Lutidines

It is found in practice that for a number of compounds reacting ma the predominant species an almost horizontal plot is obtained. For compounds presumed to be nitrated via the free bases, such as 2,6-lutidine i-oxide and 3- and 5-methyl-2-pyridone, slopes of approximately unity are obtained. Since this type of plot allows for the incomplete ionisation of nitric acid, it can be used at higher acidities than plots using — ( H + logio Hjo) which break down when the condition is no longer true. 

These arguments were originally applied to the 4-nitration of 2,6-lutidine i-oxide and quinoline i-oxide, and use of the data available... 

Because of these difficulties, special mechanisms were proposed for the 4-nitrations of 2,6-lutidine i-oxide and quinoline i-oxide, and for the nitration of the weakly basic anilines.However, recent remeasurements of the temperature coefficient of Hq, and use of the new values in the above calculations reconciles experimental and calculated activation parameters and so removes difficulties in the way of accepting the mechanisms of nitration as involving the very small equilibrium concentrations of the free bases. Despite this resolution of the difficulty some problems about these reactions do remain, especially when the very short life times of the molecules of unprotonated amines in nitration solutions are considered... 

The carbonyiation of o-diiodobenzene with a primary amine affords the phthalimide 501 [355,356]. Carbonyiation of iodobenzene in the presence of (9-diaminobenzene (502) and DBU or 2,6-lutidine affords 2-phenylbenzimida-zole (503)[357, The carbonyiation of aryl iodides in the presence of pentaflnor-oaniline affords 2-arylbenzoxazoles directly, 2-Arylbenzoxazole is prepared indirectly by the carbonyiation of (9-aminophenol[358j. The optically active aryl or alkenyl oxazolinc 505 is prepared by the carbonyiation of the aryl or enol triflates in the presence of the opticaly active amino alcohol 504, followed by treatment with thionyl chloride[359]. 

For the methyl-substituted compounds (322) the increase in AG and AHf values relative to the unsubstituted thiazole is interpreted as being mainly due to polar effects. Electron-donating methyl groups are expected to stabilize the thiazolium ion, that is to decrease its acid strength. From Table 1-51 it may be seen that there is an increase in AG and AH by about 1 kcal mole for each methyl group. Similar effects have been observed for picolines and lutidines (325). 

These results show that the measured values of H° are close to those calculated from the thiazole value and the increments. That compounds substituted ortho-ortho to nitrogen (2,4-dimethyI and 2,4,5-trimethyl-thiazole) also obey this rule shows that the methyl groups do not interact sterically (150). The same conclusion had been reached in the case of the picolines and lutidines (151). 

Important commercial alkylpyridine compounds are a-picoline (2), Ppicoline (3), y-picoline (4), 2,6-lutidine (5), 3,5-lutidine (6), 5-ethyl-2-methylpyridine (7), and 2,4,6-coUidine (8). In general, the alkylpyridines serve as precursors of many other substituted pyridines used in commerce. These further substituted pyridine compounds derived from alkylpyridines are in turn often used as intermediates in the manufacture of commercially usehil final products. 

---