PiCa values, table

The piCa values of the main alkylthiazoles are given in Table III-14. 

This tendency to pseudo-base formation increases with the complexity of the heterocyclic nucleus. Thus, whereas pseudo-base formation does not occur for the i-methylpyridinium cation, and only at a very high pH for the i-methylquinolinium and 2-methylisoquinolinium ions, the reaction takes place much more readily upon further annelation (as in lo-methyl-acridinium). The insertion of an electron-attracting substituent (such as -NOg), or a second doubly-bound ring-nitrogen atom, as in quinox-alinium and quinazolinium, has the same effect. Some examples are listed in Table 10.5, in which p/Crqh is the pH at which the pseudo-base and the quaternary cation are present in equal concentrations. Formally p/Croh is similar to a piCa value, but the equilibrium is only slowly achieved. 

For pica behavior, an ingestion rate of 10 g/day is considered by US-EPA as a reasonable value for use in acute exposure assessments, based on the available information. It should be noted, however, that this value is based on only one pica child observed in the Calabrese et al. (1989 - cited in US-EPA 1997) study, see Table 7.5. 

Hammett decided not to use the pJCas themselves for his correlation but defined a new parameter, which he called o. This O shows how electron-donating or -withdrawing a group is relative to H as a ratio of the logics or the difference of the piCas between the substituent and benzoic acid itself. If the acid required to determine a for a new substituent was not available, o could be determined by correlation with other reactions. Here are the equations and the table of o values for the most important substituents. A different value of a for any given substituent was needed for the meta and the para positions and these are called om and op, respectively. 

The pronounced electron-withdrawing nature of the 1,2,5-thia-diazole ring is evidenced by the fact that its carboxylic acid derivatives are highly acidic. The titration curve of the dicarboxylic acid exhibits two breaks with and Kai, equal to 1.59 and 4.14, respectively. The monocarboxylic acid is also strongly acidic and has a piTa of 2.47, comparable to that of ornitrobenzoic acid 2.18) and pyrazine-2-carboxylic acid (pifa 2.80). In general the piTa values for the thiadiazole carboxylic acids are in fair agreement with the pica s of the corresponding p3U azine acids (see Table II). 

Table 7.12 Mean values of performance IQ as a function of tooth lead group and history of pica (based on [24]).

The listing of piacidity from one carboxylic acid to another. For example, trifluoroacetic acid (piCa = -0.23) is 33,000 times as strong as acetic acid (pi[Pg.618]

The blood lead measures were assessed, and due to inconsistencies in measurement, only maternal blood lead during pregnancy (maternal PbB) and water lead levels during pregnancy were used as dependent variables. The mean values of these variables divided into the original three groupings are shown in Table 1, with the means of the available infant blood lead values at 1 year (infant PbBl) and at 2 years (infant PbB2). Pica in year 1 and year 2 was also measured. 

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