Hydrogenation benzene nucleus

The catalyst is inactive for the hydrogenation of the (isolated) benzene nucleus and so may bo used for the hydrogenation of aromatic compounds containing aldehyde, keto, carbalkoxy or amide groups to the corresponding alcohols, amines, etc., e.g., ethyl benzoate to benzyl alcohol methyl p-toluate to p-methylbenzyl alcohol ethyl cinnamate to 3 phenyl 1-propanol. 

The chemistry of benzenecarboxyUc acids generally is the same as that of other carboxyUc acids, which can be converted into esters, salts, acid chlorides, and anhydrides. Each carboxyl group can react separately, so that compounds in which carboxyl groups are converted into different derivatives can be prepared. Because there are aromatic hydrogens available in most of these acids, they also undergo reactions characteristic of the benzene nucleus. Some of the anhydrides have characteristic reactions. 

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. 

If an aromatic compound contains saturated aliphatic side chains nitration carried out under the above conditions takes place always in the benzene nucleus and not in the side chain. Since the carbon atoms of benzene are each united directly to only one hydrogen atom, the nitro-derivatives obtained are tertiary and therefore incapable of forming salts, nitrolic acids, or pseudonitroles, as do the primary and secondary nitro-compounds. 

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

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 ... 

As long as the benzene nucleus remains unsaturated, side chains are split at (a), (b), or (c) into various free radicals, which are immediately hydrogenated to give methanol, ethanol, and various phenolic derivatives. 

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 ... 

With benzoic acid (XXXI) the hydrogen in 2-position and the oxygen atoms of the carboxyl group must be regarded as the elec-trophilic/nucleophilic system on the grounds of the charge distribution (Fig.17). Comparison with the phenols in Table XVII shows that the benzene nucleus can contribute the electrophilic as well as the nucleophilic group to the bipolar system. The effect of further substituents depends on type and relative position. 

Orito and Imai have shown that the hydrogenation of benzene over nickel and cobalt catalysts is inhibited by alcoholic solvents and some ethers.5 As seen from the results shown in Table 11.2, benzene is hydrogenated extremely slowly or not at all in primary alcohols but very rapidly without solvent or in hydrocarbons. Benzene is hydrogenated at a considerable rate at 110°C even over Urushibara Ni A, which is known to be a poor catalyst toward the hydrogenation of aromatic nucleus,10 when used without solvent or in hydrocarbons after the water or alcohol on the catalyst has been carefully removed. 

Partial hydrogenation of aromatic rings can also be accomplished with catalysts such as [Co2(CO)g] under an atmosphere of carbon monoxide and hydrogen, but the isolated benzene nucleus cannot be re-duced. ° Anthracene (57), naphthacene, perylene (61) and pyrene (63) are converted to 9,10-dihydroan-thracene (60), 5,12-dihydronaphthacene, 1,2,3,10,11,12-hexahydroperylene (62) and 4,5-dihydropyrene (64), respectively. 

In the aromatic series it is observed that the introduction of a halogen atom into the side chain of a substance confers lachrymatory properties, while if it replaces a hydrogen atom from the benzene nucleus, a substance results which has no physiopatho-logical properties. Thus, from toluene, benzyl bromide, CgHj—CHjBr, is obtained in the first case. This has energetic lachrymatory properties, while bromotoluene, CgH4Br—CH3, has no toxic action. 

It has already been stated that Bart used catalysts in his reaction, in order to eliminate the diaze-nitrogen at low temperatures and thus decrease the formation of by-products. In Schmidt s method, in addition to non-arseuated aromatic derivatives, arsenated derivatives may also occur as by-products. In the case of the preparation of phenyl-arsinic acid, the by-product is diphenylyl-4-arsinic acid, CgH4.Ph. AsO(OH)2, and the presence of such a derivative can only be accounted for on the assumption that one of the hydrogen atoms of tire benzene nucleus becomes labile at the moment when the replacement of the diazo-group by the arsinic acid grouping takes place. 

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 ... 

