Structure Systematics of Auxin Herbicides. XXX* The Crystal Structures of (2-1sopropylphenoxy)- acetic Acid, (4-F1uorophenoxy)acetic Acid,

The crystal structures of four analogues of the auxin herbicide series, including inactive examples, have been determined by X-ray diffraction methods. The compounds (2-isopropy1phenoxy)acetic acid (I), (4-fluorophenoxy)acetic acid (2) and (2,4-dichloro-5-fluorophenoxy)acetic acid (3) are from the phenoxyalkanoic acid series while (indol-3-g1thio)acetic acid (4) is the sulfur analogue of the natural plant hormone indole-3-acetic acid. All examples exist as hydrogen-bonded cyclic dimers [0 . . .0, 2.640(7), 2.62(1), 2.637(4) and 2.665(4) for (1)-(4), respectively]. Compounds (1) and (2) are essentially planar while compounds (3) and (4) have side chains anticlinally related to the ring systems, similar to (2,4-dich1orophenoxy)acetic acid and to indole-3-acetic acid.