Anilines, dissociation

Aniline dissociation equilibrium shows that the dissociated form prevails for pH lower than the pKa value and the undissociated form for pH greater than the pKa value (Fig. 32). 

Figure 10.13 ZEKE-PFI spectra of aniline-CH4 pumping the 6a band at three different probe delay times. The top spectrum has the signature of the initially pumped complex, the broad structure in the lower trace is due to IVR to the van der Waals modes. The sharp peak at 1200 ps delay time is due to the vibrationless aniline dissociation product. Taken with permission from Smith et al. (1993).

The conversion of the diazoaminobenzene into aminoazobenzene is promoted by the addition of aniline hydrochloride even more readily than by that of free hydrochloric acid. The aniline hydrochloride dissociates in solution giving hydrochloric acid and aniline the former promotes the formation of the above equilibrium, and the latter by increasing the active mass of the free aniline further accelerates the condensation to aminoazobenzene,... 

Available information on the thermal instability of halogenated anilines during vacuum distillation is presented and analysed, and the roles of dehyrohalogenation, polymerisation, salt formastion and dissociation, aerobic oxidation, corrosion and metal-catalysed decomposition are discussed. Experimental work on these topics is presented, and preventive measures proposed. 

In addition to isolating complex 5, Markovic and Hartwig performed kinetic studies on the amination of methyl cinnamyl carbonate with aniline. The proposed mechanism involves reversible dissociation of product, reversible, endothermic oxidative addition of the allylic carbonate to form a 7i-allyliridium species, and irreversible nucleophilic attack on the 7i-allyliridium intermediate, as depicted in... 

Fig. 8. Selected examples of aromatic donor molecules that form complexes with HRP C. The apparent dissociation constant for complex formation with the resting state plant enzyme is given (original references should be consulted for details of solution conditions and error estimations). (1) 2-Naphthohydroxamic acid (228) (2) benzhydroxamic acid (228) (3) 2-hydroxybenzhydroxamic acid (salicyUiydroxamic acid) (228) (4) benzhy-drazide (228) (5) cyclohexylhydroxamic acid (228) (6) 4-methylphenol (p-cresol) (192) (7) 2-methoxyphenol(guaiacol) (192) (8) indole-3-propionic acid(24i) (9)p-coumaric acid (238) (10) aniline (243).

A detailed investigation of aniline N-methylation on Cui xZnxFc204 was carried out through in situ FTIR spectroscopy. The reactants (aniline and methanol) and possible products (NMA, DMA and o-toluidine) were adsorbed on the catalysts and analyzed [106,107]. Adsorption of methanol indicated a dissociative chemisorption as methoxy species on catalyst surface at 100°C. As the temperature increased, oxidation of methoxy species to formaldehyde to dioxymethylene to formate species was observed, and above 300°C complete oxidation takes place to CO, CO2 and H2. Indeed methanol alone on Cui xZnxFc204 and Cui.xCoxFc204 behaves in a similar way [79,107]. 

Adsorption of aniline on Cui xZnxFc204 at <100°C indicates a simple molecular adsorption through N-atom on an acid-base pair site. However, above 100°C, N-H bond dissociates and aniline chemisorbed strongly on the catalyst surface. Chemisorbed aniline... 

A summary of aniline N-methylation mechanistic features on Cui xZnxFe204 ferrospinel catalysts is given in Figure 27. It was possible, due to in-situ IR studies, to observe a dissociative adsorption and possible orientation of reactants on the catalyst surface, their conversion to product at low temperatures, and desorption-limited kinetics, all under conditions that are close to the reaction conditions. Although Cu is the active center for the aniline A-methylation reaction, and IR studies reveal that Zn acts as the main methyl species source. 

Novak et al. examined the addition of A,A-dimethylaniline to 4-biphenylyni-trenium ion (101, R = H) and found significant yields of meta adduct 103. This product was attributed to an initial addition coupling the para position of the nitre-nium ion to the para position of the aniline to give 102, followed by an acid-catalyzed 1,2-migration of the trap to the meta position (Fig. 13.50). In this case, the solvent was water. In the previous case, the solvent was acetonitrile. Presumably, the difference in the fate of the initial para adduct is governed by the solvent. The 1,2-shift requires a general base, which is abundant in the buffered aqueous solutions used by Novak et al. The homolytic dissociation is more probable in aprotic media such as MeCN. 

