Pseudo-Base Equilibria

Acidity, Basicity, Pseudo-Base Equilibria, and Resonance... 

Scheme 14. Aromaticity eind Pseudo-Base Equilibria ...

In this scheme the reversible conversion of A to O is the reaction whose rate is to be studied, whereas the reduction of O to R is the electrode process. Scheme XIV can also represent a pseudo-first-order formation of O. A specific example is the acid-base equilibrium of pyruvic acid, shown in Scheme XV. 

The study of structure and reactivity of tertiary heterocyclic enamines is associated with the problem of equilibrium of the cyclic enamine form (70) and the tautomeric hydration products 173,174) quaternary hydroxide (71), pseudo base (so-called carbinolamine) (72) and an opened form of amino aldehyde or amino ketone (73). 

Dihydroisoquinolines, e.g. (484), are basic and form quaternary salts, e.g. (521). With alkali these salts form carbinolamine pseudo-bases, e.g. cotamine (522 Y = OH), which can be oxidized to lactams or which disproportionate on standing. The quaternary ions can also react with other nucleophilic reagents, e.g. (521) + RMgBr — (522 Y = R) (521) + MeCOMe — (522 Y = CH2COMe) (521) + CN — (522 Y = CN) (521) + RNH2 —+ (522 Y = NHR). The pseudo-bases are in equilibrium with open-chain compounds since aldehyde derivatives can be prepared. 

Oxidative-reductive disproportionation is a rather typical property of some pseudo bases. Thus, l,3-dimethyl-2-hydroxybenzimidazoline (229), which exists in the solid state in the open-chain form (228), on heating at 165-185°C, is converted to mixture of 1,3-dimethylbenzimidazolone (230) (49%) and 1,3-dimethylbenzimidazoline (231) (46%) (85KGS1694). Evidently, the process proceeds via an equilibrium amount of (229) undergoing hydride transfer. 

Under weakly acid conditions the red oxonium form 9 is in reversible equilibrium with a colorless pseudo-base 10 the position of the equilibrium depends on the pH. In a test trial in a synthetic medium a solution of anthocyanins is six times more colored at pH 2.9 than at pH 3.9. 

With A+ as the oxonium form and AOH as the pseudo-base, the equilibrium is written ... 

The two-electron reduction product, fra s-[Rh(en)2(OH)(H)]+ displays a sharp H NMR signal at 30.6 t. The pseudo acid-base equilibrium, represented in equation (164), was proposed to explain the observation that this peak broadens as the pH is raised, disappearing completely by pH 13. 

Berberine as a member of the class of compounds classified by Hantzsch 73) as pseudo bases has commanded its share of discussion on this still controversial topic. The generally accepted view of Gadamer (74) that berberine exists in solution as an equilibrium mixture of three tautomeric forms (la-c) has been reexamined (75). 

In view of the above considerations, therefore, it would appear that the formation of pseudo-tropine by the action of sodium amyloxide on tropine might be a process of racemisation, and this seemed to be in harmony with the experimental facts, since the change in question is never complete. Thus, in our experiments on the preparation of pseudo-tropine, about 35 per cent, of the basic product resulting from the treatment with sodium amyloxide was found to consist of an uncrystallisable mixture of tropine and pseudo-tropine, and Willstatter mentions that the yield of pure pseudo-base obtained by him did not exceed 50 to 55 per cent, of that theoretically possible. It appeared, therefore, that, the action of sodium amyloxide on tropine resulted in the formation of an equilibrium mixture of this base and pseudo-tropine, just as the action of alkali on pilocarpine or isopilocarpine results in the production of an equilibrium mixture of these two stereoisomeric bases (Jowett, Trans., 1905, 87, 794). This, however, is not the case, since the change is irreversible, no tropine being formed by the action of sodium amyloxide on pseudo-tropine. 

The di-N-alkyl derivatives of 1,5- and 1,8-naphthyridine have significantly different properties. Diquaternary salt 99 is relatively stable in neutral solution, whereas 100 exists in equilibrium with its pseudo base (101). A similar pseudo base structure (103) has been identified in the 1,8-bridged compound (102). The 1,5-naphthyridine salt (99) is readily reduced by a one-electron transfer process to give a fairly stable green radical cation (104). 

In yet a further application of equilibrium data, the AH° for pseudo-base formation of isoquinolinium methiodide has been compared with corresponding data for 3,4-dihydroisoquinolinium methiodide, leading to a value of 48 9 kcal mole-1 for the ERE of the former.50... 

Determinations of the ERE of quinoline from thermochemical data have given results which range from 47.3 to 69 kcal mole-1.49-271-272 There appear to be no corresponding estimates for isoquinoline, but a value of 48 9 kcal mole-1 has recently been deduced from a comparison of the equilibrium constants for pseudo-base formation of iV-methylisoquinoline cation and its dihydro analog 73.273 Comparison of these ERE values with those obtained for naphthalene (61-75 kcal mole-1) shows that they parallel the somewhat lower resonance energy of pyridine relative to that of benzene. 

