Potassium amide, dissociation

The solvated electrons in the flash transient may be present either as free or paired ions. The extent of pairing may be estimated in a given experiment from the extinction coefficient of the solvated electron (taken to be 1.0 X 104 at 980 m/z) (10), from the potassium amide dissociation constant and from the ion pair dissociation constant for the process [M + e ] - M+ + e. Taking this to be about 3 X 10-3 M (1,12,14), we find for a typical flashed solution in which ADm 0.02 and Dm = 0.9, that the ratio (unpaired/paired) solvated electrons is about 30. This result is difficult to confirm, at least by spectroscopic methods, owing to the obedience to Beer s Law in the pairing region (13). 

In our liquid ammonia experiments the amide concentration (1.5 — 8 X 10 6 M) is in a range in which appreciable ionic dissociation occurs. Negative ions in aqueous solution are well known to yield solvated electrons upon excitation in their charge-transfer bands (7, 15, 19, 22), and one may expect the same process to operate also for NH2 - in ammonia. The large blue shift of the amide peak with decreasing temperature (3) supports the view that the absorption is a charge-transfer band (27). We therefore write for potassium amide in liquid ammonia (e signifies the solvated electron) ... 

It was further shown that 1 atom of nitrogen was present in each polymer chain and that added potassium ion retards the rate of formation of polymer. This indicated that initiation is by dissociated amide ion rather than potassium amide... 

Wilmarth and Dayton (18) established that this reaction is homogeneous and that the rate of exchange is first order with respect to the concentration of dissolved hydrogen. In order to establish the catalytic action of potassium amide, they took into consideration its dissociation according to... 

A few years later, Bar-Eli and Klein (i), studying the same reaction, expanded the range of concentrations of potassium amide to 200 X lO M. Using the same dissociation constant as Wilmarth and Dayton and assuming also that the activity coefficients are equal to unity, they concluded that two catalytic species are present the free amide ion NH2 and the ion pair K NH2. ... 

Styrene and other monomers can be polymerized by potassium amide in liquid ammonia. The dielectric constant of the solvent is quite high 22) and this is one of the ionic systems in which the active centers behave kinetically as free ions. Initiation involves the dissociation of potassium amide followed by addition of amide ion to the first monomer unit ... 

The polymerization rate for an anionic polymerization where termination occurs simultaneously with propagation follows in a manner similar to free-radical polymerizations. An example is the potassium amide initiated polymerization in liquid ammonia. This is one of the few anionic systems in which all active centers behave kinetically as free ions. In this system, initiation involves dissociation followed by addition of amide ion to monomer ... 

The yellow solid K3N3S2 may be a salt of the ammoniatc (XXIX), or, as was at first assumed, an equimolecular mixture of the salts (XXX) and (XXXI). Since no difference was observed in the reactivity of the H atoms of the ammoniatc during the reaction with potassium amide, the theory favored initially was that there was first a dissociation of H3N3S2 to HNS... 

Initiation involves dissociation of potassium amide followed by addition of the amide ion to styrene. Termination occurs by proton abstraction from... 

There can be little question that excitation of metal amide is the initial step in both solvent systems. However, it is not clear at the outset whether ion pairs or free ions are involved. No direct measurements of the dissociation constant of potassium ethylamide in ethylamine are available. In ammonia, the value K = 7.3 X 10 5 M has been obtained (17) from conductivity data at —33.5° C. The temperature coefficient of K appears to be small (18), and this value will be assumed also at —78 ° C. In ethylamine, the dissociation constant must be considerably smaller than this, and in the concentration range of our experiments in ethylamine (0.5 to 1.5 X 10 -3 M) most of the salt will be present as ion pairs. We therefore write for the ethylamine reaction ... 

Phosphonates. In a Homer-Emmons-based synthesis of di- and trisubstituted (Z)-Q(, -unsaturated esters, the strongly dissociated base system of potassium bis(trimethylsilyl)amide/18-Crown-6 was used to prepare the desired phosphonate anions. This base system, coupled with highly electrophilic bis(trifluoro-ethyl)phosphono esters, gave phosphonate anions which, when allowed to react with aldehydes, gave excellent selectivity for the (Z)-Q , -unsaturated esters (eq 15). ... 

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