Phenols distillation with zinc

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. 

Reaction LVIIL (a) Reduction of Phenols and Quinones by Distillation with Zinc Dust. (A., 140, 205.)—When certain aromatic oxygen compounds (phenols, naphthols, quinones, etc.), are heated with zinc dust, they are reduced to the corresponding hydrocarbons. Thus, phenol yields benzene, the naphthols naphthalene while anthracene can be obtained from anthraquinone or its hydroxy derivatives, alizarin, or quinizarin. In this way alizarin was first proved to be an anthracene derivative. (B., 1, 43.) For catalytic reduction of phenols, see C. r. 193, 1023. 

The reduction of phenols by dry distillation with zinc dust is illustrated by the conversion of 9-phenanthrol to phenanthrene (72%). The reaction is seldom of preparative value. 

Hasubanonine has the composition C H OgN, is non-phenolic, contains one carbonyl group and four methoxyl groups. On distillation with zinc-dust it gives phenanthrene, and on oxidation it yields hemipinic acid [90]. 

A phenolic hydroxyl group is removed by distillation with zinc dust, a process that has no preparative importance but can be useful for proof of structure. Phenanthrene is formed in 72% yield when 9-phenanthrol is distilled in hydrogen with zinc dust.484... 

Phenols can be reduced by distillation over zinc dust or with HI and red phosphorus, but these methods are quite poor and are seldom feasible. Catalytic hydrogenation has also been used, but the corresponding cyclohexanol (see 15-14) is a side product. ... 

Distillation of phenols with zinc dust [617] or with dry lithium aluminum hydride [618] also results in hydrogenolysis of phenolic hydroxyls but because the reaction requires very high temperatures hardly any other function in the molecule can survive, and the method has only limited use. 

Phenol containing zinc chloride catalyst has been treated with hydrogen chloride and then at 90°C, with nonene. After reaction, distillation afforded nonylphenol in high yield. The same process was applied to the synthesis of 4-tert-butylphenol (ref.5). 

Pyrocusparine contains no methoxyl group and shows crypto-phenolic properties although insoluble in aqueous alkali it readily dissolves in alcoholic potassium hydroxide (42). Zinc dust distillation of cusparine produces pyridine (39). On fusion with caustic potash, proto-catechuic add is formed (38, 39, 35). 

A solution of 17 g of 4-methoxy-2-(2-methylallyl)phenol in 56 g acetic acid was treated with 8.4 g zinc chloride followed with 28 mL concentrated HCI. This mixture was heated at reflux temperature with a mantle for 1 h. After cooling, this was poured into H20 and extracted with 2x150 mL CH2CI2. The pooled extracts were washed with several portions of 8% NaOH, until the extracts were colorless. The organic fraction was then washed with H20, and the solvent removed to yield 5.8 g of 2,2-dimethyl-5-methoxy-2,3-dihydrobenzofuran as a pale amber oil with a pungent smell. This was purified by distillation, giving a fraction of an off-white oil with a bp of 136-138 °C at 33 mm/Hg. 

Munich,[B] and applied to the analysis of nitro- explosives by M. Chenel, of the Laboratoire Centrale des Poudres, whose method of proeedure is as follows —0.5 gmi. of the finely powdered substance is digested in the cold with a solution of 1.2 gim. of phenol and 0.4 grm. phosphoric anhydride in 30 c.c. of sulphuric acid. The mixture is kept well shaken until the solution is complete. From 3 to 4 grms. of zinc-dust is then cautiously and gradually added, the temperature of the mass being kept down until complete reduction has been effected. Finally, 0.7 grm. of mercury is added, and the process continued in the usual way, according to Kjeldahl that is, the hquid is distilled until all the ammonia has passed over, and is absorbed in the standard acid. The distillate is then titrated with standard ammonia. 

O Baeyer had tried out zinc dust distillation for the first time as a synthetic route to indole. In connection with the structure determination of many natural products, this later proved a valuable method for the transformation of phenols into hydrocarbons. 

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