Azulenes protonation

Challis and Long497 have used the fast flow technique described above (p. 217) to measure the equilibrium protonation of azulene in a range of aqueous perchloric acid media at 7.5 °C and hence the rates of the forward protonation and reverse deprotonation, the overall exchange rate being the sum of these. Some representat i ve values are given in Table 141. Coupled with data obtained at other temperatures... 

RATE COEFFICIENTS FOR PROTONATION (ki FORWARD) AND DEPROTONATION (Aij REVERSE) OF AZULENE IN AQUEOUS HCIO4. AT 7.5 °C197... 

The methylene groups present after proton addition can also be clearly recognized in the N.M.R. spectra of azulene and some of its derivatives (Schulze and Long, 1964). 

The interpretation of these effects as the formation of a proton addition complex was further supported by Plattner et al. (1952) by means of spectroscopic and conductimetric investigations. In these interactions the change of the absorption spectrum is characteristic, since the blue colour of the azulene in organic solvents is changed to a yellow colour in... 

The question of how many protons are added to azulene in acid... 

In calculations on proton addition complexes of azulene and methyl-azulenes by means of the HMO method, Heilbronner and Simonetta (1952) took the effect of methyl groups into aecount by adjustment of the a and j8 values, as had already been done by PuUmann and collaborators (1950) in calculating the spectra of the methylazulenes. These calculations correctly reproduce the effeet of a methyl group on the basicity of azulene. In contrast to the ease of naphthalene, the position of addition of the proton remains independent of the position of the methyl group ... 

It may be mentioned here that 1-formyl- and 1-acetyl-azulene [241] also offer alternative C/O protonation sites, with two carbon alternatives, C-1 and C-3. Nmr spectra show that, in 1-formylazulene,... 

Protonation represents the simplest electrophilic reaction. Contrary to azulene (pKa = — 1.7)237 most pseudoazulene systems are relatively strong bases. The pK.d values are listed in Table V. Numerous pseudoazulenes can be obtained from their quaternary salts by the action of sodium acetate (see... 

This correlation is shown for a range of carbocation structures in Fig. 1, with the arenonium ions shown as filled circles. The correlation includes protonated mesitylene, studied by Marziano,107 and earlier measurements of the protonation of methoxy-substituted benzenes108 and azulenes.109 It also... 

The change in wave-heights shown in Fig. 2 can be considered as proof that an acid-base equilibrium accompanies the electrode process. This change can sometimes indicate the presence of an acid-base equilibrium when only inadequate experimental evidence can be obtained by other techniques3 (e.g. for protonation of a, (3-unsaturated ketones (51) or of azulenes (54). 

The other main source of evidence on the mechanisms of nitrosation comes from the C-nitrosation of aromatic compounds but, for this reaction, the final proton loss from the rate-determining (Challis and Higgins, 1972). Under these conditions, the overall rate and kinetic form of the reaction provide no evidence on the rate and mechanism of the nitrosation stage. Fortunately the work of Challis and his co-workers (Challis and Higgins, 1973 Challis and Higgins, 1975 Challis and Lawson, 1973) has shown that the C-nitrosation stage is rate-determining for the nitrosation of some reactive aromatic substrates (2-naphthol, azulene, 1,2-dimethylindole) in feebly acidic media and these are the reactions referred to below. 

Mason and Smith (1969) found that for a series of mono- and bicyclic aromatic hydrocarbons the changes in the fluorescence spectrum with acidity reflected the ground state protonation reaction. The p Sj )-values calculated for benzene, toluene, naphthalene, azulene, and indolizine do not correspond to observable processes since the rate of protonation is too slow to compete with deactivation of the Sj state. Photochemical deuterium and tritium exchange experiments in 1 mole dm-3 perchloric acid indicate that the radiative deactivation rate of an electronically excited aromatic hydrocarbon is faster than the rate of protonation by a factor >10s. 

On the other hand, the acid pK of the carboxylic group of the substrate is 5.5, and the pK s for ring protonation of the azulene system and for O protonation of the COOH group are known to be negative (application of the H0 scale, see Vol. 2, p. 358). Consequently, the coefficient with a value of 3.8 x 10-2 mole. I"1 in the denominator of eqn. (13) cannot be an ionization constant of the substrate, and it must have another meaning. An explanation of the experimental findings is offered on the basis of the mechanism... 

Equation (16b) is in complete agreement with the experimental equation (13). It may be concluded that a change of the rate-determining step occurs in this system, from slow proton transfer (H30+ + S XH+ + H20 k = ki) at low [H+] to rate-limiting carbon-carbon bond cleavage (X azulene + C02 k = kl/k l)KXHku) at high [H+]. 

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