Partial rate factors hydrogen exchange

A more detailed study of the nitration of quinolinium (l) in 80-05 % sulphuric acid at 25 °C, using isotopic dilution analysis, has shown that 3-) 5-) 6-, 7- and 8-nitroquinoline are formed (table 10.3). Combining these results with the kinetic ones, and assuming that no 2- and 4-nitration occurs, gives the partial rate factors listed in table 10.4. Isoquinolinium is 14 times more reactive than quinolinium. The strong deactivation of the 3-position is in accord with an estimated partial rate factor of io for hydrogen isotope exchange at the 3-position in the pyridinium ion. It has been estimated that the reactivity of this ion is at least 10 less than that of the quinolinium ion. Based on this estimate, the partial rate factor for 3-nitration of the pyridinium ion would be less than 5 x io . 

Table 10.8. Partial Rate Factors for Hydrogen Exchange in Some Substituted Aromatic Compounds...

Extensive studies on hydrogen exchange in aqueous sulfuric acid on 1 -methylpyrazole,252 isoxazole,328 and isothiazole328 and their methylated derivatives252,328 have been reported, but calculated partial rate factors derived from treatment of the form of Eq. (12) do not tally with those observed. This emphasizes the contribution from interactions between donor and acceptor groups or solvent effects which also lead to breakdown in the interpretation of nitration results.83... 

Determinations of the partial rate factors in metallation reactions have so far been limited to the measurements by Bryce-Smith (1954) on t-butylbenzene. These partial rate factors were similar in value to those for hydrogen exchange in toluene (Table 10). The role of the... 

As in hydrogen exchange (Chapter 2, Section 1. G), evaluation of standard nitration rates permits comparison of substrate reactivities under the same conditions. Previous comparisons between nitration rates of compounds of widely differing reactivity have required a stepwise procedure (71 Mil), which carries the implicit assumption that partial rate factors do not vary appreciably with acidity. 

Scheme 7.1. Log partial rate factors for acid-catalyzed hydrogen exchange. Reaction temperatures are given in parentheses.

Nitration of 4-pyridone takes place on the free base, except at high acidity where the protonated species is involved [68JCS(B)1477]. 4-Pyri-done is less reactive than 2-pyridone toward nitration (Table 9.7), a result which contrasts with hydrogen exchange where the reactivities are identical Table 9.1). As expected, nitration of 3-nitro-4-pyridone also occurs on the free base [68JCS(B) 1477]. From this work, partial rate factors of 10 10 to 10"12 were calculated for the monocations derived from 4-pyridone and 2- and 4-methoxypyridine deactivation by NH+ in these systems was estimated as >1013 [68JCS(B)1477]. 

Electrophilic hydrogen exchange in deuterioselenophenes was studied using a 4 1 mixture of acetic and trifluoroacetic acids.79 Rate constants and partial rate factors were determined for the deuterium replacement at 25°C (see Table IV). Rate constants were calculated... 

These substitutions are facilitated by electron release from the heteroatom pyrroles are more reactive than furans, which are in turn more reactive than thiophenes. Quantitative comparisons of the relative reactivities of the three heterocycles vary from electrophile to electrophile, but for trifluoroacetylation, for example, the pyrrole furan thiophene ratio is 5 x 10 1.5 x 10 I " in formylation, furan is 12 times more reactive than thiophene, and for acetylation, the value is 9.3. In hydrogen exchange (deuteriodeproton-ation), the partial rate factors for the a and p positions of A-methylpyrrole are 3.9 x 10 ° and 2.0 x 10 ° respectively for this same process, the values for furan are 1.6 x 10 and 3.2 x l(f and for thiophene, 3.9 X 10 and 1.0 x 10 respectively, and in a study of thiophene, a P ratios ranging from 100 1 to 1000 1 were found for different electrophiles. Relative substrate reactivity parallels positional selectivity i.e. the a P ratio decreases in the order furan > thiophene > pyrrole. ° Nice illustrations of these relative reactivities are found in acylations of compounds containing two different systems linked together. ... 

Partial rate factors of hydrogen exchange of azulene, cycl(2.2.3)azine, indolizine, A-methyl-isoindole, and indole have been compared <87JCS(P2)591>. 

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