JPS60258143A - Production of 2,3,5,6-tetrafluorobenzoic acid - Google Patents

Abstract

PURPOSE:To prepare the titled compound useful as a raw material for photosensitive agent, etc. in high yield without passing through an intermediate, by reacting as pentafluorobenzoic acid with zinc in an alkaline aqueous solution within a specific temperature range. CONSTITUTION:Pentafluorobenzoic acid is reacted with zinc in an alkaline aqueous solution containing lithium hydroxide, sodium hydroxide, potassium carbonate or ethylenediamine, within about 20-100 deg.C, preferably 40-70 deg.C temperature range to give the aimed 2,3,5,6-tetrafluorobenzoic acid. The amount of the alkali to be used is preferably 3-5mol based on 1mol pentafluorobenzoic acid, and the amount of the zinc to be used is preferably 1.0-2.5mol based on 1mol pentafluorobenzoic acid.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention comprises reacting pentafluorobenzoic acid with zinc in an alkaline aqueous solution at a temperature of about 20-100°C to produce 2.3.
The present invention relates to a method for producing 5.6-tetrafluorobenzoic acid.

2,3.5.6-tetrafluorobenzoic acid is useful as a raw material for photosensitizers and the like.

A method for synthesizing 2,3.5.6-tetrafluorobenzoic acid is described in J. Chem 8oc., 1965, 634.
It is written on page 1.

The above method involves first reacting pentafluorobenzoic acid with hydrazine to obtain 2,3,5,6-tetrafluoro-4
-Synthesize hydrazinobenzoic acid and then 2.3.5
．． 2,3.5.6-tetrafluorobenzoic acid is obtained by reacting 6-tetrafluoro-4-hydrazinobenzoic acid with an aqueous copper sulfate solution.

However, this method has the drawbacks of extremely low yield and long steps, making it an insufficient production method for industrial implementation.

As a result of intensive studies to improve the above-mentioned drawbacks, the present inventors discovered that 2.3,5.6-2.3,5.6- The present inventors have discovered that tetrafluorobenzoic acid can be produced in high yield from pentafluorobenzoic acid in one step without passing through any intermediates, and have completed the present invention.

In the present invention, the alkaline substances used to prepare the alkaline aqueous solution include alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, potassium hydroxide, or cesium hydroxide, potassium carbonate, and calcium oxide. Any substance that dissociates or changes into an alkaline substance in an aqueous solution can be used. Furthermore, any organic amines such as ethylenediamine and triethylamine, and anything showing alkalinity such as ammonia can be used.

The present inventors estimate that when KOH is used as the alkaline substance, the method of the present invention reacts according to the reaction formula (1) below.

Therefore, the amount of alkali used should theoretically be 2 times or more by mole per 1 mole of pentafluorobenzoic acid, but preferably 3 to 5 times by mole. Although the alkali concentration varies depending on the concentration of pentafluorobenzoic acid to be charged, it is generally desirable to adjust it to 5 to 20% by weight.

Any zinc can be used, such as commercially available metal zinc powder. The amount of zinc to be used should theoretically be 1 mol or more per 1 mol of pe/tafluorobenzoic acid according to the above reaction formula (1), but it is usually 1 mol or more.
．． It is preferable to make it exist in an amount of 0.5 to 2.5 moles.

In the present invention, the reaction can be carried out at a reaction temperature in the range of 20 to 100°C, but the lower the reaction temperature is, the lower the reaction rate is, so the reaction time needs to be longer, resulting in lower productivity. Further, if the reaction temperature is high, a large amount of tetrafluoro-4-hydroxybenzoic acid is produced as a by-product, which is not preferable.

Therefore, it is particularly preferable to conduct the reaction at a temperature in the range of 40 to 70°C. Alternatively, the reaction may be carried out while raising the temperature stepwise from a low temperature to a high temperature. "°□: The reaction time is usually preferably carried out in a range of 5 hours to 48 hours.

After the reaction is completed, solid matter is separated from the product by means such as r-filtration, and then an acidic aqueous solution such as sulfuric acid or hydrochloric acid is added to the aqueous solution to adjust the pH to 1 to 6 to perform so-called acid precipitation 2.3. 5. Generate 6-tetrafluorobenzoic acid.

To separate 2.3.5.6-tetrafluorobenzoic acid from the aqueous solution containing 2,3.5.6-tetrafluorobenzoic acid obtained after acid precipitation, an extraction solvent such as ether, methylene chloride, 2 using benzene, toluene, etc.
, 3,5.6-tetrafluorobenzoic acid is extracted into an organic layer, separated and then evaporated to dryness. If necessary, this product can be further purified by methods such as recrystallization.

Hereinafter, the present invention will be explained in more detail with reference to Examples.
The present invention is not limited to these.

Example 1 Potassium hydroxide 21.2 in a 500 WLl three-necked flask
2 (0,377 mol) was charged, and then 20.0 f (0,377 mol) of pentafluorobenzoic acid was charged.
,0943 mol) and zinc powder 12.49 (0.19 mol)
was added and heated at 60°C for 24 hours with stirring.

Next, after cooling, the solid matter was separated by filtering, and then a 6N aqueous sulfuric acid solution was added dropwise to this mother liquor so that PI (PI) was 1.7 to neutralize it. Benzoic acid was obtained. 200 cc of ether was added to the obtained aqueous solution containing 2,3,5,6-tetrafluorobenzoic acid, and 2.3.5.6-tetrafluorobenzoic acid was extracted from the organic layer.

This extraction operation was repeated three times, and the ether layer was extracted with water.
After washing with OD and evaporating to dryness. The evaporated product was analyzed using liquid chromatography (column packing: ODS system, carrier solvent: methanol/sodium dihydrogen phosphate mixed aqueous solution) using 2,5-dil-benzoic acid as an internal standard. As a result, the desired yield was 81.6 mol% of 2.3.5.6-tetrafluorobenzoic acid and 9.6 mol% of tetrafluoro-4-human tetraxybenzoic acid based on the charged pentafluorobenzoic acid. Substances had changed.

This product was recrystallized using water to give 151 to 152
Pure 2,3.5.6-tetrafluorobenzoic acid with a melting point of .degree. C. was obtained. Elemental analysis using this pure product confirmed that the substance was 2t L 5+ 6-tetrafluorobenzoic acid.

Elemental analysis value C (%) H (regret) F (ri) Theoretical value 43
．． 34 1.03 39.18 Analysis value 42.9 1.
1 38.9

Claims (1)

[Claims] (A process for producing 2,3,5,6-tetrafluorobenzoic acid, which comprises reacting 11-pentafluorobenzoic acid with zinc in an alkaline aqueous solution at a temperature range of about 20 to 100'C. (2) The method according to claim (1), wherein the reaction is carried out at a temperature range of 40 to 70°C.