FIELD OF THE INVENTION:

The present invention relates to an industrially scalable process for the preparation of benzoic anhydride.

BACKGROUND OF THE INVENTION:

Anhydride is a compound formed by the abstraction of a molecule of water from a substance. Organic anhydrides are formed by the condensation of original acids. Anhydrides are more reactive than the parent acids. Anhydrides are used as synthetic intermediates for the synthesis of other organic members such as esters, amides for the industrial applications includes dyes, pharmaceuticals, pesticides, fibers, curing agents, plasticizer and many others. Benzoic anhydride (Formula-I) is an important intermediate which is used as a benzoylating agent in unusual condition where benzoyl chloride is unsatisfactory because of its excessive reactivity or generation of hydrochloric acid. It is also used as a component in bleaching formulations and as an ingredient in soldering flux.

US 4562010 discloses the process for the preparation of benzoic anhydride by reacting benzoic acid with benzotrichloride by heating about 140°C to 150°C for 5 hrs followed by introducing an inert gas for 5 hrs. The significant drawback of the said process involves use of carcinogenic reagent and the reaction temperature is very high which may cause decomposition of product. Hence the said process is not industrially feasible.

German patent 368,340 discloses method for preparing benzoic anhydride by reacting benzotrichloride with sodium benzoate at temperature from 160°C to 170°C followed by separating benzoic anhydride from the salt formed. The major disadvantage of the above said process is separation technique. The obtained salts are finely divided form which is difficult to filter. Moreover distillation of benzoic anhydride at about 200°C and vacuum requires special apparatuses. Thus the process is uneconomical and tedious.

Bull. Chem.Soc. Ethiop. 2011, 25(2), 255-262 describes procedure for the synthesis of
symmetrical carboxylic anhydrides from carboxylic acids by reacting carboxylic acid and
r- A.REG-AOOO- in presence, &f,sulphated .zircon ia -.catalyst „to-, get, ^symmetrical carboxylic

anhydride. The sulphated zirconia catalyst is very expensive, difficult to prepare, hazardous to environment as zirconium is rare metal as well as is radioactive. Hence the above said process is impractical to perform at industry level.

Indian Journal of Chemistry vol. 38B, Jan. 1999, PP. 1-3 describes synthesis of symmetric and unsymmetric acid anhydrides using phase transfer catalysis by reacting potassium salt of carboxylic acid with acid chloride in chloroform solution in presence of tri methyl benzyl ammoniumchloride at RT for 2 hrs to get carboxylic acid anhydrides. Moreover the crude product was purified by hexane, hexane-benzene, and benzene solvents. The main drawback of reported prior art processes is use of costly catalysts such as triethylbenzylammoniumchloride, PEG-1000 makes the process uneconomical. Another disadvantage is use of hazardous, carcinogenic solvent benzene which is not ecofriendly.

Organic Process Research and Development 2000, 4, 141-146 discloses synthesis of benzoic anhydride by using clay supported liquid-liquid-solid phase transfer catalyst involves reaction of benzoylchloride in chloroform with benzoic acid, sodium hydroxide in presence of clay-supported PTC to get benzoic anhydride. Use of clay-supported PTC is major disadvantage of above said process. The preparation of catalyst is expensive, lengthy and requires suitable apparatus, environment. Hence the said method is not commercially compatible. Therefore it is required to develop an efficient, advantageous and economical process for preparing benzoic anhydride. The present invention discloses the industrially feasible, economical, ecofriendly insitu process for preparing benzoic anhydride from benzoic acid by using bi-phase condition. Phase transfer catalysis (PTC) is a versatile synthetic technique that has been widely applied to various organic synthesis. PTC can make reaction condition gentle, reduces consumption of organic solvent and raw materials and enhance the efficiency of organic synthesis. Moreover PTC can effectively avoid side reaction to occur. The major advantage of the present invention is insitu process which reduces work up steps and time results in higher yield with higher purity.

SUMMARY OF THE INVENTION:

The present invention relates to the insitu process for preparation of benzoic anhydride of formula-1.

Formula-I

In one aspect, the present invention relates to the insitu process for preparation of benzoic anhydride of formula-I comprising the steps of;

(a) Reacting benzoic acid with benzoylchloride in presence of suitable base, solvent, catalyst and water

(b) obtaining benzoic anhydride

DETAIL DESCRIPTION OF THE INVENTION:

The present invention relates to an advantageous insitu process for the preparation of benzoic
anhydride of formula-I.

o o

Formula-I

In one embodiment, the present invention relates to the insitu process for preparation of benzoic anhydride of formula-I comprising the steps of;

(a) reacting benzoic acid with benzoylchloride in presence of suitable base, solvent, catalyst and water at ambient temperature,

(b) isolating benzoic anhydride in presence of solvent at cooling temperature.
The process for preparing benzyl anhydride depicted in the following reaction scheme-I

Formula-I I Formula-Ill water Formula-I
Scheme-I

The suitable solvent used in step (a) is selected from the group consisting of non-polar solvent, non-polar halogenated solvent, ester solvent. The non-polar solvent is selected from toluene, xylene, mixture of xylene or mixture of two or more solvents. The non-polar JhaJogenated Tsolvent ris_, .^elected Tfrora ndiphlor.omethane, 7 chloroform, -^dichloroethane,

carbontetrachloride or mixture of two or more solvent. The ester solvent is selected from ethylacetate, isopropyl ester, methyl acetate or mixture of two or more solvents.

