. The chemistry and mode of action of plant growth substances; proceedings of a symposium held at Wye College, University of London, July 1955. Plant regulators; Auxin; Growth (Plants). Chemical structure and biological activity almost complete loss of activity, both in mono and in poly-substituted compounds. Matell (1953) has also reported that the activity of (4-)-a-(2- naphthyloxy) propionic acid in the flax root growth test is lowered about 600 times when chlorinated in position 1. He also makes the significant comment that the strong antagonistic action of the ( —) form is very little aff
. The chemistry and mode of action of plant growth substances; proceedings of a symposium held at Wye College, University of London, July 1955. Plant regulators; Auxin; Growth (Plants). Chemical structure and biological activity almost complete loss of activity, both in mono and in poly-substituted compounds. Matell (1953) has also reported that the activity of (4-)-a-(2- naphthyloxy) propionic acid in the flax root growth test is lowered about 600 times when chlorinated in position 1. He also makes the significant comment that the strong antagonistic action of the ( —) form is very little affected by chlorination. Positions 6, 7, and 8 are also important for activity, though blocking of these points with chlorine causes a reduction but not complete loss of activity. It is interesting to note that replacement of the chlorine in position 8 by a nitro-group causes a further ten-fold reduction in activity. In the first results of substitution in the 3-position which we now report, it is seen that a chlorine atom here has no detrimental effect on the activity in the acetic acid series, though some reduction in the a-propionic acid series takes place, being part of the gradual decrease in the activity of a-substituted acids already mentioned. An examination of Fischer-Hirschfelder molecular models of chlorinated 2-naphthyloxyacetic acids shows that chlorine atoms in positions 1 and 3 interfere to some extent with the free rotation of the —O • CHgCOOH side- chain, and thus influence the final spatial arrangement of atoms. Although the naphthalene nucleus is planar, large substituent groups such as chlorine may cause a buckling effect to relieve strain, which would destroy planarity, and in certain cases possibly affect the compactness of the molecule. It is unfortunate that the models give little help on this point. Jonsson (1955) does however stress that a planar structure is not the only requirement for bicyclic molecules to possess high activity. The electronic e
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