2,4-D
2,4-D (or 2,4-Dichlorophenoxyacetic Acid) is a common systemic herbicide used in the control of broadleaf weeds. It is the third most widely used herbicide in North America and the most widely used herbicide in the world. 2,4-D was introduced in 1946, when it was the first commercially successful selective herbicide. It allowed for greatly enhanced weed control in wheat, maize (corn), rice, and similar cereal grass crops.
Uses today include:
- weed control in lawns and other turf
- no-till burndown
- control of weeds and brush along fences and highway and railroad rights of way
- conifer release, that is, control of broadleaf trees in conifer plantings
- grass hayfields and pastures
- cereal grains
- Occasionally, in corn and sorghum.
2,4-D is sold in various formulations under a wide variety of brand names. It continues to be used because of its low cost despite the availability of more selective, more effective, safer products.
Physical Properties
Note: properties given are for the acid. Commercial formulations used today use the trimethylamine salt or one of several esters, for which properties differ.
Relative molecular mass 221.04
Appearance: Colourless, white to yellow powder when pure
Melting point 140.5 C
Water Solubility: 900 mg/L @ 25 C
Relative vapour density 7.6
Vapour pressure: 0.02 mPa @ 25 C, 52.3 Pa @ 160 C
CAS Number: 94-75-7
Toxicity
The LD50 in rats has been reported as 375 and 666 mg/kg in two different studies. Single oral doses of 5 and 30 mg/kg body weight did not cause any acute toxic effects in human volunteers.
The amine salt formulations can cause irreversible eye damage.
Manufacture
2,4-D is a member of the chlorination of phenol. The production process creates several contaminants including isomers, monochlorophenol, and other polychorophenols and their acids.
The product Agent Orange, used extensively throughout the Vietnam War, contained 2,4-D. However, the controversies associated with the use of Agent Orange were associated with a contaminant (dioxin) in the 2,4,5-T component.
