Preparation of 2,5-dichlorophenol

2,5-Dichlorophenol is readily and facilely prepared from 2,5-dichloro-4-hydroxybenzenesulfonic acid or salt thereof, said sulfonic acid/sulfonate itself being characteristically produced by hydrolysis of a precursor trichlorobenzenesulfonic acid, by cleaving the sulfo/sulfonate moiety therefrom by treatment, e.g., under reflux, with a constant boiling point hydrobromic acid.

BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
The present invention relates to the preparation of 2,5-dichlorophenol, 
and, more especially, to the preparation of 2,5-dichlorophenol from 
1,2,4-trichlorobenzene. 
2. Description of the Prior Art: 
2,5-Dichlorophenol is a known valuable compound, particularly useful as an 
intermediate in the production of a wide variety of crop chemicals for, 
e.g., crop and plant protection. 
SUMMARY OF THE INVENTION 
It has now surprisingly been found, and which is a major object of the 
present invention, that a high purity (e.g., but little byproduct), high 
quality 2,5-dichlorophenol is readily and facilely prepared by (i) first 
sulfonating 1,2,4-trichlorobenzene into 2,4,5-trichlorobenzenesulfonic 
acid, (ii) next hydrolyzing said 2,4,5-trichlorobenzenesulfonic acid by 
known technique to give 2,5-dichloro-4-hydroxybenzenesulfonic acid or salt 
thereof, and then (iii) removing the sulfo or sulfonate moiety therefrom 
by treatment with constant boiling hydrobromic acid, advantageously under 
reflux. 
DETAILED DESCRIPTION OF THE INVENTION 
More particularly according to the present invention, it has now 
surprisingly been found that removal of the sulfo group is most readily 
and effectively carried out by treating the precursor sulfonic acid with 
an excess of constant boiling point hydrobromic acid (about 48 percent 
strength). Heretofore, the sulfonic acid function, or sulfo group, was 
typically cleaved under extreme conditions of high temperature, mandating 
reaction in an autoclave, the inner wall members of which were easily 
damaged (by sulfuric acid, phosphoric acid), or which otherwise did not 
produce a satisfactory result (hydrochloric acid). But by the use of 
constant boiling point hydrobromic acid consistent herewith, it is 
unexpectedly found that the sulfo group cleavage reaction need not be 
carried out in an autoclave, but only under reflux at a base temperature 
of about 140.degree. C. (reflux temperature 126.degree. C.). The 
hydrobromic acid employed, moreover, can easily be recovered, and reused 
or recycled.

In order to further illustrate the present invention and the advantages 
thereof, the following specific example is given, it being understood that 
same is intended only as illustrative and in nowise limitative. 
EXAMPLE 
(1) Preparation of 2,4,5-trichlorobenzenesulfonic acid 
119 g (1.0 mol) of technical grade chlorosulfonic acid were added dropwise, 
under stirring, to 218 g (1.2 mol) of technical grade 90% strength 
1,2,4-trichlorobenzene over a period of time of about one hour. The 
reaction was permitted to proceed at 150.degree. to 160.degree. C. for 
three hours. During the reaction a slow stream of dried air or dried 
carbon dioxide was passed through the reaction mixture. The resulting 
hydrogen chloride of reaction was transferred with the gas stream into a 
receiving flask containing water. The hydrochloric acid trapped in the 
receiving flask constituted about 96% of theoretical. The reaction 
mixture, after being cooled to 120.degree. C., was mixed with 200 ml of 
water. Steam was then charged therein to remove excess trichlorobenzene. 
37 g (0.2 mol) of trichlorobenzene were recovered. Water was added to the 
reaction mixture to provide a total volume of 700 g which was then 
titrated with sodium hydroxide solution. 
Total acid: 1.068 mol, 
Sulfuric acid: 0.066 mol (determined as barium sulfate) 
The total yield consequently was 1.00 mol of 2,4,5-trichlorobenzenesulfonic 
acid (262 g). 
(2) Preparation of the sodium salt of 2,5-dichloro-4-hydroxybenzenesulfonic 
acid 
An iron autoclave was charged with 590 g of water, 273 g of 48% strength 
aqueous sodium hydroxide solution (3.3 mol) and 700 g of the 37.4% 
strength aqueous trichlorobenzenesulfonic acid solution obtained in (1). 
The mixture was heated at 180.degree. C. for 5 hours, the pressure being 
7-8 bar. 
Upon completion of the reaction, the disodium salt of 
2,5-dichloro-4-hydroxybenzenesulfonic acid was present, which, upon 
addition of dilute sulfuric acid, liberated sodium 
2,5-dichloro-4-hydroxybenzenesulfonate. 
The yield was about 98% of the theoretical. 
(3) Preparation of the 2,5-dichlorophenol 
139 g (0.5 mol) of sodium 2,5-dichloro-4-hydroxybenzenesulfonate were added 
to 400 ml of approximately 48% strength aqueous hydrobromic acid. The 
mixture was stirred under reflux for 4 hours, during which the base 
temperature was about 140.degree. C. and the reflux temperature about 
126.degree. C. The solution which was still warm, about 60.degree. C., was 
then stirred with 300 ml of toluene. The toluene phase was separated off, 
dried and evaporated to dryness. The residue consisted of 70 g of crude 
2,5-dichlorophenol (86% of theoretical). 
Analysis: calculated Cl, 43.5% found Cl, 43.2% 
As the solution containing hydrogen bromide cooled, a slight amount of 
sodium bromide precipitated. The hydrogen bromide solution could be 
reused. 
While the invention has been described in terms of various preferred 
embodiments, the skilled artisan will appreciate that various 
modifications, substitutions, omissions, and changes may be made without 
departing from the spirit thereof. Accordingly, it is intended that the 
scope of the present invention be limited solely by the scope of the 
following claims, including equivalents thereof.