New intermediate for fungicides

The use of the non-chlorophenolic compound 2-(Thiocyanomethyl)-thio benzothiazole (TCMTB) as a fungicide is more common today due to new legislation that now restricts the use of Pentachlorophenol (PCP). In the past, most tanners would have chosen a cheap fungicide as long as it was effective. However, decisions are now being influenced by new legislation and environmental issues. Before, phenolics were the most popular choice, but these are no longer acceptable due to the possibility of the formation of chlorinated dibenzodioxine, a poisonous compound. After the new restrictions were introduced, substituted benzothiazoles became the norm.

BCM can be prepared in various ways, but the new method of production begins by a reaction of excess methylene chloride, that contains a catalytic amount of aluminium, with liquid bromine. The heat of the reaction is generated by refluxing the methylene chloride. An efficient condenser with a water cooling circulation system is necessary to prevent any loss of methylene chloride. Aluminium chloride is a byproduct and can be used in other reactions such as the Friedel Craft.

The basic equation by which BCM may be derived is:

6CH₂Cl₂ + 3Br₂ + 2Al --> 6CH₂ClBr + 2AlCl₃

The CLRI and the Indian National Chemical Laboratory carried out experiments where 102g (1.2 mol) methylene chloride and 5.4g (0.02 mol) aluminium were placed in a three-necked round bottom flask fitted with an efficient stirrer, funnel and a condenser with an efficient cold water circulation system. 48g (0.6g atom) bromine was gradually added through the funnel. The mixture was left to stir for 6-7 hours. The start of the reaction was slow after an initiation period and the temperature was between 25-30°C. Mild refluxing was allowed. The AlCl3 was separated and the crude reaction product distilled by fractional distillation. The distillate contained 50% methylene chloride (40-42°C), 40% bromochloromethane (68-70°C), and 10% methylene bromide (98-100°C). Methylene chloride can be recycled.

The new method of producing bromochloromethane is cleaner and more environmentally-friendly. Raw materials such as methylene chloride are easily available, and aluminium in the form of small turnings is inexpensive. Bromine is also readily available. Care is required to maintain the proportions and the temperature by circulating sufficient cold water to prevent any loss of methylene chloride reflux.

Aluminium is the catalytic agent from which one chlorine atom is taken from the methylene chloride and replaced by bromine. The rate of the principal reaction can be controlled by the rate the aluminium is added. Furthermore, since the methylene chloride is in excess at all times, the tendency for the formation of the byproduct methylene bromide is reduced to a minimum.

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