Weller24 has estimated enthalpies of exciplex formation from the energy separation vg, — i>5 ax of the molecular 0"-0 and exciplex fluorescence maximum using the appropriate form of Eq. (27) with ER assumed to have the value found for pyrene despite the doubtful validity of this approximation the values listed for AHa in Table VI are sufficiently low to permit exciplex dissociation during its radiative lifetime and the total emission spectrum of these systems may be expected to vary with temperature in the manner described above for one-component systems. This has recently been confirmed by Knibbe, Rehm, and Weller30 who obtain the enthalpies and entropies of photoassociation of the donor-acceptor pairs listed in Table XI. From a detailed analysis of the fluorescence decay curves for the perylene-diethyl-aniline system in benzene, Ware and Richter34 find that... 

Fig. 7. Schematic representation of the mass spectrum at a photon energy of 10.15 e.v. (a) for aniline vapor (Ip = 7.69 e.v.), (b) for benzylamine vapor (f B = 8.64 e.v.). The thick bars correspond to the dissociation of the metastable ions.

The electrochemical oxidation of aromatic aldehydes (1) must be studied in strongly alkaline media. Acidity functions for strongly alkaline aqueous solutions of alkali metal and quaternary ammonium hydroxides, corresponding to dissociation of proton (H ), are well established (2, 3). Substituted anilines and diphenylamines (4,5) and indoles (6) were used as acid-base indicators for establishment of such scales, but whether an acidity scale based on one type of indicator can be rigorously applied to acid-base equilibria involving structurally different acidic groups for reactions in strongly alkaline media remains questionable. For substituted anilines, behavior both parallel (7) and nonparallel (8) to the H scale based on indole derivatives has been reported. The limited solubility of anilines in aqueous solutions of alkali metal hydroxides, the reactions of the aniline derivative with more than one hydroxide ion, irreversible substitution reactions (9), and the possibility of hydroxide ion addition rather than... 

Provided that the influence of the water-ethanol composition on the reaction involving addition of hydroxide ions to benzaldehydes can be characterized by any parameter Yj (the application of Y- used for benzoic acid dissociations in ethanol-water mixtures (28) might be doubtful), application of the relation pYt - pyo = C(Yi - Y0) would indicate that the value of C (0.638) for benzoic acids (28) and —0.573 for anilines (29) is close to zero for the benzaldehyde reaction (4). 

A simple case where the general a constants in Table 8.5 do not succeed in correlating acidity constants is when the acid or base function is in direct resonance with the substituent. This may occur in cases such as substituted phenols, anilines, and pyridines. For example, owing to resonance (see Fig. 8.4), a para nitro group decreases the pKa of phenol much more than would be predicted from the o para constant obtained from the dissociation of p-nitrobenzoic acid. In such resonance cases (another example would be the anilines), a special set of o values (denoted as oJpara) has been derived (Table 8.5) to try to account for both inductive and resonance... 

We have stated earlier that because of proximity effects, no generally applicable aj values may be derived for ortho substitution. Nevertheless, one can determine a set of apparent 0)ortho values for a specific type of reaction, as for example, for the dissociation of substituted phenols. Table 8.7 gives such apparent O)ortho constants for estimating pKa values of substituted phenols and anilines. Of course, in cases of multiple substitution, substituents may interact with one another, thereby resulting in larger deviations of experimental from predicted pKa values. Some example calculations using the Hammett equation are given in Illustrative Example 8.2. 

Pyridine-halogen complexes (73) dissociate on heating halogen is lost so readily that these compounds act as mild halogenating agents toward phenol or aniline, for example (see Section 3.2.1.3.8). 

In acetic acid it is possible to measure separately the equilibrium constant of proton transfer to form an ion pair and the constant for dissociation of ion pairs to form free ions. [I. M. Kolthoff and S. Bruckenstein, J. Amer. Chem. Soc., 78, I (1956) S. Bruckenstein and I. M. Kolthoff, J. Amer. Chem. Soc., 78, 10 (1956)]. G. W. Geska and E. Grunwald, J. Amer. Chem. Soc., 89, 1371, 1377 (1967) applied this technique to a number of substituted anilines and concluded that the equilibrium constant of the ionization step, rather than the overall acid dissociation constant, is the quantity that should be considered in discussions of effects of structural changes on acidity. 

Lee s group has published extensive results on aminolysis of sulfonates.278 281 Thus the reactions of anilines witii 2-cyano-2-propyl and 1-cyanocyclooctylarenesulfonates in acetonidile have been stiidied.278 A dissociative, S k2 mechanism with a loose TS is supported from die usual LFERs. An 5k2 mechanism is also found for the reaction in acetone of (Z)-benzyl (X)-benzenesulfonates witii (Y)-pyridines.279 Nucleophilic substiditions witii die cycloalkyhnetiiylsulfonates (306) and anilines in MeOH were also sfridied.280 Finally die reaction of tiiiopheneethyl arenesulfonates (307) with anilines and AvA -dimethylanilines in MeCN has been reported on.281 Frontside-atiack in an 5n2 mechanism witii a four-centre TS is supported. 

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