MCPA

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MCPA (2-methyl-4-chlorophenoxy-acetic acid, IUPAC (4-chloro-2-methylphenoxy)acetic acid, C{{sub|9}}H{{sub|9}}ClO{{sub|3}}) is a powerful and selective phenoxy herbicide. The pure compound is brown-colored powder.

History
Synthesis of MCPA was first reported by Synerholme and Zimmerman in 1945 and by Templeman and Foster in 1946. Templeman and Foster were searching for a compounds with similar or greater selective activity than à-napthylacetic acid in inhibiting the growth of weeds while not adversely affecting the growth of cereal grains. They synthesized MCPA from the corresponding phenol by exposing it to chloroacetic acid and dilute base in a straightforward substitution reaction.
 * 2-CH3-4-ClC6H3OH + ClCH2CO2H + base → 2-CH3-4-ClC6H3OCH2CO2H + baseH+Cl-

Chemical use
Because it is inexpensive MCPA is used in various chemical applications. Its carboxylic acid group allows the formation of conjugated complexes with metals (see below). The acid functionality makes MCPA a versatile synthetic intermediate for more complex derivatives.

Commercial use
MCPA is used as an herbicide, generally as its salt or esterified forms. Used thus, it controls broadleaf weeds, including thistle and dock, in cereal crops and pasture. It is selective for plants with broad leaves, and this includes most deciduous trees. Clovers are tolerant at moderate application levels. It is currently classified as a restricted use pesticide. Its toxicity and biodegradation are topics of current research. Though not extremely toxic, it has recently been determined that MCPA can form complexes with metal ions and thereby increase their bioavailability, though there is also work being done to utilize this ability.

Brand names
The following commercial products contain MCPA.
 * Agritox
 * Agroxone
 * Chiptox
 * Chwastox
 * Cornox
 * Methoxone
 * Rhonox
 * Tigrex
 * Verdone Extra (UK)
 * Weed-Rhap
 * Weed'n'Feed
 * Zero Bindii & Clover Weeder (Aus)