Hydrated Naphthylamines.— We have referred to the hydrated naphthylamines in our discussion of the constitution of naphthalene (p. 772). The tetra-hydro products are of two kinds (i) Those termed aromatic in which the hydrogen is added to the benzene nucleus which does not contain the amino group and which possess the characters of aromatic amines. (2) Those termed alicyclic in which the hydrogen is added to the benzene nucleus which does contain the amino group and which possess the characters of aliphatic amines. Now while the alpha- and g/a-naphthylamines each yield both kinds of tetra-hydro products if subjected to proper treatment yet by the same treatment, viz., with sodium amalgam in amyl alcohol, the atpha-naphthylamine yields an aromatic tetra-hydro compound while the i>e/o-naphthylamine yields mostly an alicyclic compound. 

Character of Center Nucleus.—As was stated in connection with anthracene itself we can not say positively as to the character of the center nucleus in either the hydrocarbon or the quinone. In anthracene the aliphatic character of this center nucleus is indicated by its formation from an ethane residue, by the tetra-brom ethane synthesis. This does not, however, preclude the possibility of its becoming a true benzene nucleus when condensed with two benzene rings, for benzene itself may be made from aliphatic hydrocarbons, from acetylene by polymerization (p. 478), and from hexane through hexa-methylene with the loss of hydrogen after the formation of the cyclo-paraffin (p. 469). Also naphthalene, in which there is no doubt of the benzene character of the two nuclei, may have one nucleus formed from an aliphatic chain as in the syntheses given (p. 767) from phenyl butylene bromide, from phenyl vinyl acetic acid and from tetra-carboxy ethane. In the same way the facts in regard to anthraquinone do not prove... 

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. 

Cyclohexane is one of the important raw materials utilized in the production of nylon and, therefore, a cheap catalyst having a 100% selectivity for hydrogenation of the benzene nucleus is very much desired. 

The rate of decomposition is much higher in hydrogen than in the air, and the nickel particles formed on the surface are so fine and so closely surrounded by the nickel aluminate carrier that the catalytic activity of the reduced nickel is so modified that it loses the ability to hydro-decompose the C—C bond completely but still retains the high activity to hydrogenate the benzene nucleus. The number of fine nickel particles produced on the surface by the reduction at 500-600° should be very small, because they are completely poisoned by the addition of a minute amount of sulfur compound in hydrogen. 

A new method of preparation of a supported nickel catalyst that has no free nickel oxide and, therefore, has a high selective activity after reduction for hydrogenation of the benzene nucleus was thus obtained. 

Nickel silicate prepared by the SHOP method has little activity for hydrogenation in general, but exhibits a high selective activity for the hydrogenation of the benzene nucleus when heated above 500° for 1 hour in hydrogen. It is noticeable that this catalyst has no ability to hydrosplit the C—C bond. 

The dehydration and calcination of the precipitate from SHCP gives anhydrous silicate or aluminate which itself has little catalytic activity for hydrogenation in general. It is, however, gradually decomposed and reduced to a small extent in a stream of hydrogen at 500-600° and exhibits mild catalytic activity, i.e., a very high selective activity for the hydrogenation of the benzene nucleus at low temperature. 

The palladium-exchanged catalyst has the ability to promote the hydrogenation of the benzene nucleus and no ability to decompose the C—C bond in the cyclohexane produced. The rate of hydrogenation of benzene is proportional to the first power of the partial pressure of hydrogen, Ph. and is independent of that of benzene. 

The piu ified activated charcoal, whose BET surface area amounted to 1140 m2/gm, was soaked with a dilute solution of PdCl2-2 HCl, left for several hours, and then filtered and washed completely. It was confirmed that all palladium ions were completely adsorbed on the charcoal and all chloride ions were completely washed out from the charcoal. The catalyst thus prepared exhibited an excellent catalytic activity for the liquid-phase hydrogenation of the benzene nucleus at room temperature without preliminary reduction with hydrogen. This catalyst, furthermore, after evacuation for several hours at 200-300°,... 

PaHadiumammine complex cations exchange-adsorbed on the surface of active charcoal (AC) were decomposed under 150° in the air and then were reduced easily with hydrogen at low temperature. Palladium metal particles were dispersed very finely and homogeneously on the surface of AC and again exhibited a very high selective activity for the hydrogenation of the benzene nucleus. 

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