Orienting Influences in the Benzene Ring. A Review of Experimental Evidence.

Abstract

The entry of a second group or radical into the nucleus of a monosubstituted benzene is well characterized by the lack of random in respect to the position attacked. Indeed, such apparently is also the case for any aromatic substitution reaction in which there are one or more substituents present and two or more positions open to substitution. Thus it is a very general phenomenon and one of no slight complexity. In its simplest form, where a single substituent enters the ring of a monosubstituted benzene, it is the fact that all three possible isomers are not formed in equal amounts which gives rise to this problem. (It is to be noted that possibly twice as much of the ortho and meta isomers as of the para isomer would be the normal circumstance in the absence of directive influences.) Furthermore, the ratio of isomers formed varies enormously with the character of the original substituent. These facts summarize briefly the well-known problem of orienting influences in the benzene ring.That each aromatic substituent possesses a characteristic directive influence has been known for many years, and not a few hypotheses have been offered to explain the observation that a group was either ortho-para or meta directing. But it is not easy to classify the many substituents in this way, for an almost infinite variation in the ratios of the isomers is apparently possible. Though such a situation is caused in large measure by the great number of possible substituents, there are other factors as well. The influence of temperature, reaction medium, and catalysts should not be heedlessly neglected. The r61e of these factors and the consequent difficulty of attaining comparable reaction conditions for the estimation of the relative directive strengths of various substituents has 1 The major part of this paper was accomplished while the author was a t The Johns Hopkins University. Revision and additions were made during the course of his work a t The Victoria University of Manchester, Manchester, England, as a

Why do the [PhSiO1.5]8,10,12cages self-brominate primarily in the ortho position? Modeling reveals a strong cage influence on the mechanism

“…Aromatic halogenation reactions are among the oldest known organic reactions. [1][2][3][4][5][6][7][8][9][10][11][12] There are multiple reviews on various aspects of aromatic halogenation. [9][10][11][12] Multiple kinetic and mechanistic studies have been reported as well as extensive studies on the catalysis of these reactions.…”

Quantum chemical explanation and APT atomic charge index of substituent orienting effect proposed by DFT calculation and MO Theory

“…Since the beginning of last century, chemists have begun to study the substituent orienting effect. [7][8][9][10][11][12][13][14][15][16][17] By the 1940s, Pauling's electron resonance state description 5 is widely used to explain the substituent orienting effect in organic chemistry. This is because any chemical experiment or spectral analysis cannot give data to reveal the origin of substituent orienting effect, and thus, it is a very difficult topic in the field of organic reactions.…”

“…Note that the two nitrations also should consider the impact of spatial obstruction and electrostatic interactions. The nitration reaction of molecule 8 in Figure 9 has been expressed in Equation ( 3), and the nitration 3 of 9 in Figure 9 is expressed as following Equation (8). Figure 9 11 is the optimized structure and its APT charges of C 6 H 5 -CH 2 Cl.…”

Orientation of Substitution in the Benzene Nucleus.

“…This essentially induces a relative alternating charge distribution around the ring with an overall electron density less than that of benzene; consequently, the derivative reacts more slowly than benzene and chiefly at the meta position. For instance, in the two series (7,67) given below there is an increased amount of meta isomer produced in nitration with increasing electronegativity of the groups in the «-position. Field polarization also occurs and probably is significant for highly polar groups.…”