Aniline Formation

in part, may be a reflection of the fact that an arylnitrene prefers to abstract hydrogen from starting azide rather than from the solvent. In the one reaction studied, thermolysis of phenyl azide-d in pentane gives two main products, aniline (containing 40% N-D) and iV-pentylanilines (containing 18% N-D). Even more surprisingly, thermolysis of phenyl azide in MejCD affords aniline containing only 7% of N-D. 

Formal dimerization of arylnitrenes to form azo compounds is not as ubiquitous as hydrogen abstraction (to form amines) but it frequently accompanies desired processes and occasionally predominates. Amounts of azo compound 

The mechanism of this reaction has yet to be established. Smith (1969) reviewed the possible pathways in some detail, so only the main contenders will be considered here. Attack by a thermolytically or photolytically generated nitrene on undecomposed azide is statistically much more probable than simple dimerization of two nitrenes. Experiments where arylnitrenes have been generated in the presence of an undecomposed azide lend no support for this route however. l-Methyl-3,5-diphenyl-4-azidopyrazole was decomposed at 80°C in the presence of / -anisyl azide (which is stable at 80°C) in decalin but no crossed azo product was detected. In another experiment, phenyl azide was recovered almost quantitatively from the triethylphosphite deoxygenation of nitrosobenzene in the presence of this azide.  

An intramolecular version of this process, the photolytic conversion of 2,2 -diazidobiphenyl (36) to benzo[c]cinnoline, is also an unfavored reaction when carried out at room temperature. 4-Azidocarbazole is the main product but photolysis of 36 in a glass at 77 K gives a quantitative yield of benzo[c]cinnoline. Presumably conformational effects play a crucial role in this reaction but it should be pointed out that nitrene intermediacy has not been confirmed. 

The second mechanism worthy of consideration involves initial abstraction of one hydrogen by triplet nitrene to form an anilino radical which may subsequently either dimerize or abstract another hydrogen. This is illustrated for the thermal decomposition of p-anisyl azide in cumene. Indirect support for the hydrazo intermediate 37 comes from the observation that thermolysis of phenyl azide in decalin affords hydrazobenzene and benzidine as well as azobenzene and aniline. Anilino radical intermediates also are implicated in the oxidation 

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This behaviour was rationalised by a stepwise reduction mechanism, in which a high catalyst or KOH concentration gives a high hydride concentration and leads to the aniline formation and suppression of intermolecular reactions to the dimeric azo-compound. 

Klehr argued against this thiolytic cleavage of sulfenamides, as he did not observe accelerated aniline formation from Ph—NH—SR in the presence of 1-thioglycerol24,38. However,... 

The photoreduction of nitrobenzene using p5o ex filtered light from a medium pressure mercury arc was studied in petroleum, toluene, ether, 2-propanol, tert-butyl alcohol, diethylamine, triethylamine, aqueous solutions of 2-propanol and diethylamine and also in aqueous t-butylalcohol containing sodium boro-hydiide 3 >. Varying amounts of aniline, azo- and azoxybenzene were obtained. In the presence of a fourty-fold excess of benzophenone, a six-fold increase in the rate of aniline formation in ethereal solution was observed, and aniline formation was completely suppressed by addition of biacetyl or octafluomaphthalene Since unreacted nitrobenzene could be recovered in these experiments, it is demonstrated that the triplet state of nitrobenzene was quenched. 

Boron trifluoride etherate was used in conjunction with the reducing agent borane to rearrange aromatic O-triisopropylsilyl ketoximes to cyclic and acyclic aniline derivatives. The steric hindrance of the substituents on the silicon atom, the size of the aliphatic ring and the presence of alkoxy substituents on the aryl group played important roles in the aniline formation. 

The reactions of chlorobenzene and benzaldehyde with ammonia over metal Y zeolites have been studied by a pulse technique. For aniline formation from the reaction of chlorobenzene and ammonia, the transition metal forms of Y zeolites show good activity, but alkali and alkaline earth metal forms do not. For CuY, the main products are aniline and benzene. The order of catalytic activity of the metal ions isCu> Ni > Zn> Cr> Co > Cd > Mn > Mg, Ca, Na 0. This order has no relation to the order of electrostatic potential or ionic radius, but is closely related to the order of electronegativity or ammine complex formation constant of metal cations. For benzonitrile formation from benzaldehyde and ammonia, every cation form of Y zeolite shows high activity. 

To manufacture aniline from chlorobenzene and ammonia, cuprous oxide or diamino cuprous chloride has been used as the catalyst and the reaction is usually carried out in the liquid phase under pressure (7). There are few reports on the reaction in gas phase. Jones (8) found that CuX was active for aniline formation while ZnX led to the formation of dichlorobenzenes. The reaction of benzaldehyde with ammonia over zeolite has never been reported. 

Reaction of Chlorobenzene with Ammonia. Preliminary experiments showed that chlorobenzene did not show any reaction over zeolites in a helium stream and that CuY zeolites have good activity for aniline formation. Therefore, the reaction over CuY was studied in detail. 

The catalytic activity for the aniline formation from chlorobenzene and ammonia of the Y zeolites with various cations was studied at 395° C (Table I). It is clear that the transition metal-exchanged zeolites have the catalytic activity for the reaction, while alkali metal and alkaline earth metal zeolites do not. The fact that alkaline earth metal-exchanged zeolites usually have high activity for carbonium ion-type reactions denies the possibility that Bronsted acid sites are responsible for the reaction. Thus, catalytic activity of zeolites for this reaction may be caused by the... 

