given for variations of manufacturing process and for the characteristics of both the processes and sols obtained. Furthermore, the following surface modifications of particles are demonstrated silica sols stabilized with ammonia, amine, and quaternary ammonium hydroxide aluminum-modified or cation-coated silica sol and lithium silicate. Finally, future trends in silica sol manufacturing are discussed from the viewpoint of not only raw materials and improvement of the procedures but also the function of the silica sols and their particle shape. 

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