Biological activity of 2,4-D esters on rubber vine (Cryptostegia grandiflora): dependence on vapour pressure and molecular weight and isomerism of the alcohol substituent

Abstract

The relationship between molecular structure and biological efficacy was investigated for 16 esters of 2,4-D [(2,4-dich1orophenoxy)acetic acid] on rubber vine (Cryptostegia grandiflora). These included the normal (n) or straight-chain esters from C-1 (methyl) to C-8 (octyl), the n-decyl, n-dodecyl, isobutyl, amyl (iso-pentyl), 2-ethylhexyl (iso-octyl), and the methoxy-, ethoxy-, and butoxyethyl esters. For the normal series esters, biological efficacy was found to be a function of both the molecular weight and the vapour pressure of the esters. This relationship was linear for the higher molecular weight, low volatile esters, biological efficacy decreasing with increasing molecular weight and the accompanying decrease in vapour pressure of the esters. The low molecular weight, volatile esters were more active than the higher molecular weight, low volatile esters, and increases in the vapour pressure of these low molecular weight, volatile esters were sufficient to account for the deviation from linearity of those esters. When all esters are considered, the same relationships hold but the branched-chain (iso) and chemically substituted (alkoxy alcohol) esters are less effective than the corresponding normal esters. Possible reasons for these results are discussed.