Atropic acid

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Atropic acid is an Impurity of Ipratropium bromide and Atropine Sulfate. Atropic acid and β,β-dimethylatropic acid are plant growth regulators.

The synthesis of atropic acid involves several steps. Firstly, tropic acid is oxidized with hot KMnO4 to form benzoic acid. Secondly, tropic acid reacted with HBr to produce C9H9O2Br. Thirdly, C9H9O2Br is reacted with alcoholic KOH to yield atropic acid, C9H8O2. Finally, atropic acid is catalytically hydrogenated to form hydratropic acid, C9H10O2.

Atropic acid is formed by the dehydration of tropic acid. Hence addition of water to atropic acid gives tropic acid.
Atropic acid, on oxidation yields benzoic acid. The formation of benzoic acid reveals that atropic acid and tropic acid contain atleast one benzene nucleus with a side chain containing carboxylic acid in their structure.

As atropic acid is an unsaturated acid it mean atropic acid may be either structure (I) or (II) .
Hence, the structure (II) is atropic acid which is confirmed by oxidation with KMnO4 to form phenylglyoxal.