A composition for preparing aryl carboxylic acid; said composition comprising: i. at least one ionic liquid of formula I, in an amount of 0.05 to 50 % of the total mass of the composition; wherein, Ar is substituted or un-substituted aryl group, Z is (CH 2) n, wherein n is an integer from 0 to 10, Q + is an organic cation selected from the group consisting of quaternary ammonium, cholinium, sulfonium, phosphonium, guanidinium, imidazolium, pyridinium, pyrrolidinium, morpholinium, quinolinium, isoquinolium, pyrazolium and piperidinium; R is C 1 to C 20 linear or branched alkyl group or Ar-Z, A - is an anion selected from the group consisting of chlorides, bromides, fluorides, iodides, mesylates, tosylates, sulfates, alkyl sulfonates, phosphates, phosphonates, alkyl phosphates, nitrates, nitrites, carbonates, acetates, bicarbonates, hydroxides and oxides; ii. at least one catalyst, in an amount of 0.01 to 5 % of the total mass of the composition, wherein the catalyst comprises at least one metal compound, the metal being selected from the group consisting of cobalt, magnesium, chromium, copper, nickel, vanadium, iron, molybdenum, tin, cerium, zirconium, cesium and titanium; iii. at least one non-oxidizable polar solvent selected from the group consisting of acetic acid, methyl acetate, ethyl acetate, propyl acetate, benzyl acetate, and water; and iv. at least one oxidizable compound selected from a multi-alkylated arylene compound with no two successive ring positions bearing alkyl groups, toluene, and methylnaphthalene, said composition when used in the preparation of aryl carboxylic acid results in the formation of less than 4000 ppm of partially oxidized derivatives of said multi-alkylated arylene compound, toluene, and methylnaphthalene; wherein said derivatives are selected from p-tolualdehyde, m-tolualdehyde, naphthalene carboxaldehyde, benzaldehyde and partially oxidized derivatives of said multi-alkylated arylene compound comprising at least one carboxylic acid moiety and at least one aldehyde moiety. i. at least one ionic liquid of formula I, in an amount of 0.05 to 50 % of the total mass of the composition; wherein, Ar is substituted or un-substituted aryl group, Z is (CH 2) n, wherein n is an integer from 0 to 10, Q + is an organic cation selected from the group consisting of quaternary ammonium, cholinium, sulfonium, phosphonium, guanidinium, imidazolium, pyridinium, pyrrolidinium, morpholinium, quinolinium, isoquinolium, pyrazolium and piperidinium; R is C 1 to C 20 linear or branched alkyl group or Ar-Z, A - is an anion selected from the group consisting of chlorides, bromides, fluorides, iodides, mesylates, tosylates, sulfates, alkyl sulfonates, phosphates, phosphonates, alkyl phosphates, nitrates, nitrites, carbonates, acetates, bicarbonates, hydroxides and oxides; Ar is substituted or un-substituted aryl group, Z is (CH 2) n, wherein n is an integer from 0 to 10, Q + is an organic cation selected from the group consisting of quaternary ammonium, cholinium, sulfonium, phosphonium, guanidinium, imidazolium, pyridinium, pyrrolidinium, morpholinium, quinolinium, isoquinolium, pyrazolium and piperidinium; R is C 1 to C 20 linear or branched alkyl group or Ar-Z, A - is an anion selected from the group consisting of chlorides, bromides, fluorides, iodides, mesylates, tosylates, sulfates, alkyl sulfonates, phosphates, phosphonates, alkyl phosphates, nitrates, nitrites, carbonates, acetates, bicarbonates, hydroxides and oxides; ii. at least one catalyst, in an amount of 0.01 to 5 % of the total mass of the composition, wherein the catalyst comprises at least one metal compound, the metal being selected from the group consisting of cobalt, magnesium, chromium, copper, nickel, vanadium, iron, molybdenum, tin, cerium, zirconium, cesium and titanium; iii. at least one non-oxidizable polar solvent selected from the group consisting of acetic acid, methyl acetate, ethyl acetate, propyl acetate, benzyl acetate, and water; and iv. at least one oxidizable compound selected from a multi-alkylated arylene compound with no two successive ring positions bearing alkyl groups, toluene, and methylnaphthalene,