0
THE ORIENTATION AND MECHANISM OF ELECTROPHILIC AROMATIC SUBSTITUTION1 LLOYD N. FERGUSON Howard University, Washington, D. C.
FOR over fourscore years now, chemists have been fascinated by the fart that substitution reactions with monosubstituted benzenes yield varying amounts of ortho, meta, and para isomeric disubstitution products, depending primarily upon the nature of the group already attached to the benzene nucleus. I t was early recognized that groups could be classified as chiefly ortho- and para-directing or meta-orienting, and much effort was spent in arranging substituents in order of their orienting effects. This is a topic which every student of organic chemistry takes up briefly, learning a t least one of the many empirical rules2 by which one may predict the orienting influences of the various types of substituents. What has increased the attractiveness of this topic is the recognition that most of the chemical and physical properties of aromatic molecules also are dependent upon the nature of the nuclear substituents. In this connection, there have been observed good correlations between orientation of aromatic substitution and such properties as dipole m ~ r n e n t s ,wave ~ length4 and intensity5 of ultraviolet light absorption, infrared absorption freqnen~ies,~nuclear magnetic shielding,' dissociation constants of phenylacetic acids, benzoic acids, phenols, anilines, pbenylboric acids, etc.: oxidation potentials of quinones,# and many chemical reaetivities as expressed through the Hammett equation.'Q This suggests that essentially the same basic factors are responsible for each of these properties; consequently, the broad problem has been to identify these factors and to study their oueration. Based on talks given, spring, 1954, before the Lund Chemical Society, University of Lund, Lund, Sweden, and the Danish Chemiml Society, University of Copenhagen, Copenhagen, Denmark. 2 FERGUSON, L. N., Chrm. Reva., 50,47 (1952). 8 Rr, T., AND H. EYEING, J. Chem. Phys., 8,433 (1940); L. E. S m o ~ Trans. , Faraday Sac., 30, 789 (1934). Worm L.. AND J. M. VANDESBELT. J. Am. Chem. Soc.. 69.
Many reviews have appeared on aromatic substitution~.11. 12. 18 but the subject is one of such continued popularity that it seems worth while to present to the r student some of the most recent concepts and experimental results in this area. Although substitution may take place by either elec-, trophilic (electron-seeking) or nucleophilic reagents, or by free radicals (homolytir) attacking the aromatic molecule, the type first mentioned is by far the most common and extensively studied, so that this article will be limited to elertrophilic substitution. Furthermore, the discussion will be directed to the two fnndamental questions of interest to a chemist: (1) what is the mechanism of reaction and (2) why do groups show preferential orientation in substitution? Nitration with the usual nitric-sulfuric acid mixture has been shown quite conclusively to proceed by the following mechanism:" ZHdO,
+ HONO* d 2HSO4- + H80t f ArH
+ +X02
slow ----t
+ArHh'02 L ArNO.
+SO2 ( 1 )
+ArHNOl
(2)
+H+
(3)
where ArH is an aromatic compound and +ArHSOpis a pentadienate cation" whose structure is a resonance hybrid of forms such as
Several types of experimental data provide evidence that the nitronium ion is the nitrogen species that attacks the aromatic molecule.16 For instance, sulfuric l1 INGOLD, C. K., "Structure and Mechanism in Organic Chemistry," Cornell University Press, Ithaca, N. Y., 1953, Chap.
717 7 A.
\---.,-
PUTT, J. R., J. Chem. P h ~ s . 19, , 263 (1951). a SOLOWAY, A. H., AND S. L. FRIEGS, J. Am. Chem. Sac., 73, 6
PRICE,C. C., Chem. Revs., 29,37 (1941). For earlier reviews, see references cited in Ref. 2. For reviews on nurleonhilic substitution. see J. MILLER.REVS.Pure and Appl. ~ h e m . ~ ( ~ u s l r a l 1, i a )17i , (1951); J. F. B ~ N X E T T AND R. E. ZAHLER,Chem. Rcus., 49, 2i3 (1951). For a recent treatment of homolytie substitution, see D. H. HEYAND G. H. WILLIAMS, D i s c u s ~ i m Faraday ~ Soe., 14, 216 (1952). l4 This name for the intermediate complex of electrophilie substitution is taken from IN. J. S. D E ~ A R"The , Electronic Theory of Organic Chemistry," Clarendon Press, Oxford, 1949, p. 164. '"FROST, A. A,, AXD R. G. PEARSON,"Kinetics and Mechanism," John Wile). & Sons, Inc., New York, 1953, p. 297 ff. 18
0 0 ~ 1 ) ,*"".,. ' MEYER,L. H., AND H. S. GUTOWSKY, J. Phys. Chem., 57,481
mnn """"
~~~
(1953). BRANCH, G. E. K., A N D M. CALVIN,'