The UV and H NMR spectral properties of 1-methylquinoxalinium iodide suggest that at pH 9-12, the pseudo base 10 is in equilibrium with the covalent hydrate 11 formed by addition of a molecule of water across the C-3, N-4 bond. At higher pH the anion 12 is formed, and this is less susceptible to covalent hydration across the C-3, N-4 bond. ... 

Imino-6,7-dihydro derivatives of the 5,6-benzologue of (10) yield an equilibrium system involving the tautomeric forms of pseudo bases (21), (22), (23), and anhydro base (24) <90CB493, 91CB111, 94H(37)205l). Different tautomers (26), (27) and forms (28), (29) of pseudo bases (25, X = 0... 

The catalytic properties of these reduced species have been actively studied by Gillard and co-workers. Among other things, they have shown that H2 also initiates Delepine s reaction of [RhClj] ", that Oj dramatically impedes the catalyst, and that halide substitution at trans-[Rhpy4Cl2] (equation 167) is also catalyzed by primary and secondary alcohols. Consistent with the pseudo acid-base equilibrium (equation 164), the rate of catalyzed halide substitution is pH dependent, with little catalytic activity occurring in acidic solutions. There is an induction period for the reaction (the more alcohol in the solution the shorter the period), suggesting that deoxygenation of the solution must occur before the reduced catalyst can be effective. 

Table 10.5 Equilibrium between quaternary cations and their pseudo-bases...

Application of the above-mentioned basicity method is limited, and it can be applied only to systems whose aromaticities are destroyed on protonation. Heteroaromatics such as pyridine and quinoline conserve their aromaticity during protonation therefore a different method is applied, that of considering the basicities of pseudo-bases derived from /V-meth-ylheteroaromatic cations and the corresponding nonaromatic analogues. The principle is shown for isoquinoline in Scheme 14. The model compound used for comparison is 2-methyl-3,4-dihydroisoquinolinium cation. The pAa values for the equilibrium 1 — 2 and 3 — 4 were measured they correspond to an enthalpy difference A/A of 9.2 kcal/mol (X and Y are the differences in the resonance energies between 1 and benzene and 3 and benzene, respectively). 

This method has been extended to pseudo-bases derived from the azoles as illustrated for thiazole (Scheme 15). The equilibrium constants for pseudo-... 

In aqueous media, most of the natural anthocyanins behave like pH indicators, being red at low pH, bluish at intermediate pH and colorless at high pH. It has been demonstrated that in acidic or neutral media, four anthocyanin structures exist in equilibrium the fiavylium cation (AH-I-), the quinoidal base, the carbinol pseudo base and the chalcone. In general, anthocyanins are present in the fiavylium form at... 

In solutions of pH 1.0 and lower, anthocyanins exist solely as red-coloured flavylium salts. When increasing pH, the equilibrium shifts in favour of colourless carbinol pseudo base and the red colour fades. Around the range of pH values of 4.0 to 4.5, anthocyanins are completely colourless. Another increase in pH is manifested by the purplish-red colour, which is caused by formation of a neutral quinoid base that requires the presence of free hydroxyl groups on one of C-5, C-7 or C-4 carbons. In solutions of pH 7 a blue coloured quinoid base is formed. After some time or following an increase in pH value, a gradual decrease of blue colour intensity occurs as a result of yellow chalcone formation. If the solution is acidified to around pH 1.0, the blue quinoid and colourless carbinol bases are converted back into red flavyhum cations. The transformation of chalcones is slower and not quantitative. 

In Equations (6.1) to (6.3), is the reactor outlet temperature in °C, and Tapp is the approach temperature calculated from the gas analysis for the Boudouard reaction in K. The according pressure-based equilibrium constant is represented by p,B in bar and was fitted (ln(/ p) over 1/T) with a third-order polynomial expression, is the total system pressure in bar and xqo is the mole fraction of CO in the product gas. All variables are imported automatically and the equations are solved iteratively. Of course, the Boudouard reaction itself is not valid as soon as carbon is set as inert. However, the calculation procedure provides a temperature and pressure dependent empirical ( pseudo-Boudouard ) expression that relates to CO2/CO and permits a robust correlation for this generic model with a smooth transition to the zone where carbon is present. Including this modification, the model results for the validation case indicate the right order of magnitude for the CO2 concentration (1.22% deviation of the molar flows). The hydrogen balance of the case from the literature had a feilure rate of 3.13% hence, the model with a closed balance predicts the same excessive amount All error for the other components could be reconciled to less than 1.3% each. 

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