Methyl aniline, ionization

We turn now to the ionization process of a series of methyl mono-substituted ring systems pyridines, anilines, phenols, and substituted benzoic acids. Table 2 shows the free energies, A G, the heats, A H, and entropies of ionization, A S, for a number of these compounds. The compounds fall into two classes, according to the different ionic nature of the ionization process. The pyridine and aniline ionizations are of the type... 

According to previous reviews, ammonia and amines also retard hydro-formylation. This statement has to be corrected according to more recent work [7, 130, 131, 1042]. Contrary to the statement mentioned, the hydro-formylation reaction is accelerated by small amounts of certain amines having an ionization constant of 10 [130], as e.g. pyridine, chinoUne, picoline, lutidine, aniline, toluidine, xylididine, methyl aniline and aliphatic and aromatic amides such as N,N-dimethyl formamide, N-methyl pyrroli-done and acetanilide. If larger amounts of these amines are used with HCo(CO)4 the effect becomes smaller. Very large amounts hinder the reaction (see table 3). 

Maroz A, Hermann R, Naumov S, Brede O. (2005) Ionization of aniline and its N-methyl and N-phenyl substituted derivatives by (free) electron transfer to n-butyl chloride parent radical cations. JPhys Chem A 109 4690 696. 

Ionized aniline possesses an isomeric stable species, the 4- (or 3-) dehydroanilinium distonic ions (m/z 93). It is thus expected that their identification could afford some additional pieces of information related to the localization of the initial site of protonation. The distonic ions can readily be prepared by protonation of iodoanilines followed by collisional deiodination. Such a protonation-deiodination sequence is also readily performed in the quadrupole-time of flight instrument and the CID spectra of all the m/z 93 ions are collected in Table 5. As a reference for aniline radical cations, AZ-methylaniline was used and was observed to intensively expel a methyl radical after protonation and excitation (high cone voltage). 

We turn now to the methyl disubstituted pyridines (lutidines), phenols (xylenols) and anilines, for which the AG, AH and AS values for the ionization process are available. 

Despite the fact that d is not a simple quantity, but a function oi AH and A S, its variation with change of structural parameters obeys a simple relationship, as may be seen from the additivity of A O values. This term is used here in the sense that the thermodynamic quantities for disubstituted compounds may be predicted from the changes produced when single substituents are introduced separately. The free energies of ionization of some methyl disubstituted pyridines, anilines and phenols are shown in Table 4 (to which we shall refer in detail later). From A G values for the parent and mono-substituted compounds it is possible to estimate A G values for disubstituted compounds. Thus, from the A G values for p ridine and the picolines, the following increments are calculated 2-methyl 1 01, 3-methyl 0-56, 4-methyl 1 03. 

In calculating la, Mermens et al. (1984) used a values appropriate to ionisation of anilinium ions. Their la descriptor is therefore equivalent to the Ia descriptor used here for the nitrobenzenes. However, for ortho methyl and ethyl substituents. Mermens et al. use 0.10 and 0.05, respectively, as the a values. These values seem to apply uniquely to ionization of anilinium ions -more usually these substituents have negative values. Using the Za descriptor, calculated from the a values given in TABLE 3, an improvement is obtained in the OSAR for the anilines ... 

This technique has, for example, shown that HNC rather than HCN is lost from metastable aniline molecular ions and that ionized methyl acetate loses CH20H radicals rather than OCHj as the lowest-energy fragmentation route. 

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