The ionic radius or the electrostatic potential (e/r) is often used as the measure of the polarizing power of the cations (2, 3, 6). For example, Broiisted acidity has a good correlation with these properties (3). The correlation between the catalytic activity in the aniline formation reaction and either ionic radius or electrostatic potential was very poor. 

Figure 2. Catalytic activity for aniline formation as a function of electronegativity and formaiion constant of ammine complex...

This is the same order of the catalytic activity of transition metal in exchanged zeolites for aniline formation. Irving and Williams (15, 16) pointed out also that there is a clear correlation between complex stability and the second ionization potential. As a matter of fact, a good correlation was found between the catalytic activity and the second ionization potential of divalent ions. (We thank the reviewer for pointing out this correlation.)... 

As described above, the catalytic activity of metal ion-exchanged zeolites for aniline formation has a good correlation with electronegativity and with the formation constant of ammine complexes of metal cations. The order of the activity agrees with the Irving-Williams order. These facts give irrefutable evidence that the transition metal cations are the active centers of the reaction. 

Weber (1996) performed additional experimental studies on 4-aminoa-zobenzene (4-AAB). The compound was chosen based on the hypothesis that azo compounds capable of reduction by zero-valent iron and an amino group would allow for determination of surface-reaction occurrence. The compound 4-AAB reduces and forms aniline. The reduction of 4-AAB was performed without cleaning the iron with hydrochloric acid, which is a standard method in most experiments. Complete loss of 4-AAB occurred in 2 hr. When the iron was washed with hydrochloric acid, the reaction occurred so quickly that aniline formation could not be measured. An additional experiment with bound 4-AAB was conducted to determine if the reaction... 

The following four steps outline the mechanism for aniline formation. 

Three years later Elbs1 reverted to the experiments of Gattermann. On repeating the, same he obtained, indeed, the same results, but simultaneously observed that considerable quantities of aniline are always formed besides the p-ami-dophenol. When Elbs used glacial acetic acid as a diluent of the sulphuric acid he found a considerable increase in the yield of p-amidophenol, but the yield of aniline kept apace of that of the latter. If he used a lead in place of a platinum cathode, the reduction was accelerated, being favorable to the aniline formation at the expense of the p-amidophenol. 

Elbs 2 defends the first view. He explains the aniline formation at lead and zinc cathodes in sulphuric-acid solution in the following manner ... 

The nature of hydrogen abstraction process is normally indicated by the fact that the hydrogen-supplying solvent undergoes dehydrogenative coupling to a considerable extent but always in a poorer yield as the aniline formation would predict. Thermolysis of phenylazide 6 in p-xylene 7 affords aniline 8 a) and syn-di-p-tolylethane (P) 61>. Thermolysis of p-methoxyphenylazide (70) in cumene 11) gave the aniline 12 and bicumyl 13) 23>. 

The reaetion of amines with 2,4-dinitrophenyl sulfate can result in the formation of phenol and sulfate ion (by S—O bond fission), or alternatively in the production of A-substituted anilines and hydrogen sulfate ions (by C—O bond fission). Under non-micellar conditions, C—O bond cleavage is the dominating reaetion, while eationie micelles are able to induce complete suppression of aniline formation. This dramatie effect has been explained in terms of a change in the miero-environment of both the reactants and activated complexes through contributions from hydrophobic and electrostatic interactions [407]. 

The vapor phase N-alkylation of aniline with ethanol took place over an H-ZSM-5 catalyst at temperatures between 250° and 500°C. The maximum selectivity for N-monoethylation occurred at the lower temperatures with increasing amounts of diethyl aniline produced at higher reaction temperatures. v/ith montmorillonite catalyst, reaction at 400°C gave a 64% selectivity for N-ethyl aniline formation at 77% conversion. A vanadium exchanged montmorillonite was more active but less selective giving N-ethyl aniline in 48% selectivity and N, N-diethyl aniline in 37% selectivity at 97% conversion.94... 

The relative rates of these reaction sequences were found to depend on the nature of the medium (homogeneous or heterogeneous), rate of addition of water, temperature, reactivity of the amine (formed by the decomposition of the carbamic acid) with the isocyanate, concentration, and other factors. For example, in the reaction with phenyl isocyanate, cold water and heterogeneous medium favored the formation of diaryl urea while with boiling water the main product was aniline. Dilution also favored aniline formation. 

Methylation of aniline forms a range of products varying from N-alkylated to C-alkylated compounds. ALPOs and substituted ALPOs facilitate N-alkylation whereas ZSM-5 catalyse both C- and N-alkylations. ZSM-5 catalyst forms only mono methylated products. Both mono and di methylated products are formed with SAPOs and ALPOs. KY and CsY catalyse only mono methyl aniline formation. Conversions are high with ZSM-5, KY and CsY. Mechanism of reaction is proposed. 

Catalytic hydrogenations of aromatic nitro compounds with a stable hydroxylamine intermediate often have two different kinetic phases hydrogen uptake is rapid up to ca 60 %, then distinctly slower in the second phase. This means that reduction of the hydroxylamine to the aniline, formally a hydrogenolysis, is difficult in these cases. In the presence of the promoters discussed in Section 8.5.4.3, the second phase is less pronounced or disappears. This suggests a mechanism which could be called catalytic by-pass (see Figure 4). Experiments in the absence of hydrogen indicated that the vanadium promoters catalyze the disproportionation to give aniline and the nitroso intermediates that re-enter the catalytic cycle. As a consequence, the hydroxylamine does not accumulate and aniline formation is accelerated. 

Nitration of polystyrene resins followed by reduction with SnCl resulted in aniline formation which was further converted into the corresponding isothiocyanate. This polymer was used as an insoluble reagent for Edwan-degradation. o... 

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