Phenol sulphonation process

At one time the requirement for phenol (melting point 41°C), eould be met by distillation of eoal tar and subsequent treatment of the middle oil with eaustic soda to extraet the phenols. Such tar acid distillation products, sometimes containing up to 20% o-cresol, are still used in resin manufacture but the bulk of phenol available today is obtained synthetically from benzene or other chemicals by such processes as the sulphonation process, the Raschig process and the cumene process. Synthetic phenol is a purer product and thus has the advantage of giving rise to less variability in the condensation reactions. 

In the sulphonation process vaporised benzene is forced through a mist of sulphuric acid at 100-120°C and the benzene sulphonic acid formed is neutralised with soda ash to produce benzene sodium sulphonate. This is fused with a 25-30% excess of caustic soda at 300-400°C. The sodium phenate obtained is treated with sulphuric acid and the phenol produced is distilled with steam (Figure 23.1). 

Marqueyrol and Loriette [1] investigated the process of phenol sulphonation with varying sulphuric acid concentration, time and temperature of sulphonation. Their results are presented below (Table 117). 

The second stage in the preparation of picric acid is the nitration of the phenol-sulphonic acid obtained in the first stage of the process. 

The sulphonation process is the oldest (1890) of the commercial phenol syntheses. Whilst it is still in use, it is essentially obsolete because of the requirement of auxiliary chemicals and formation of valueless by-products. 

Today the sulphonation route is somewhat uneconomic and largely replaced by newer routes. Processes involving chlorination, such as the Raschig process, are used on a large scale commercially. A vapour phase reaction between benzene and hydrocholoric acid is carried out in the presence of catalysts such as an aluminium hydroxide-copper salt complex. Monochlorobenzene is formed and this is hydrolysed to phenol with water in the presence of catalysts at about 450°C, at the same time regenerating the hydrochloric acid. The phenol formed is extracted with benzene, separated from the latter by fractional distillation and purified by vacuum distillation. In recent years developments in this process have reduced the amount of by-product dichlorobenzene formed and also considerably increased the output rates. 

The process is effected in two stages. Phenol is first sulphonated and then the sulphonation product is nitrated with nitric acid (p. 126). 

For the first stage of the process, i.e. sulphonation of phenol, sulphuric acid is used in some excess (4-4.5 moles of sulphuric acid per 1 mole of phenol). Initially a sulphuric acid ester is formed ... 

The spent acid from the nitration of phenol by the methods described contains several by-products, among them 2,4-dinitrophenol-6-sulphonic acid in the proportions of 22 parts per 100 parts of phenol used for the process, which corresponds to a 8% loss of the phenol, and oxalic acid in the proportion of 5-6 parts per 100 parts of phenol. These are the principal by-products that lower the yield of picric acid. 

The first step of the process - sulphonation of phenol - is effected in an iron sulphonator, equipped with a jacket, a heating coil and a stirrer. The construction of the sulphonator is in principle similar to that applied in Great Britain, as described above (Fig. 112, p. 506). 

Chloro-l-hydroxynaphthalene is converted into the sulphonate 246 on eosin-sensitized irradiation in the presence of sodium sulphite. A study of the chain substitution of the chloro group in 4-chloro-l-hydroxynaphthalene by aqueous sodium sulphite has shown that two mechanisms for the photoinitiation have been identified and two intermediates have been detected a radical anion of 4-chloro-l-naphthoxide and the sulphite radical anion. Thus, an SjjajI mechanism is suggested and is one that involves reaction with the radical anion of sulphite. An example of the S n 1 process between a phenol and the (2-cyanoaryl)azo-f-butylsulphides has been reported. The 1 reactivity of several compounds (Scheme 27) have demonstrated that 247 is a product however, this is also photochemically reactive and is converted into the cyclic ether 248 . 

Phenolic sulphoxides and sulphones are formed in the transformation process of thiobisphenolic antioxidants (Chap. IV B). Sulphoxide restores the antioxidant capacity though to a lesser extent than that of sulphide266,267) while sulphone has no antioxidant properties. However, corresponding phenoxyls are formed from both these compounds22,260) they are less stable than phenoxyls from the starting sul-... 

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