Toluene from p-toluidine

Benzene may be prepared similarly from plienyldiazonium chloride (Section IV,60). 

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Problem 19.28 Prepare (a) 2-bromo-4-hydroxytoluene from toluene, (b) 2-hydroxy-5-methylbenzaIdehyde from p-toluidine, (c) m-methoxyaniline from benzenesulfonic acid. M... 

Gomberg and Pemert4 recommended the use of the sodium diazotates for the preparation of biaryls from aniline and from p-toluidine and the normal diazo method for negatively substituted amines such as the bromo- and nitro-anilines. Grieve and Hey 7 found little difference in the yields of biaryls obtained by the two procedures when they coupled diazotized aniline with six different components benzene, toluene, m-xylene, chlorobenzene, nitrobenzene, and ethyl benzoate. Where it is applicable, this modification offers the advantage that the dropwise addition of alkali is eliminated. 

Toluene from Toluidine.—It is often desirable to obtain tbe hydiocarbon from the base. The process of diazotisntion offers the only convenient method. The diazonium salt may be reduced by alcohol (Reaction 1, p. 162) or, as in the piesent instance, by sodium stannite. Less direct methods are the con-veision of the diazonium compound into (i) the hydrazine (see p. 174), (2) the acid and distillation with lime (p. 200), (3) the halogen derivative and reduction with sodium amalgam, 01, finally (4) the phenol and distillation with zinc dust. 

Similarly, benzene and hydrazoic acid in the presence of a mixture of fluorosulphonic and trifluoromethanesulphonic acid give aniline quantitatively. From toluene 100% of a mixture of o-, m- and p-toluidines is obtained and bromobenzene yields 42% o-bromoaniline and 47% p-bromoaniline, Butyl azide gives iV-butylarylamines under these conditions89. Photolysis of 1-aminoquinolinium perchlorate (76) in aromatic hydrocarbons (benzene, toluene, ethylbenzene or mesitylene) in the presence of 18-crown-6 affords arylamines, e.g. aniline and p-toluidine, by way of the nitrenium ion90 cf Reference 91. 

While the direct halogenation of toluene gives a mixture of isomers that is difficult to separate into the pure isomers, the isomeric o- and p-nitrotoluenes 6a and 6b, formed by nitration, are easy to separate from each other. Thus reduction of the single o- or p-nitrotoluene 6 to the o- or p-toluidine 7a or 7b respectively, followed by conversion into the corresponding diazonium salt 8 and a subsequent Sandmeyer reaction leads to the pure o- or p-halotoluene 9. 

For the preparation of biphenyl compounds with substituents in only one ring, it is usually advisable to employ a substituted aniline and couple with benzene rather than to use aniline and couple with a benzene derivative, since with benzene only a single biaryl compound is produced. Thus, 4-methylbiphenyl can be isolated readily in pure form from the reaction between diazotized p-toluidine and benzene, but not from the mixture of 2- and 4-methylbiphenyl formed from diazotized aniline and toluene. 

Azo compounds are formed to some extent in the biaryl reactions. From the reaction between diazotized p-toluidine and benzene, azo-p-toluene, CH3C6H4N=NC6H4CH3, was isolated, indicating that, in this particular instance at least, the azo compound was derived entirely from the diazonium compound. In addition to these types, of compounds, a considerable amount of high-boiling material is usually formed in the reactions. 

In 1902 Mai discovered that aniline, p-toluidine, and benzidine could be deaminated by reducing their diazonium chlorides with 10% aqueous hypophosphorous acid. Benzene, toluene, and biphenyl were obtained in 40, 67, and 60% yields, respectively. From a-naphthalenediazonium chloride, naphthalene was produced very slowly and in unstated yield.8 The reaction was not investigated further, and the encouraging results that had been obtained did not give rise to widespread use of hypophos-... 

Some idea of the utility of the hydrazine procedure may be inferred from the yields obtained on deaminating the following compounds aniline (45-75%),110 1Ui 118 p-toluidine (55-65%),uo 116 3-chloro-4-amino-toluene (40%),117 5-bromo-6-aminoquinoline ( satisfactory yield ).181 ... 

Haussermann 2 reduced nitrobenzene and nitrotoluenes both in alkaline and acid solution, the former with iron, the latter with platinum, electrodes. By reduction in alkaline solution, he obtained as principal product hydrazobenzene and hydrazo-toluene respectively in sulphuric-acid solution he got from nitrobenzene, as chief products, benzidine sulphate and azoxy-benzene, besides an easily changeable body which was not further determined. o-Nitrotoluene3 under like conditions gave o-tolui-dine sulphate besides small quantities of o-toluidine p-nitro-toluene yielded principally p-toluidine. 

Scheme 10.14. The formation of aminobenzene (aniline) and o- and p-methylaminobenzenes (o- and p-toluidines) from the nitration of a mixture of benzene and methylbenzene (toluene) and subsequent reduction of the mixture with iron (Fe) in acetic acid (CH3CO2H).

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