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:
[phenol; toluene; pyridine chemical structures]
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:
[TNT chemical structure]
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.