PRESIDENTIAL ADDRESS. 419 
radical C is introduced into each of the miono- substitution derivatives C,H, A 
and ©,H,'B be known, it is not possible to calculate the proportion in which the 
isomerides C,H, ABC will be produced when the radical C is substituted in the 
compound C,H,’ AB. 
Although the validity of the ortho-para- and of the meta- laws may be im- 
peached, they serve as a first approximation, and many theories have been pro- 
pounded to account for them Armstrong has suggested that in ortho-para- 
substitution the additive compound is formed by association of the addendum 
with the carbon atom carrying the radical already substituted in the molecule,*’ 
whereas in meta- substitution it arises by union of the addendum with this 
radical,** transformation to the respective disubstitution derivatives being 
effected possibly in step-by-step progression, as conjectured by Lapworth.” 
Holleman, who also adopts the additive hypothesis, is of the opinion that the 
radical already present in the molecule may promote or retard the association 
of the addendum with the pair of carbon atoms, to one of which it is itself 
attached. By the operation of the first of the alternatives an ortho- and by con- 
jugation a para- derivative will arise; from the second a meta- derivative will 
result, when scission of the additive compound ensues. Holleman’s is the only 
hypothesis which has been submitted to the test of quantitative investigation, 
and although, as already mentioned, the results do not suggest that finality has 
been reached, it marks an advance in the study of this obscure problem.*° 
No discussion of substitution in the benzene series would be adequate with- 
out reference to the remarkable behaviour of amines and phenols. Unlike other 
mono- substitution derivatives, which do not differ markedly from benzene in 
reactivity, these furnish mono-, di-, and tri-derivatives very readily. With 
aniline or acetanilide, substitution occurs first of all in the side chain, being 
followed under appropriate conditions by removal of the substituent from the 
amino- group and entry into positions relatively ortho-, para-, or both ortho- and 
para- to it. The earliest of these changes to be studied was the transformation 
of methylaniline into para-toluidine; many of them have been discovered by 
Chattaway and his collaborators, and until a critical study of the chlorination of 
Bepunee was undertaken by Orton and Jones,*! it was held that they were of 
the type : 
NHAc CLNHAc NClAc NHAc 
6-O=0-¢ 
From the dynamics of the reaction, which occurs only in the presence of hydro- 
chloric acid, it is now known that in the production of acetparachloroaniline 
intra-molecular transformation from the side chain to the ring does not take 
* Kinetic studies of the chlorination and bromination of toluene, CH aCH., 
however, gave no indication of the production of an intermediate additive com- 
pound of the hydrocarbon and addendum (cf. Holleman, Polak, van der Laan, 
and EKuwes, Rec. trav. chim., 1908, 27, 435; Bruner and Dluska, Bull. Acad. 
Sci., Cracow, 1907, 693; Bancroft, J. Physical Chem., 1908, 12, 417; Cohen, 
Dawson, Blockey, and Woodmansey, /'rans. Chem. Soc., 1910, 97, 1623. 
*“ H. E. Armstrong, 7'rans. Chem. Soc., 1887, 51, 258. 
* A. Lapworth, Z’rans. Chem. Soc., 1898, 98, 454; 1901, '79, 1265. 
** It should be mentioned that other views, based on the loosening or 
strengthening of the affinity of the hydrogen atoms situated in ortho-, para- or in 
meta- positions, brought about by the disturbing influence of the radical already 
present in the molecule on the valency of the carbon atom to which it is 
attached, and therefore on that of the other five carbon atoms, have been 
advanced by Flirscheim (J. prakt. Chem., 1902 [ii.], 66, 321), Tschitschibabin 
(tbid., 1912 [ii.], 86, 397), and others. 
" K. J. P. Orton and W. J. Jones, British Association Report, 1910, p. 96; 
Trans. Chem. Soc., 1909, 95, 1456. 
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