Protonic Species in Liquid Ammonia

When we use any substance as a solvent for a protonic acid, the acidic and basic species produced by dissociation of the solvent molecules determine the limits of acidity or basicity in that solvent. Thus, in water, we cannot have any substance or species more basic than OH or more acidic than H30 in liquid ammonia, the limiting basic entity is NHf, the acidic is NH4. Many common inorganic acids, for example HCl, HNO3, H2SO4 are all equally strong in water because their strengths are levelled to that of the solvent species Only by putting them into a more acidic... 

The label am " indicates a species dissolved in liquid ammonia.) An example of proton transfer in the gas phase is the reaction of hydrogen chloride and ammonia gases. They produce the fine powder of ammonium chloride often seen coating surfaces in chemical laboratories (Fig. 10.5) ... 

According to the Arrhenius theory of acids and bases, the acidic species in water is the solvated proton (which we write as H30+). This shows that the acidic species is the cation characteristic of the solvent. In water, the basic species is the anion characteristic of the solvent, OH-. By extending the Arrhenius definitions of acid and base to liquid ammonia, it becomes apparent from Eq. (10.3) that the acidic species is NH4+ and the basic species is Nl I,. It is apparent that any substance that leads to an increase in the concentration of NH4+ is an acid in liquid ammonia. A substance that leads to an increase in concentration of NH2- is a base in liquid ammonia. For other solvents, autoionization (if it occurs) leads to different ions, but in each case presumed ionization leads to a cation and an anion. Generalization of the nature of the acidic and basic species leads to the idea that in a solvent, the cation characteristic of the solvent is the acidic species and the anion characteristic of the solvent is the basic species. This is known as the solvent concept. Neutralization can be considered as the reaction of the cation and anion from the solvent. For example, the cation and anion react to produce unionized solvent ... 

The alkali metals in liquid ammonia give deep coloured solutions which have been shown to contain solvated electrons. The unsaturated system takes up an electron to give an anion radical. There is evidence for this species from electron spin resonance studies. It accepts a proton from the solvent to give a radical which is reduced to a carbanion by another sodium atom. Finally the addition of a proton gives the reduced product. This proton is supplied by a protic solvent like enthanol and not from NH3. 

The two free hydroxy groups are First protected with acetic anhydride. In a second step the acetyl group is reductively cleaved by a Birch reduction with lithium in liquid ammonia.19 Lithium dissolves in the ammonia with the formation of solvated electrons. Stepwise electron transfer to the aromatic species (a SET process) leads first to a radical anion, which stabilizes itself as benzylic radical 38 with loss of the oxygen substituent. A second SET process generates a benzylic anion, which is neutralized with ammonium chloride acting as a proton source (see Chapter 12). 

A saturated ketone is obtained when an a, unsaturated ketone is treated with lithium in liquid ammonia, followed by water or a similar protonating species. Most steroidal A -3-ketones and similar compounds give products with the trnns-fused all-chair structure of minimum energy, which led Barton [32] to the generalisation that the more stable product is produced by protonation of an intermediate carbanion (14) in its thermodynamically preferred conformation (15) (see p. 54). Further studies, including the reduction of substituted... 

It is to be noted that, after geminate recombination, when diffusion takes place on the nanosecond and longer time scale, reactions between the radiolytic species still occur and, in the absence of any other solutes, those reactions are responsible for the disappearance of the solvated electron. Besides, the metastability of the "blue" solutions of alkali metals in liquid ammonia is due to the fact that the solvated electron does not react with another solvated electron and that it reacts extremely slowly with the protonated form (NH/) which is at an extremely low concentration. 

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