The ortho: para ratio in the nitration of biphenyl
The ortho: para ratio in the nitration of biphenyl has been accurately determined under various conditions by means of vapour phase chromatography. The low ratio (ca. 0·6) commonly observed in nitric acid–sulphuric acid is found to be due predominantly to a homogeneous reaction, rather than to a heterogeneous reaction as has been previously suggested; under suitable conditions a predominantly heterogeneous reaction can be obtained which by contrast gives a high ratio (ca. 1·5). A previous theory attributing variable ratios in biphenyl nitration to the degree of homogeneity of the reaction mixture is accordingly modified, and it is shown that nitric acid in acetic anhydride and dinitrogen pentoxide in acetonitrile do give uniquely high and closely similar ratios as in the nitration of anilides and ethers, so that a common explanation is necessary. It is proposed that nitration with preformed nitronium ions gives rise to normal ortho: para ratios, whereas nitration with all species of the type NO2X (some of which are positively charged) gives enhanced ratios due to an additional mode of ortho-substitution involving, in the case of biphenyl, nucleophilic displacement of X (or X–) by the π-electrons of the unsubstituted ring to give a π-complex involving NO2+. This π-complex goes over to the most accessible σ-complex which is, because of the nonplanarity of the phenyl rings, that formed at the ortho-position in the other ring.
Nitration by nitric acid–acetic acid produced widely varying ratios according to the technique employed, and this is attributed to different nitrating species being present under the different conditions. Nitration under free-radical conditions gave up to 40% of meta-product in the mononitrobiphenyls, but overall yields were extremely low because of side reactions.
Conditions for the optimum yield of ortho- and of para-nitrobiphenyl are given.