Organic reactivity in liquid ammonia

Abstract

Liquid ammonia is a useful solvent for many organic reactions including aliphatic and aromatic nucleophilic substitution and metal-ion catalysed reactions. The acidity of acids is modified in liquid ammonia giving rise to differences from conventional solvents. The ionisation constants of phenols and carbon acids are the product of those for ion-pair formation and dissociation to the free ions. There is a linear relationship between the pK(a) of phenols and carbon acids in liquid ammonia and those in water of slope 1.68 and 0.7, respectively. Aminium ions exist in their unprotonated free base form in liquid ammonia. The rates of solvolysis and aminolysis by neutral amines of substituted benzyl chlorides in liquid ammonia show little or no dependence upon ring substituents, in stark contrast with the hydrolysis rates of substituted benzyl halides in water which vary 10(7) fold. However, the rates of the reaction of phenoxide ions and amine anions with 4-substituted benzyl chlorides gives a Hammett ρ = 1.1 and 0.93, respectively. The second order rate constants for the substitution of benzyl chlorides by neutral and anionic amines show a single Brønsted β(nuc) = 0.21 whereas those for substituted phenoxide ions generate a Brønsted β(nuc) = 0.40. The rates of aromatic nucleophilic substitution reactions in liquid ammonia are much faster than those in protic solvents indicating that liquid ammonia behaves like a typical dipolar aprotic solvent in its solvent effects on organic reactions. Nitrofluorobenzenes (NFB) readily undergo solvolysis in liquid ammonia but oxygen nucleophiles, such as alkoxide and phenoxide ions, displace the fluorine of 4-NFB in liquid ammonia to give the corresponding substitution product with little or no competing solvolysis product. The Brønsted β(nuc) for the reaction of 4-NFB with para-substituted phenoxides is 0.91, indicative that the decomposition of the Meisenheimer σ-intermediate is rate limiting. The aminolysis of 4-NFB occurs without general base catalysis by the amine and the second order rate constants generate a Brønsted β(nuc) of 0.36, which is also interpreted in terms of rate limiting breakdown of the Meisenheimer σ-intermediate.

Lignin solubilisation and gentle fractionation in liquid ammonia

“…We feel that this can be the result of the different chemical behaviour of organic compounds in liquid ammonia, and possibly due to flash precipitation when the liquid ammonia is released. [36][37][38]…”

A comprehensive N+ scale of nucleophilicity in an equation including a Swain‐Scott response/selectivity parameter

“…A limiting factor in correlations and predictions of nucleophilic reactivity is the availability of nucleophilicity parameters for specific combinations of nucleophiles and solvents. In addition to widely employed plots versus p K a , comprehensive data are available for N values (Eqns and ),, and the same sources can be used to obtain values of N + ″ from data for 2 , Z = NMe 2 (Eqn – see Table and Reference . N + ″ is recommended as a useful similarity model, and neither N + ″ nor N are ‘general scales of nucleophilicity’ …”

Reaction kinetics in liquid ammonia up to 120°C: techniques and some solvolysis and substitution reactions

“…[10][11][12] One example is shown in Scheme 3: [13] Our interest in ammonia as a reaction solvent has been driven primarily by the potential to use it as an alternative to dipolar aprotic solvents that are expensive and difficult to recover. [14] Work in our group [15] has shown that, at 25°C, nucleophilic substitution of fluorine by oxy-and nitrogen anions on 4-fluoronitrobenzene (4-FNB) is much faster than solvolysis. For example, at 25°C, the rate constant for the reaction of sodium phenoxide with 4-FNB in liquid ammonia is 5.28 × 10 À2 M À1 s À1 , compared with the solvolysis rate of 2.18 × 10 À7 M À1 s À1 .…”