Phase I - Introduction

Phase I reactions can reduce or modify the phytotoxicity of herbicides, predispose the molecules to Phase II reactions and increase polarity. Therefore, Phase I reactions are considered the primary factor in selectivity. These reactions may also bioactivate the herbicide. Phase I reactions are not synthetic reactions in that the herbicide does not increase in size, but rather this phase is needed to introduce a reactive substituent group not already present. Some functional or reactive groups and their names follow (the hyphen indicates where the group attaches to the remainder of the molecule):

Amide = -CONH2

Amino = -NH2

Carboxylic acid = -COOH,

Cyano = -C=N,

Halogens (chloride, bromide, iodide, fluoride) = -Cl, -Br, -I, -F,

Hydroxyl = -OH,

Methoxy = -OCH3,

Methyl = -CH3,

Methylthio = -SCH3,

Sulfhydryl = -SH

The major reactions involved in Phase I conversions are oxidation, reduction or hydrolysis which form free amino, hydroxyl, or carboxylic acid groups. Many of the products from these reactions are acted on immediately by Phase II reactions as well.

Summary Note:

A Phase I reaction is the first step needed to make a herbicide less toxic; this reaction modifies the molecule so it is more water soluble and ready for Phase II and Phase III reactions which further detoxify the chemical.