Weak bases derived from ammonia

Pentafluoroaniline. Pentafluoroaniline [771 -60-8] i2is been prepared from amination of hexafluoroben2ene with sodium amide inbquid ammonia or with ammonium hydroxide in ethanol (or water) at 167—180°C for 12—18 h. It is weakly basic (p = 0.28) and dissolves only in concentrated acids. Liquid crystals have been prepared from Schiff bases derived from pentafluoroaniline (230). 

Besides water, the most common weak base is ammonia, NH3, whose proton transfer equilibrium with water appears in Section 16-. Many other weak bases are derivatives of ammonia called amines, hi these organic compounds, one, two, or three of the N—H bonds in ammonia have been replaced with N—C bonds. The nitrogen atom in an amine, like its counterpart in ammonia, has a lone pair of electrons that can form a bond to a proton. Water does not protonate an amine to an appreciable extent, so all amines are weak bases. Table 17-4 lists several examples of bases derived from ammonia. 

Just as ammonia, NH3, is a weak base, there are a large number of nitro-gen-containii organic compounds called amines that are also weak bases. In the early days of organic chemistry, basic amines derived from natural sources were known as vegetable alkali, but they are now referred to as alkaloids. The study of alkaloids provided much of the impetus for the growth of organic chemistry in the nineteenth century, and it remains today a fascinating area of research. 

In addition to the usual C, H, and O, amines contain nitrogen (N). Amines are derived from ammonia (NHj) by substituting an alkyl group for one or more of the hydrogen atoms on ammonia. To name an amine, the alkyl groups are named, followed by the word amine. Examples include methyl amine, dimethyl amine, methyl ethyl amine, and trimethyl amine. Like ammonia, amines are weak bases. 

Salts are compounds formed by the union of acidic and basic radicles. When dissolved in water thej are ionised into their constituent positive and negative ions. If the positive ion is derived from a weak base, such as ammonia, it tends to lose a proton, and so increase the H-ion concentration of the solution. 

In contrast to the acid/base behaviour of polymeric bulk water, monomeric water is a relatively weak acid and base in the gas phase compared to its substituted derivatives (R—OH, R—O—R, etc.), whose conjugated base or acid ions are stabilized by polarization of the alkyl groups. The gas-phase basicity of water is 138 kJ/mol (33 kcal/mol) below that of ammonia. Its gas-phase acidity is comparable to that of propene and it is less acidic than phenol by about 167 kJ/mol (40 kcal/mol). With respect to the well-known acid/base properties of water, ammonia, and phenol in aqueous solution, one has to conclude that enormous solvation energies must contribute to the difference from the behaviour of isolated water molecules. See Section 4.2.2 for further discussions and references. 

Ruthenium has been investigated by many laboratories as a possible catalyst for ammonia synthesis. Recently, Becue et al. [T. Becue, R. J. Davis, and J. M. Garces, J. Catal., 179 (1998) 129] reported that the forward rate (far from equilibrium) of ammonia synthesis at 20 bar total pressure and 623 K over base-promoted ruthenium metal is first order in dinitrogen and inverse first order in dihydrogen. The rate is very weakly inhibited by ammonia. Propose a plausible sequence of steps for the catalytic reaction and derive a rate equation consistent with experimental observation. 

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