Aromatics monosubstituted benzenes

The course of aromatic substitution has been placed on a more scientific basis by the following rules of Hammick and Illingworth (jfour. Chem. Soc., 930. 2358), If a monosubstituted benzene derivative has the formula CgHsXY, where X is the atom joined to the benzene ring and Y is an atom or group of atoms attached to X, then —... 

First the five protons (integral) of the //NMR spectrum (Sfj = 7.50 - 7.94) in the chemical shift range appropriate for aromatics indicate a monosubstituted benzene ring with typical coupling constants 8.0 Hz for ortho protons, 1.5 Hz for meta protons.). The chemical shift values especially for the protons which are positioned ortho to the substituent Sn = 7.94) reflect a -M effect. Using the CH COLOC plot it can be established from the correlation signal hclS = 66.AI7.94 that it is a benzoyl group A. 

Recently, a kinetic study has been made of the substitution of diazotised sulphanilic acid in the 2 position of 4-substituted phenols under first-order conditions (phenol in excess) in aqueous buffer solutions at 0 °C131a. A rough Hammett correlation existed between reaction rates and am values, with p about -3.8 however, the point for the methoxy substituent deviated by two orders of magnitude and no explanation was available for this. The unexpectedly low p-factor was attributed to the high reactivities of the aromatic substrates, so that the transition state would be nearer to the ground state than for reaction of monosubstituted benzene derivatives. 

When a substituent is introduced directly into an aromatic molecule, it may enter into certain of the available positions more readily than into others. This phenomenon of orientation has been exhaustively studied, and empirical rules have been found which describe the experimental results fairly satisfactorily. In a monosubstituted benzene CeHjR, for example, the introduction of... 

Hanzlik RP, Hogberg K, Judson CM. Microsomal hydroxylation of specihcally deuter-ated monosubstituted benzenes. Evidence for direct aromatic hydroxylation. Biochemistry 1984 23( 13) 3048-3055. 

Soma et al. (12) have generalized the trends for aromatic compound polymerization as follows (1) aromatic compounds with ionization potentials lower than approximately 9.7 eV formg radical cations upon adsorption in the interlayer of transition-metal ion-exchanged montmorillonites, (2) parasubstituted benzenes and biphenyls are sorbed as the radical cations and prevented from coupling reactions due to blockage of the para position, (3) monosubstituted benzenes react to 4,4 -substituted biphenyls which are stably sorbed, (4) benzene, biphenyl, and p-terphenyl polymerized, and (5) biphenyl methane, naphthalene, and anthracene are nonreactive due to hindered access to reaction sites. However, they observed a number of exceptions that did not fit this scheme and these were not explained. 

The most familiar set of organic reactions is perhaps the electrophilic aromatic substitutions. For monosubstituted benzenes the major products from the process are either o- or p-disubstituted benzenes or m-disubstituted analogs. 

We will restrict our consideration to reactions of substituted benzenes and to nitrogen heteroaromatic systems in which the reaction takes place first with the n system. The simplest example of reaction of a monosubstituted benzene with an electrophile (Lewis acid) is shown in Scheme 11.1. The electrophile may attach itself to the n system (step A) in four distinct modes, ipso, ortho, meta, and para. The reactivity of the aromatic ring and the mode of attachment of the electrophile will be influenced by the specific nature of the substituent group, which may be X , Z, or C type. Detachment of the electro-... 

The irradiation of benzenes with alkenes provides a fascinating array of photochemical reactions, not least because it converts the aromatic substrates into polycyclic, non-aromatic products. In principle, benzene can undergo reaction across the 1,2-(ortho). 1,3-(meta), or 1,4-(para) positions the 1,3-cycloaddition is structurally the most complex, but it is the predominant mode of reaction for many of the simplest benzene/alkene systems. The products are tricyclic compounds with a fusion of two five-membered rings and one three-membered ring, and an example is the reaction of benzene with vinyl acetate (3.411. For monosubstituted benzenes there can be a high... 

Most of the substituent increments presented in Table 4.82 can be derived from 3C shifts of benzenoid carbons in monosubstituted benzenes as listed in Table 4.53. Additional substituent increments are available for fused aromatic rings such as naphthalene and... 

A quantitative description of the reactivity of monosubstituted benzenes to electrophilic substitution based on considerations of inductive effect parameters and con-jugative effect parameters from the 13 C chemical shifts of the aromatic compounds has been proposed.3 MO calculations on the proton migration in the ipso adducts formed in the reaction of CH3+ and SiH3+ with benzene have been described.4 With SiH3+ the ipso adduct is the most stable of possible isomers, whereas for CH3+ the >ara-protonated isomer is the most stable. 

Quantitative rate data ample for an adequate test of the applicability of a linear free-energy relationship are now available. Prior to an examination of this question, however, it is convenient to present all the experimental information necessary for a discussion of the problem. The partial rate factors for sixty reactions of toluene, the most intensively studied aromatic compound, were summarized in Table 2. Other monosubstituted benzenes, although less completely investigated than toluene, provide results encompassing a broad range of relative reactivity. The data for the reactions of the simple aromatic... 

Major variations in the reaction conditions are required to effect substitution within a reasonable time interval. Thus, the data for the highly deactivated aromatics are subject to the same limitations characterizing experimental assessment of the relative reactivity of the highly activated monosubstituted benzenes. 

Activated monosubstituted benzenes The major doubt concerning a general linear free-energy relationship for aromatic substitution is contained in the question whether resonance contributions to an electron-deficient transition state are sufficiently... 

Deactivated monosubstituted benzenes Traditionally, qualitative discussions of electrophilic substitution emphasize the deactivating influence of positive or partial positive charges on substituent atoms (Ingold, 1953). Only in recent years, however, have valid quantitative measurements been performed to allow the elucidation of the extent of the deactivation of the aromatic nucleus by such groups. The available information, still very limited in scope, is summarized in Table 18. 

The ll NMR spectrum of each isomer shows peaks corresponding to live aromatic protons, so compounds A and B each contain a monosubstituted benzene ring. Only four compounds of molecular formula C8HnN meet this requirement. 

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