418 TRANSACTIONS OF SECTION B. 
Arising probably from this even distribution of valency, it is characteristic 
of benzene to furnish additive compounds in which six atoms of hydrogen or 
a halogen, but not two or four, become attached symmetrically to the molecule ; 
substitution, however, occurs when a catalyser is present, such as the aluminium- 
mercury couple for halogenation, or sulphuric acid for nitration or sulphonation, 
leading initially to the production of mono- substituted derivatives. Whether 
the catalyser by association with the benzene molecule *’ limits this additive 
capacity, or whether its function is to promote the elimination of the halogen 
acid or water respectively,’” is still a subject of discussion, but in the absence 
of a reaction of additive type it is not easy to account for facts such as the 
production of a certain amount of trinitrophenol when benzene is nitrated in the 
absence of sulphuric acid. 
—H,0 H:OH — HNO, OH"! 
SS a 
NO, H-NO, 
(in presence of (in absence of 
sulphuric acid sulphuric acid) 
The much-debated questions still remain: Why and by what mechanism, 
when a second or third substituent is introduced into the molecule, is the 
orientation of the isomeric products determined by the radical or radicals already 
present? For disubstitution, the ortho-para- and the meta-laws have been 
deduced, and the radicals which respectively promote mainly ortho-para- sub- 
stitution on the one hand, and meta- substitution on the other have been 
catalogued.*? But these laws take account only of the orientation of the chief 
product or products, whereas all three derivatives, ortho, meta, and para, have 
been detected in most of the reactions studied, and their relative proportion in 
many cases is known to depend on the conditions, being affected by such 
influences as variation in temperature or in the medium employed. Nitration 
of acetanilide, for example, furnishes a mixture of ortho- and para-nitroacet- 
anilide, but of aniline in the presence of much sulphuric acid yields chiefly 
meta-nitroaniline.** And, to illustrate the inadequacy of the meta-law, the fact 
that sulphonation of benzenesulphonic acid with concentrated sulphurie acid at 
230°-240° furnishes an equilibrium mixture of the meta- and para-disulphonic 
acids in the proportion of 2:1 may be quoted.** 
In the exploration of this field many workers have participated, but the 
results, recorded almost as often in patent specifications as in journals, are 
seldom quantitative, so great is the difficulty at times in isolating the minor 
product or products of the change. Recently, however, by a most ingenious 
use of melting-point curves and density determinations, Holleman and his 
collaborators have carried out an exhaustive series of substitutions with small 
quantities of material and under known conditions;** yet after a survey 
of the whole field the conclusions reached are :— 
1. That uncertainty cannot be removed until some basis exists for different 
reactions to be carried out under comparable conditions.** 
2. That even if the relative amounts of the isomerides formed when a 
3° B. N. Menschutkin, Abstr. Chem. Soc., 1912, 102, i. 98-100. 
4° Cf H. E. Armstrong, Trans. Chem. Soc., 1887, 51, 263. 
41 The phenol by nitration forming the trinitro- derivative (picric acid), 
Armstrong and Rossiter, also Groves, Proc. Chem. Soc., 1891, 7, 89. 
2 Cf. Noelting, Ber., 1876, 9, 1797; Armstrong, 7’rans. Chem. Soc., 1887, 
51. 258; Crum Brown and Gibson, ibid., 1892, 61, 367. 
* Hubner, Annalen, 1881, 208, 299. 
44-J. J. Polak, Ree. trav. chim., 1910, [ii.] 14, 416; R. Behrend and M. 
Mertelsmann, Annalen, 1911, 878, 352. 
45 A. F. Holleman, Die direkte Hinfiihrung von Substituenten in den 
Benzolkern (Veit & Co., Leipzig, 1910), p. 215. 
46 Wor example, nitration is effected chiefly at low temperatures, but sulphona- 
tion of mono- substituted benzenes at temperatures higher than the ordinary, 
which if employed in nitration would lead to mixed products. 
