Heteroaromatic reactivity. Part IV. The kinetics of nitration of cinnoline 2-oxide and quinoline 1-oxide in sulphuric acid. The mechanism of nitration of N-heteroaromatic oxides, with special reference to 2,6-lutidine 1-oxide

Abstract

The rates of nitration, in sulphuric acid at 80°, of cinnoline 2-oxide (70·0–81·2% H₂SO₄) and 2-methoxycinnolinium cation (75·2–81·5 % H₂SO₄) have been measured. The plot of log₁₀k₂ against % H₂SO₄ from the nitration of cinnoline 2-oxide is steeper at higher than at lower acidities, and comparison with other substrates suggests that both the oxide and its cation are being nitrated, the latter reaction growing more important as the acidity increases. In the range 64·4–90·0% H₂SO₄ the proportions of 5- and 8-nitrocinnoline 2-oxide formed increase, and that of 6-nitrocinnoline 2-oxide decreases. The 5- and 8-nitro-compounds are probably formed wholly or mainly by nitration of the cation, and the 6-nitro-compound wholly or mainly from the free base.

As shown by other workers, the nitration of quinoline 1-oxide gives predominantly the 5- and 8-nitro-compounds at lower temperatures, and predominantly the 4-nitro-compound at higher temperatures. The kinetics of these two processes have been separated, and it has been shown that the rate of (5 + 8)-nitration depends upon acidity in a quite different way from that of 4-nitration. Also, orientation of nitration depends upon acidity as well as temperature; at a fixed temperature the proportion of 4-nitration decreases and that of 5- and 8-nitration increases with increasing acidity. The usual criteria show that 5- and 8-nitration involve 1-hydroxyquinolinium, and as applied previously would suggest that 4-nitration involves the free base.

The latter conclusion and earlier ones regarding the involvement of the free bases in the production of 4-nitropyridine 1-oxide and 4-nitro-2,6-lutidine 1-oxide have been reassessed through a critical consideration of the activation parameters for these nitrations. Whilst it is certain that these 4-nitrations do not involve the protonated oxides, it is also clear that they do not proceed by nitration of the small equilibrium concentrations of free bases. Possible mechanisms are discussed.