Ammonia, reaction with conjugated acids

In his conjugate theory, which used terms and ideas corps copule borrowed from Gerhardt, Jons Jacob Berzelius (1779-1848) viewed metal-ammines as conjugated or copulated compounds consisting of ammonia and a conjugate or copula. The conjugate cannot be removed by reaction with an acid, and it neither decreases nor increases the saturation capacity of a base, that is, a metal conjugated with ammonia can still combine with other substances. 

The formation of the above anions ("enolate type) depend on equilibria between the carbon compounds, the base, and the solvent. To ensure a substantial concentration of the anionic synthons in solution the pA" of both the conjugated acid of the base and of the solvent must be higher than the pAT -value of the carbon compound. Alkali hydroxides in water (p/T, 16), alkoxides in the corresponding alcohols (pAT, 20), sodium amide in liquid ammonia (pATj 35), dimsyl sodium in dimethyl sulfoxide (pAT, = 35), sodium hydride, lithium amides, or lithium alkyls in ether or hydrocarbon solvents (pAT, > 40) are common combinations used in synthesis. Sometimes the bases (e.g. methoxides, amides, lithium alkyls) react as nucleophiles, in other words they do not abstract a proton, but their anion undergoes addition and substitution reactions with the carbon compound. If such is the case, sterically hindered bases are employed. A few examples are given below (H.O. House, 1972 I. Kuwajima, 1976). 

When an acid and a base react, the products are a new acid and base. For example, the acetate ion, C1T3COO-, in reaction 6.7 is a base that reacts with the acidic ammonium ion, N1T45", to produce acetic acid and ammonia. We call the acetate ion the conjugate base of acetic acid, and the ammonium ion is the conjugate acid of ammonia. 

The two-step procedure includes formation of a N-substituted phthalimide 3, and its subsequent cleavage to the primary amine 5. Phthalimide (which can be obtained from reaction of phthalic acid with ammonia) shows NH-acidity, since the negative charge of the phthalimide anion (the conjugated base) is stabilized... 

Problem 2.11 Nitric acid (HN03) reacts with ammonia (NH3) to yield ammonium nitrate. Write the reaction, and identify the acid, the base, the conjugate acid product, and the conjugate base product. 

The conjugation in 2,2, 4,4, 6,6 -hexanitroazobenzene (HNAB) (90) is also reflected in its thermal stability (m.p. 220 °C). The synthesis of HNAB from picryl chloride and 2,4-dinitrochlorobenzene is discussed in Sections 4.8.1.2 and 4.8.1.3 respectively. 3,3, 5,5 -Tetraamino-2,2, 4,4, 6,6 -hexanitroazobenzene (149) has been synthesized by an unusual but efficient route which involves the nitration-oxidative coupling of 3,5-dichloroaniline (147) on treatment with nitric acid, followed by reaction of the resulting product, 3,3, 5,5 -tetrachloro-2,2, 4,4, 6,6 -hexanitroazobenzene (148), with ammonia. Both the tetrachloro (148) and tetraamino (149) derivatives exhibit high thermal stability. 

Pyridines form stable salts with strong acids. Yellow ionic picrates were used for characterization in the past. Pyridine itself is often used to neutralize acid formed in a reaction and as a basic solvent. The basicity of pyridine (as measured by the dissociation constant of its conjugate acid, pKa 5.2) is less than that of aliphatic amines (cf. NH3, pA"a 9.5 NMe3, pKd 9.8). This reduced basicity is probably due to the changed hybridization of the nitrogen atom in ammonia the lone electron pair is in an sp3-orbital, but in pyridine it is in an s/r-orbital. The higher the s character of an orbital, the more it is concentrated near the nucleus, and the less available for bond formation. Nitriles, where the lone electron pair is in an. vp-orbital, are of even lower basicity. 

Amines are weak bases, and ammonium ions are weak acids. The parent of all amines is ammonia, NH3. When a base such as methylamine reacts with water, the product is the conjugate acid. That is, methylammonium ion produced in Reaction 6-27 is a weak acid ... 

For reactions with ammonia analogous products such as HN=PCl3 and HN =P(NH,)3 were proposed, but characterization was hampered by incomplete reactions, separation-resistant mixtures, and sensitivity to moisture. Furthermore, gradual polymerization occurred with loss of ammonia to yield phosph.im , a poorly characterized solid of approximate formula (PN2H)r us the ullimuie product. If instead of free ammonia its less reactive conjugate acid is used, reaction with PCI, proceeds at a moderate rate and the results are more definitive ... 

SAMPLE SOLUTION (a) Always start with an equation for an acid-base reaction. Ammonia is a Bronsted base and accepts a proton from the —OH group of acetic acid. Ammonia is converted to its conjugate acid, and acetic acid to its conjugate base. 

The reduction of conjugated olefins under oxo conditions is reminiscent of the reduction of the same systems by sodium and liquid ammonia and by metal-acid combinations. These reductions are thought to consist of electron transfers, involving one or two electron shifts, the electrons being supplied by the metal going into solution (Hammett, 20). With stilbene and sodium in liquid ammonia, the reaction may be represented (Schlenk and Bergmann, 49) ... 

This is more a solution process than a chemical reaction. In contrast, weak bases, like weak acids, react slightly with water to form ions. The reaction of ammonia with water, described earlier, is one example. Conjugate ions of molecular acids are also bases, as just described. Base strength determines the extent to which a base interacts with water to form ions. The stronger the base, the weaker its conjugate acid. 

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