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Nitration and aromatic reactivity - Hoggett J.G.

Hoggett J.G., Moodie R.B., Penton J.R. Nitration and aromatic reactivity - Cambridge, 1971. - 252 p.
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It has been mentioned ( 4.4.2) that nitronium tetrafluoroborate reacts
with pyridine to give i-nitropyridinium tetrafluoroborate. This compound
and several of its derivatives have been used to effect what is called
the 'transfer nitration' of benzene and toluene.46 i-Nitropyridinium
tetrafluoroborate is only sparingly soluble in acetonitrile, but its
homologues are quite soluble and can be used without isolation from the
solution in which they are prepared. i-Nitropyridinium tetrafluoroborate
did nitrate toluene in boiling acetonitrile slowly, but not at 25 . In
contrast, i-nitro-2-picolinium tetrafluoroborate readily
72
4-4]
Nitronium salts in organic solvents
effected quantitative nitration at ordinary temperatures; presumably the
2-methyl group, by preventing planarity, weakened the bonding between the
nitro group and the pyridine ring. The results of competitive nitrations
of benzene and toluene are given in table 4.5. The identification of the
active electrophile, and the elucidation of other aspects of these
reactions, will be of great interest.
table 4.5 Competitive nitrationsi45 of toluene and benzene with
i-nitropyridinium tetrafluoroborates in acetonitrile at 25 0
Substituents in Relative Isomer proportions | 0 :p-ratio
pyridinium salt rates (partial rate factors)
< meta para
ortho
2-Me 36-5 63-8 yz CC'I 0-965
(69-9) (3'5) (72'3)
2,6-Me2 39-0 63 9 3-0 33"! 0-965
(747) (3'S) (774)
2,4,6-Me3 41-4 63-1 3-1 33-8 0-93
(78-5) (3-8) (84-0)
4-MeO-2,6-Me2 44-5 64-1 2*6 33-3 0-965
(85-5) (3*5) (88-9)
1,5-Dinitro- 13-2 62-1 2*1 35-8 0-865
quinolinium*
(24-6) (o-8) (284)
* In nitromethane; the salt was not very soluble in acetonitrile.
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