Hydrogen bonding in liquid ammonia

Ammonia liquefies at atmospheric pressure only when cooled to -33° but at moderate pressures it can be liquefied at ordinary temperatures. There is considerable intermolecular hydrogen bonding in liquid ammonia, which is a colourless mobile liquid but is troublesome to handle because the fumes are noxious. This is why great care is required in its handling. 

In many of its properties, liquid ammonia resembles water. Both are polar and involve extensive hydrogen bonding in the liquid state. It is interesting to note that hydrogen bonding in liquid... 

Table 16.2 shows a compound profile for ammonia, NH3. Ihis is the familiar pyramidal molecular hydride. It is capable of forming strong hydrogen bonds with itself and other molecules, most notably water. Much heat is needed to break the hydrogen bonds of liquid ammonia, and this results in a high heat of vaporization. 

The Birch reductions of C C double bonds with alkali metals in liquid ammonia or amines obey other rules than do the catalytic hydrogenations (D. Caine, 1976). In these reactions regio- and stereoselectivities are mainly determined by the stabilities of the intermediate carbanions. If one reduces, for example, the a, -unsaturated decalone below with lithium, a dianion is formed, whereof three different conformations (A), (B), and (C) are conceivable. Conformation (A) is the most stable, because repulsion disfavors the cis-decalin system (B) and in (C) the conjugation of the dianion is interrupted. Thus, protonation yields the trans-decalone system (G. Stork, 1964B). 

Single-bond cleavage with molecular hydrogen is termed hydrogenolysis. Palladium is the best catalyst for this purpose, platinum is not useful. Desulfurizations are most efficiently per-formed with Raney nickel (with or without hydrogen G.R. Pettit, 1962 A or with alkali metals in liquid ammonia or amines. The scheme below summarizes some classes of compounds most susceptible to hydrogenolysis. 

Triple bonds can also be selectively reduced to double bonds with DIBAL-H, " with activated zinc (see 12-36), with hydrogen and Bi2B-borohydride exchange resin, ° or (internal triple bonds only) with alkali metals (Na, Li) in liquid ammonia or a low-molecular-weight amine.Terminal alkynes are not reduced by the Na—NH3 procedure because they are converted to acetylide ions under these conditions. However, terminal triple bonds can be reduced to double bonds by the... 

Reduction of (24) by Lindlaar hydrogenation gives a cis double bond and sodium in liquid ammonia gives the trans double bond,... 

Acetylides are formed by treating terminal acetylenes with a strong base, sodium amide in liquid ammonia being the one most commonly employed. Acetylenes with a hydrogen atom attached to the triple bond are weakly acidic (pATa about 25) due to the stability of the acetylide anion (see Section 4.3.4),... 

In systems of conjugated double bonds catalytic hydrogenation usually gives a mixture of all possible products. Conjugated dienes and polyenes can be reduced by metals sodium, potassium, or lithium. The reduction is accomplished by 1,4-addition which results in the formation of a product with only one double bond and products of coupling and polymerization. Isoprene was reduced in 60% yield to 2-methyl-2-butene by sodium in liquid ammonia [357]. Reduction of cyclooctatetraene with sodium in liquid ammonia gave a... 

Double bonds conjugated with benzene rings are reduced electrolytically [344] (p. 23). Where applicable, stereochemistry can be influenced by using either catalytic hydrogenation or dissolving metal reduction [401] (p. 24). Indene was converted to indane by sodium in liquid ammonia in 85% yield [402] and acenaphthylene to acenaphthene in 85% yield by reduction with lithium aluminum hydride in carbitol at 100° [403], Since the benzene ring is not inert toward alkali metals, nuclear reduction may accompany reduction of the double bond. Styrene treated with lithium in methylamine afforded 25% of 1-ethylcyclohexene and 18% of ethylcyclohexane [404]. 

Results of the reduction of unsaturated alcohols depend on the respective positions of the hydroxyl and the double bond. Since the hydroxyl group is fairly resistant to hydrogenolysis by catalytic hydrogenation almost any catalyst working under mild conditions may be used for saturation of the double bond with conservation of the hydroxyl [608]. In addition, sodium in liquid ammonia and lithium in ethylamine reduced double bonds without affecting the hydroxyl in non-allylic alcohols [608]. 

Unsaturated amines are hydrogenated at the multiple bonds by catalytic hydrogenation over any catalyst. The double bond in indole was saturated in catalytic hydrogenation over platinum dioxide in ethanol containing fluoro-boric acid and indoline was obtained in greater than 85% yield [456. AUylic amines such as allylpiperidine are also reduced by sodium in liquid ammonia in the presence of methanol (yield 75%) [709. ... 

The hydrogen bonds in crystalline and liquid ammonia are weaker than those in ice and water for two reasons the small ionic character of the N—H bond gives it only small hydrogen-bond-forming power, and the one unshared electron pair of the NH8 molecule must serve for... 

Polyacrylamides are chemically stable towards acids (TFA, hydrogen fluoride [163]), bases, and weak oxidants or reducing agents. Problems, which might be related to cleavage of the amide bonds in poly(7V,/V-dialkylacrylamidcs), were encountered upon treatment of such supports with sodium in liquid ammonia [169]. 

It is interesting to consider the electron cavity size in other polar solvents relative to that in ammonia. For strongly hydrogen bonded solvents, such as water, the surface tension energy associated with cavity formation is larger than in liquid ammonia. It is thus reasonable to assume that the size of the electron cavity (if a cavity exists at all) will be substantially smaller in water than in liquid ammonia (19,35). 

Although these catalytic partial hydrogenations of alkynes may well be regarded as the procedure of choice for (Z)-alkenes,25 other catalytic systems have been explored. These include a sodium hydride-sodium alkoxide-nickel(n) acetate reagent,26 and a sodium borohydride-palladium chloride-polyethylene glycol system.27 Diisobutylaluminium hydride (DIBAL) has also been used for the conversion of alkynes into (Z)-alkenes.28 ( )-Alkenes are formed when the internal triple bond is reduced with sodium in liquid ammonia.29... 

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