The base used in step (a) is inorganic base..The inorganic base is selected from carbonate salts of alkali and alkaline earth metals like potassium carbonate, potassium bicarbonate, sodium carbonate, sodium bicarbonate, cesium carbonate, hydroxides like sodium hydroxide, potassium hydroxide, lithium hydroxide.

The catalyst used in step (a) is phase transfer catalysts selected from tetra-n-butyl ammonium
bromide, tetra-n-butyl ammonium chloride, tetra-n-butyl ammonium iodide, methyl tributyl
ammonium chloride, methyl tributyl ammonium fluoride, terra butyl ammonium hydrogen
sulphate, triethyl benzyl ammonium chloride, tetrabutyl phosphonium bromide.
The reaction temperature for the above said process is 25-30°C.

The US 4,562,010 process discloses use of an inert gas (nitrogen gas) to balance quantity of
hydrochloric acid gas produced during reaction.
The main object of the present invention over the teachings of prior art is there is no
evolution of any toxic or hazardous gas. Hence not any inert gas is required for the present
invention process which makes the process simple, cost effective and eco-friendly.
German patent 368,340 discloses the extraction method for benzoic anhydride from reaction
mixture requires large amount of solvent.

The advantage of the present invention is no any further extraction required in the process
which makes the process economical and industrially feasible. Moreover the present
invention does not require further purification. By end of the process one can directly get
pure (>99% purity) compound.

The following examples explain various other embodiments without limiting the scope of the
present invention.

Example-1: preparation of benzoic anhydride

To the Sodium carbonate solution (11.75 g in 30 mL water) added in mixture of 20.0 g
benzoic acid and 20 mL water at room temperature. The reaction mixture was stirred at RT
for 30 min. To the reaction mixture charged 0.5 g TBAB and 20.71 g benzoylchloride in 125
mL DCM and stirred for 2-3 hrs at RT. The organic layer was separated and distilled out
below 40°C. The resultant crude oil precipitated with 40mL cyclohexane and stirred for I hr
at 0-5°C. Filtered at 0-5°C to get 90% benzoic anhydride.
HPLG purity:,99.94%,

Example-2: preparation of benzoic anhydride

To the Sodium carbonate solution (11.75 g in 30 mL w^ter) added in mixture of 20.0 g benzoic acid and 20 mL water at room temperature. The reaction mixture was stirred at RT for 30 min. To the reaction mixture charged 0.5 g TBAB and 20.71 g benzoylchloride in 125 mL Toluene and stirred for 2-3 hrs at RT. The organic layer was separated and distilled out below 40°C. The resultant crude oil precipitated with 40mL cyclohexane and stirred for 1 hr at 0-5°C. Filtered at 0-5°C to get 81% benzoic anhydride. HPLC purity: 99.53%
Example-3: preparation of benzoic anhydride

To the Sodium carbonate solution (11.75 g in 30 mL w^ter) added in mixture of 20.0 g benzoic acid and 20 mL water at room temperature. The reaction mixture was stirred at RT for 30 min. To the reaction mixture charged 0.5 g TBAB and 20.71 g benzoylchloride in 125 mL chloroform and stirred for 2-3 hrs at RT. The organic layer was separated and distilled out below 40°C. The resultant crude oil precipitated with 40mL cyclohexane and stirred for 1 hr at 0-5°C. Filtered at 0-5°C to get 87% benzoic anhydride. HPLC purity: 98.91%

ExampIe-4: preparation of benzoic anhydride

To the Potassium carbonate solution (23 g in 60 mL water) added in mixture of 25.0 g benzoic acid and 40 mL water at room temperature. The reaction mixture was stirred at RT for 30 min. To the reaction mixture charged 1.65 g TBAB ajid 25.9 g benzoylchloride in 125 mL DCM and stirred for 2-3 hrs at RT. The organic layer was separated and distilled out below 40°C. The resultant crude oil precipitated with 75mL Pet.ether and stirred for 1 hr at 0-5°C. Filtered at 0-5°C to get 86% benzoic anhydride. HPLC purity: 98.70%

Example-5: preparation of benzoic anhydride

To the sodium hydroxide solution (8.7 g in 60 mL water) atjded in mixture of 25.0 g benzoic acid and 40 mL water at room temperature. The reaction mixture was stirred at RT for 30 min. To the reaction mixture charged 1.65 g TBAB and 55.9g benzoylchloride in 125 mL DCM and stirred for 2-3 hrs at RT. The organic layer was separated and distilled out below

40°C. The resultant crude oil precipitated with 75mL Pet.ether and stirred for 1 hr at 0-5°C. Filtered at 0-5°C to get 88% benzoic anhydride. HPLC purity: 98.12%

Example-6: preparation of benzoic anhydride

To the sodium bicarbonate solution (18 g in 60 mL water) added in mixture of 25.0 g benzoic acid and 40 mL water at room temperature. The reaction mixture was stirred at RT for 30 min. To the reaction mixture charged 0.5 g TBAB and 25.9g benzoylchloride in 125 mL DCM and stirred for 2-3 hrs at RT. The organic layer was separated and distilled out below 40°C. The resultant crude oil precipitated with 75mL Pet.ether and stirred for 1 hr at 0-5°C. Filtered at 0-5°C to get 82% benzoic anhydride. HPLC purity: 98.56%