NOVELTY - Preparing 2-oxindole compound (I), comprises: preparing first slurry comprising inorganic base in an organic solvent; adding substituted nitro-benzene compound (III) and malonic ester derivative (IIIA) to first slurry to obtain substituted 2-(2-nitrophenyl)-malonic ester derivative (II); contacting (II) with solvent to obtain second slurry and adding metal and acid to obtain (I) and water-insoluble impurity (A1); adding mineral acid to second slurry to convert (A1) into water-soluble metal compound (A2); isolating (I) free of (A1) and (A2) from second slurry; purifying; and isolating pure (I).    USE - The process is useful for preparing (I) which is 6-chloro-2-oxindole (claimed) that is used as an intermediate for preparing ziprasidone.    ADVANTAGE - The process: utilizes mineral acid and provides (I) having sulfated ash content of less than 0.1% and HPLC assay of more than 97.5%, preferably more than 98%, with high yields; is simple, efficient, economical, industrially scalable and non-hazardous; utilizes high purity (II) that improves the yield and purity of (I) by avoiding the formation of other impurities; utilizes metal/acid reduction that minimizes the formation of dehalogenated product as impurity; and reduces process unit operations.    DETAILED DESCRIPTION - Preparing 2-oxindole compounds of formula (I), comprises: preparing a first slurry comprising inorganic base in an organic solvent; adding substituted nitro-benzene compound of formula (III) and malonic ester derivative of formula (R1-O-C(=O)-CH2-C(=O)-O-R2) (IIIA) to the first slurry to obtain substituted 2-(2-nitro-phenyl)-malonic ester derivative of formula (II); contacting (II) with a solvent to obtain a second slurry and adding a metal (M) and an acid of formula (HX1) to obtain (I) and water insoluble impurity of formula (M(OH)X1) (A1); adding mineral acid to the second slurry to convert (A1) into corresponding water soluble metal compound of formula (MX1n) (A2) that gets removed during processing of reaction mass; isolating (I) free from (A1) and (A2) from the second slurry; purifying; and isolating pure (I).    R = H, optionally cyclic alkyl, aryl (optionally substituted), heteroaryl, haloalkyl (CF3), alkoxy, haloalkoxy, thioalkyl or halo (preferably Cl);    X = halo;    R1, R2 = optionally cyclic 1-4C alkyl, preferably CH3;    M = metal;    X1 = acid moiety; and    n = equivalent to the valency of M.    An INDEPENDENT CLAIM is also included for substantially pure (I) with sulfated ash less than 0.1% and HPLC assay more than 98%.