Hydrogen substitution, benzene nucleus

Influence of Methyl Substitution on the Relative Rates of Hydrogenation of Benzene Nucleus... 

How is the course of halogen substitution in the benzene nucleus to be explained It is not at all probable that direct replacement of hydrogen occurs, such as we must assume in the formation of benzyl chloride and in the reaction between methane and chlorine, since the hydrogen attached to the doubly bound carbon atom of olefines exhibits no special reactivity. However, various facts which will be considered later (p. 164) indicate that benzene reacts with halogen in fundamentally the same way as does ethylene. The behaviour of ethylene towards bromine is the subject of the next preparation. 

A variety of substituted benzenes are known that have one or more of the hydrogen atoms of the ring replaced with other atoms or groups. In almost all of these compounds the special properties associated with the benzene nucleus are retained. A few examples of benzenoid hydrocarbons follow, and it will be noticed that the hydrocarbon substituents include alkyl, alkenyl, and alkynyl groups. Many have trivial names indicated in parentheses ... 

The presence of the hydroxyl group in phenols facilitates the substitution of the nuclear hydrogen atoms by halogen the number and position of the substituent atoms varies with the nature of the phenol. This method is an indirect means of identification, as the formation of a substitution derivative is not a characteristic reaction of the phenol group but of the benzene nucleus. Phenol reacts with bromine to give 2,4,6-tribromophenol ... 

The presence of an indoline nucleus was established by color reactions, substitution reactions, e.g., bromoajmaline, mp 192°, andUV-absorption spectroscopy. Upon catalytic hydrogenation, the benzene ring was reduced with remarkable ease to yield hexahydroajmaline, mp 149°—150°, [a]D + 92° (CHCI3) there was, however, no other unsaturation apart from this (1). 

If this mechanism is really operative, the "abstraction product" formed in benzene is not necessarily due to a triplet nitrene precursor. Recently a careful study of the thermolysis of methylazide in substituted benzenes demonstrated that the unsubstituted primary sulfonamide is a product of hydrogen abstraction by the nitrene 72). On the other hand there are remarkable differences in isomer ratios (o m p) of the ring-substituted anilides formed depending on the spin state of the reacting nitrene. The triplet was shown to attach the aromatic nucleus mainly in the o-position, as is expected from a highly electrophilic diradical. Dehydrogenations by carbonylnitrenes have been reported by several authors for a variety of systems. In the direct photolysis of ethylazidoformate 29 in cyclohexene, the amide 30 and the bicyclohexenyl 31 were isolated 35b Both products result from an abstraction reaction. 

All the mono- and diarylarsines may be prepared by the reduction of the corresponding arsinic acids, whether the benzene nucleus is substituted or not. Such reduction may be effected with zinc dust and hydrochloric acid, and, in the case of phenylarsinic acid, electrolytic reduction in aqueous alcohol solution has also been used. The primary arsines show no basic properties and readily undergo oxidation in air, forming oxides, acids, and arseno- compounds. Halogens react with these arsines, replacing the hydrogen ... 

The introduction of the hydroxyl group into the benzene nucleus increases the ease with which its hydrogen atoms can be replaced by substitutions. Add 1 ml of one per cent solution of bromine in carbon tetrachloride to 2 ml of 10 per cent solution of phenol. Recall the formation of bromobenzene. 

Catalytic hydrogenations over CojfCOjg (using Hj and CO) or with stoichiometric quantities of preformed hydridocarbonyl complex CoH(CO)4 are useful for the partial selective reductions of polycyclic aromatic compounds. Isolated benzene rings are not affected. Naphthalene is reduced to tetralin, at 200°C under a pressure of 20 X 10 kPa and anthracene to 9,10-dihydroanthracene (99%). The substituted phenanthrene nucleus is stable under these conditions as illustrated by hydrogenation of perylene 1 and pyrene 2. ... 

By the substitution of other radicals or elements for the remaining hydrogen atoms of the benzene nucleus, or for the hydrogen atoms of the amiaogen gitiup, NH, a great number of derivatives, induding many iso-meres, are produced. 

Strychnine shows reactions typical of an anilide. For example, bromine (40, 63, 81, 192, 243) and chlorine (158, 184) (under controlled conditions) substitute for a hydrogen in the free pam-position of the benzene nucleus however, halogenation wall proceed further to give tri- and tetrahalogenated derivatives (158, 188). p-Nitrostrychnine (108, 243) and 3, 5 -dinitro-strychnine (37) have been prepared. Hydrolysis of dinitrostrychnine to... 

Vomicine is a monoacidic, tertiary base, with one double bond, dihydro-vomicine (8) being readily obtained by catalytic reduction (Pt02 in acetic acid). The ease with which one hydrogen atom is replaced by bromine would indicate that substitution has occurred in the benzene nucleus para to Na (8). It is only on such a hypothesis that the failure of bromovomicine to form a substituted bivomicyl is understandable (12). 

Aromatic Compoimds. Suhsiituiion, Direct substitution of iodine into the benzene nucleus is feasible only in the presence of an oxidizing agent. The hydrogen iodide formed simultaneously with the iodo compound is unstable, the iodine in the benzene nucleus is loosely bound and conditions must therefore be provided to avoid reversibility. 

These substances are derivable from benzene and its homo-logues by the substitution of one or more univalent groups (NHj) radical phenyl (CeHj), or its homologues, derivable from the benzene nucleus, for the hydrogen of ammonia. They all are strongly basic in character. 

A modification of this process consists in reducing arylstibinic acids and treating the reduced product with sodium hahde m the presence of acid. The method is used for acids containing an amino-group substituted in the benzene nucleus, e.g. m-aminophenyldichlorostibine hydrochloride is formed when 7 -nltrophenylstibinic acid and stannous chloride are treated with a cold saturated solution of hydrogen chloride in alcohol. To transform tliis to the di-iodostibine hydriodide, its aqueous solution is acidified with hydrochloric acid and treated with a solution of sodium iodide. 

A satisfactory benzene structure must account for the symmetry of the molecule as revealed by the equivalence of the six hydrogen atoms and the observed numbers of isomers of substituted benzenes. It must also account for the stability of the benzene nucleus, as shown qualitatively by its resistance to degradation and to addition, and by its ability to undergo and survive substitution reactions. We shall not at first stress the demands of stability, for it is now clear that classical organic chemistry could not meet them from its own resources. 

The mechanism of electrophilic aromatic substitution has two steps. First, the electrophile E attacks the benzene nucleus, much as it would attack an ordinary double bond. The resonance stabilized cationic intermediate thus formed then loses a proton to regenerate the aromatic ring. Note two important points in the formulation of this general mechanism. First, always show the hydrogen at the site of the initial electrophilic attack. Second, the positive charge in the resulting cation is indicated by three resonance forms and is located ortho and para to the carbon that has been attacked, a result of the rules for drawing resonance forms (Sections 1-5 and 14-1). 

This last result bears also on the mode of conversion of the adduct to the final substitution product. As written in Eq. (10), a hydrogen atom is eliminated from the adduct, but it is more likely that it is abstracted from the adduct by a second radical. In dilute solutions of the radical-producing species, this second radical may be the adduct itself, as in Eq. (12) but when more concentrated solutions of dibenzoyl peroxide are employed, the hydrogen atom is removed by a benzoyloxy radical, for in the arylation of deuterated aromatic compounds the deuterium lost from the aromatic nucleus appears as deuterated benzoic acid, Eq. (13).The over-all reaction for the phenylation of benzene by dibenzoyl peroxide may therefore be written as in Eq, (14). 

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