Azide equilibrium measurements

There have been more equilibrium measurements for reactions of carbocations with azide than halide ions. Regrettably, there is little thermodynamic data on which to base estimates of relative values of pARz and pAR using counterparts of Equations (17) and (18) with N3 replacing Cl. Nevertheless, a number of comparisons in water or TFE-H20 mixtures have been made87,106,226,230 and Ritchie and Virtanen have reported measurements in methanol.195 The measurements recorded below are for TFE-H20 and show that whereas pA" 1 is typically 4 log units more positive than pA R. pA Rz is eight units more negative. The difference should be less in water, perhaps by 2 log units, but it is clear that azide ion has about a 1010-fold greater equilibrium affinity for carbocations than does chloride (or bromide) ion. 

Fig. 4.12 Stability ranges of copper azides (from equilibrium measurements at 25 °C, the existence of CuNs is possible in absence of oxidizing agents) [107]...

Tliis conclusion is supported by the data on the azide-tetrazole equilibrium for s-triazolo[2,3-d]tetrazoles (128) [79JCS(P1)2886]. Methylation of neutral azide resulted in the 1-methyl derivatives of 3-azidotriazole 128A only, whereas on methylation of the anion 128A, the tetrazoles 129 and 130 were also trapped in 25 and 10% yields, respeetively. Tire predominanee of 128T over 128T" was attributed to these two bieyelie anions but no ealeu-lations on relative energies have been performed. Tire azide-tetrazole equilibrium eonstants were measured for 128A7T in DMSO-dg 0.45 at 27°C (R = H), and 0.78 at 23°C and 1.80 at 80°C (R = Ph). 

With a view to determining the equilibrium constant for the isomerisation, the rates of reduction of an equilibrium mixture of cis- and rra/i5-Co(NH3)4(OH2)N3 with Fe have been measured by Haim S . At Fe concentrations above 1.5 X 10 M the reaction with Fe is too rapid for equilibrium to be established between cis and trans isomers, and two rates are observed. For Fe concentrations below 1 X lO M, however, equilibrium between cis and trans forms is maintained and only one rate is observed. Detailed analysis of the rate data yields the individual rate coefficients for the reduction of the trans and cis isomers by Fe (24 l.mole sec and 0.355 l.mole .sec ) as well as the rate coefficient and equilibrium constant for the cw to trans isomerisation (1.42 x 10 sec and 0.22, respectively). All these results apply at perchlorate concentrations of 0.50 M and at 25 °C. Rate coefficients for the reduction of various azidoammine-cobalt(lll) complexes are collected in Table 12. Haim discusses the implications of these results on the basis that all these systems make use of azide bridges. The effect of substitution in Co(III) by a non-bridging ligand is remarkable in terms of reactivity towards Fe . The order of reactivity, trans-Co(NH3)4(OH2)N3 + > rra/is-Co(NH3)4(N3)2" > Co(NH3)sN3 +, is at va-... 

For phenanthrene hydrate the derived value of p fR is —11.6. This is comparable to values for the benzhydryl (-12.5) or / -methylphenethyl (-12.8) cations.22,69,73 The evaluation of p fR as well as pKa allows derivation of p h2o = pA"R — pKa = 9.3. This equilibrium constant offers a measure of the stability of the 9,10-double bond of phenanthrene and thus the aromaticity of its central benzene ring. Comparison with the double bond of 2-butene, for which p h2o = —3.94,86 indicates a 1013-fold greater stability, for the aromatic double bond. It should be noted that the value of p h2o does not depend on azide trapping. In the difference between p fR and pKA the rate constant kAz cancels out and K o = kAkn2o/knkp. 

This concludes the discussion of the stabilities of carbocations with hydrocarbon-based structures and also of different methods for deriving equilibrium constants to express these stabilities. The remainder of the chapter will be concerned mainly with measurements of stabilities for oxygen-substituted and metal ion-coordinated carbocations. Consideration of carbocations as conjugate acids of carbenes and derivations of stabilities based on equilibria for the ionization of alkyl halides and azides will conclude the major part of the chapter and introduce a discussion of recent studies of reactivities. 

Aliphatic azides undergo a little known allylic-type rearrangement (reaction 81). This has been studied in a variety of solvents for R = metliyl and hydrogen and the percentages of the isomers at equilibrium have been measured In contrast to the analogous iso-merizations of allylic chlorides the rates of rearrangement are insensitive both to alkyl substitution and to solvent and no accompanying... 

Kramer [48] calculated the equilibrium constant to be 3.5 X 10" and the pressure of hydrazoic acid is 1.9 X 10- mm at a carbon dioxide pressure of 1 mm. Aqueous hydrazoic acid in equilibrium with the gas phase at 25°C would be 2.3 X 10" M with respect to hydrazoic acid. The equilibrium constants were measured [48] for militaiy-grade azide in 50 50 alcohol-water solutions and are summarized in Table I. 

Photoelectron spectroscopy was applied to study the azide-tetrazole equilibrium of (1). The observed bands were correlated with calculated (MNDO) ionization potentials <81MI 815-01, 82MI 815-01,90ZN(B)59>. Measurements carried out at different temperatures revealed that the equilibrium is clearly shifted to the azide with higher temperatures. 

In agreement with the rate constants measured for amino-acids in solution, azide radicals attack primarily mostly tryptophan whereas halides like dibromide may also react with methionine. Thiocyanate radicals are also specific of tryptophan residues. However this process is a reversible equilibrium between lysozyme (119). 

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