3.25.97 | herbintro.html
Introduction to Herbicides
Chemical Families
One of the most productive ways to study herbicides is by chemical structural
"family". Any system of organizing herbicides is, in some sense, arbitrary. The
ideal way of organizing herbicides is in a way that facilitates learning and retention.
Organizing herbicides by chemical structural family does this. It also provides a
framework to fit new herbicides into as they are introduced.
Recently, there was a documentary on agriculture on Public Television. The narrator told a
story about his young son and where food came from. He was standing in the produce section
of a very large supermarket. Did food come from this store? The son was incredulous at
this suggestion and patiently told his father that food really came from boxes stored in
the back of the store. In some ways, farmers and others who interact with agrichemicals
have the same incomplete awareness of where herbicides originated. Many people associate
herbicides with local dealers, and sales and technical representatives of the companies in
their local areas (cans from the coop?). The real origins of herbicides are far deeper
than that. If an understanding of the science of herbicides is to be obtained, the
historical, economic and social roots of herbicides must be understood.
Chemical Roots
There are several chemical antecedents shared by most of the herbicides we have today.
These roots are in acetic acid, ammonia and coal tar. In the early part of the 20th
century, the Haber process of atmospheric nitrogen fixation to produce ammonia for
fertilizers and explosives was also developed. Ammoniacal forms of fixed nitrogen (-NH3)
and urea (H2N-CO-NH2) are common forms of nitrogen fertilizers. Some herbicides are
structurally related to these chemicals. Of far more ancient origin is the domestic
synthesis of ethanol (CH2OH) in many alcoholic beverages. If left in the air too long,
ethanol is oxidized to form acetic acid, or vinegar: CH2 COOH. Some herbicides are
structurally related to acetic acid.
Organic Chemical Industry History
Where did herbicides come from? Herbicides were a logical outgrowth of the development
of the organic chemical industry that has its roots in the 19th century. In a sense, we
owe today's herbicides to two famous people of the 1880's in England: Queen Victoria and
Jack the Ripper. There is a very poor slum district in the east side of London, England
known as Whitechapel. In the 1880's, a series of brutal murders of women were committed
there. Legend has it that they were committed by Jack the Ripper, a fact never confirmed.
In any case, these murders alarmed Queen Victoria. She was counciled that they were
impossible to prevent due to the very dark alleys and streets of that part of the city.
There was a need for a good municipal street lighting system. Unfortunately, this was
before the advent of electrical lighting, and the Queen wanted a solution. As a result, a
lighting system was constructed in the city of London utilizing coal gas, a by-product of
soft coal (bituminous) gasification:
(soft coal) + (12000°) + (no air) = coal gas + coal tar
The gas was piped throughout the city for lighting, and the coal tar remained as a waste
product of the process. The coal tar accumulated in large heaps and became a major
problem. The chemical composition of coal tar is highly varied, but the major constituents
are:
[add: phenol; toluene; pyridine]
Some herbicides are structurally related to the phenols, toluene and pyridine. German
chemists were the first to realize the potential commercial value of coal tar. Until that
time, the clothing dye industry of the world was based on natural products. The Germans
experimented with the constituents of coal tar and their discoverys lead to the founding
of the synthetic dye industry. Soon after that the use of these organic derivatives for
modern explosives was discovered. An example is TNT, trinitrotoluene:
[add: TNT]
Certainly, a lot of the mass carnage of the First World War was made possible with the
modern rifles and artillery that relied on these more effective explosives. One of the
first chemical companies in the world was a German company known before World War II as
I.G. Farben. The history of this company is fascinating and it's story is the story of the
organic chemical industry. The power this company held can be seen in that it was not
broken up after the First World War as part of the Treaty of Versailles, despite its key
role in supplying the German army with munitions. After the Second World War it was
finally dismantled into several separate chemical companies including BASF, Hoechst and
Bayer.
During WWII I.G. Farben was actively involved in the Nazi war effort. They kept the German
army running, even when their supplies of petroleum and rubber were cut off in the later
part of the war. They accomplished this by using coal, of which there were ample supplies
in Germany itself, for the production of synthetic gasoline and synthetic rubber for
tires. I.G. Farben ran many of the Nazi death camps for their labor to construct, and run,
the factories associated with these camps. Many of the insecticides we have today,
including the organo-phoshate pesticides, owe their origins to chemical warfare agents and
chemicals used for mass genocide in concentration camps in the Nazi Holocaust of WWII. The
legacy of coal tar, and other chemicals, was to provide the basis upon which our modern
herbicides are synthesized. Natural products with herbicidal activity, allelochemicals,
are far more complex than those of conventional herbicides.
Chemical Research led to Herbicides
Many other important chemicals were developed in the early part of the 20th century.
These include Bakelite, an early form of plastic. All of the chemical products discovered
provided the commercial incentive that stimulated intensive research for new and novel
chemicals. This search lead inevitably to the discovery of the first selective herbicides.
We will start our story, then, with the first selective commercial herbicide 2,4-D.
