Hydrogenation heat displacement

Reduction of Metallic Oxides.—Hydrogen can displace many metals from their oxides, the reduction taking place at the ordinary temperature, as with silver and palladium oxides, or on heating, as with the oxides of copper, cadmium, lead, antimony, nickel, cobalt, and iron. Sometimes these reductions are incomplete, an equilibrium being attained. Such equilibria depend on the experimental conditions, an example being the action of steam on heated iron (p. 15). 

Naphthalene is classified as aromatic because its properties resemble those of benzene (see Sec. lO.lO). Its molecular formula, C,oHs, might lead one to expect a high degree of unsaturation yet naphthalene is resistant (although less so than benzene) to the addition reactions characteristic of unsaturated compounds. Instead, the typical reactions of naphthalene are electrophilic substitution reactions, in which hydrogen is displaced as hydrogen ion and the naphthalene ring system is preserved. Like benzene, naphthalene is unusually stable its heat of combustion is 61 kcal lower than that calculated on the assumption that it is aliphatic (see Problem 10.2, p. 323). 

Similarly, sodium reacts with such aromatic compounds as fiuorene, indene, and triphenylmethane to give the corresponding sodium salts. These are just about the least acidic compounds from which hydrogen is displaced by metallic sodium, and rather drastic conditions are required in these cases. It is necessary, for instance, to heat fiuorene to from 190° to 200 °C. to achieve the direct displacement of hydrogen (29) ... 

Conversely, at low temperatures it is the molecular state, with a heat of adsorption of approximately 21 kJ mol that predominates. In contrast, hydrogen is only present in a dissociated state with a heat of adsorption of 75 to 96 kJ mo It can therefore be seen that a low temperatures (<400 K) hydrogen will displace the molecular nitrogen, while at higher temperatures, when nitrogen dissociation occurs, the nitrogen will inhibit hydrogen adsorption. ... 

Anhydrous hydrogen fluoride (as distinct from an aqueous solution of hydrofluoric acid) does not attack silica or glass. It reacts with metals to give fluorides, for example with heated iron the anhydrous iron(II) fluoride is formed the same product is obtained by displacement of chlorine from iron(II) chloride ... 

Tin tetrachloride has been used to prepare the stericaHy hindered triisopropylchlorosilane [13154-24-0] (119). Organobromosdanes are obtained under similar conditions through reaction with cupric and mercuric bromide. These reactions are most suitable for stepwise displacement of hydrogen to form mixed hydridochlorosilanes or in systems sensitive to halogen (120). Hydrides have also been displaced using organic bromides. Heating triethylsilane and... 

The waste gas remaining after removal of ammonia and recovery of hydrogen cyanide contains enough hydrogen and carbon monoxide that it is flammable and has enough heat value to make it a valuable fuel. It is usually used to displace other fuel ia boilers. 

The initial series of major tranquilizers consists of alkylated derivatives of 4-aryl-4-hydroxypiperidines. Construction of this ring system is accomplished by a set of rather unusual reactions. Condensation of methylstyrenes with formaldehyde and ammonium chloride afford the corresponding hexahydro-1,3-oxazines (119). Heating these oxazines in the presence of acid leads to rearrangement with loss of water to the tetrahydropyridines. Scheme 1 shows a possible reaction pathway for these transformations. Addition of hydrogen bromide affords the expected 4-bromo compound (121). This last is easily displaced by water to lead to the desired alcohol (122) The side chain (123) is obtained by Friedel-Crafts acylation of p-fluorobenzene with 4-chloro-butyryl chloride. Alkylation of the appropriate arylpiperidinol with 123 affords the desired butyrophenone derivative. Thus,... 

Substances which react with water to liberate flammable gas, e.g. carbides (liberate acetylene), alkali metals (hydrogen), organometallics (hydrocarbons - see Table 6.8), and where the heat of reaction is sufficient to ignite the gas. Thus metals which are less electronegative than hydrogen (see Table 6.10) will displace this element from water or alcohols, albeit at different rates. 

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