As the industrial revolution grasped the United States in the late 1800s, the agricultural revolution

soon followed.  Farmers became more and more critical about their crops, and soon they began

noticing how weeds and other unbeneficial plants interfered in the growing and harvesting of their

crops.  As early as the 1930s, agricultural chemical research had begun and revealed a number of

synthetic organic compounds that were able to regulate plant growth.  Farmers were undoubtedly

impressed with these new compounds, but they weren't the only ones.  The military powers of the

world looked upon these compounds as potential military applications that may prove to be very

beneficial if used during war as defoliants.  The most effective herbicide was 2,4 -

dichlorophenoxyacetic acid, which will be referred to in this text as (2,4-D).  The molecular structure

of 2,4-D follows in figure one.

Figure 1  2,4 - Dichlorophenoxyacetic acid

You may recognize the name 2,4-D as it is used even today as an effective herbicide.  Greater

understanding of the molecular interactions between this compound and other herbicides has provided

requirements that limit how concentrated the herbicide can be when it is sold in today's market.

     During World War II, the United States Army completed defoliant research at Fort Detrick,

Maryland.  With what we know today, Fort Detrick was the first place that

2,4,5-trichlorophenoxyacetic, referred to as (2,4,5-T) in this text, was synthesized.  This defoliant was

regarded as more effective, easier to apply, and safer than any other weed killers.  Once World War II

ended, 2,4,5-T was marketed and sold widely around the world.  Britain even used 2,4,5-T to destroy

enemy crops and cover during their colonial war with Malaysia.  The molecular structure of 2,4,5-T

follows in figure 2 below.
 

Figure 2 2,4,5 - Trichlorophenoxyacetic acid

Compare its structure to that of 2,4-D (figure one) and you will notice that these two molecules differ

by the meta substitution (with respect to the phenoxy functional group) of one additional chlorine

substituent.  This additional chlorine substituent has important implications when a comparison

molecular reactivity is explored.  The ether functional group is a strongly activating ortho, para

director. The terms ortho, para, and meta (o-, p-, m-) are used extensively in organic chemistry to

describe the position of heteroatoms on the benzene ring.  Please refer to the link for a detailed

explanation of these terms and how they relate to benzene regiochemistry.

     In 1960, the U.S. Army tested many herbicides both in the laboratory and in the field.  They were

looking for a herbicide that would defoliate woody and broad-leaved vegetation.  When the U.S.

became more involved in Vietnam in 1964, a compound known today as Agent Orange was conceived.

     Agent Orange was actually a one-to-one mixture of 2,4-D and 2,4,5-T.  Agent Orange was used

extensively during the war to (1) clear paths through the jungle to make the war easier to fight by

improving observation and (2) destroy enemy crops.  Altogether, the years from 1962 to 1971, involved

spraying nearly 19 million gallons of herbicides in Vietnam; at least 11 million gallons were Agent

Orange.  This military project was called Operation Ranch Hand.  In 1969, some 3.25 million gallons

of Agent Orange alone were sprayed on the jungles of Vietnam.

     The toxicity of Agent Orange centers around the by-product that is formed when Agent Orange is

synthesized.  Referred to today as dioxins, they are a group of chlorinated aromatic hydrocarbons that

are formed in trace amounts during production of many chlorinated compounds.  In the case of agent

orange (2,4,5-T), the dioxin; 2,3,7,8 tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) is formed as illustrated

in reaction one below:
 

Reaction 1 Synthesis of Agent Orange and the toxic Dioxin; 2,3,7,8-TCDD   2,4,5-Trichlorophenol      2,4,5-Trichlorophenoxyacetic acid  2,3,7,8-Tetrachlorodibenzo-p-dioxin

Reaction one above also explains a great deal to us when we observe the reaction and tear apart each

intermediate that is formed during the course of the reaction.  The bulk of the product that is formed is

2,4,5-T.

     The dioxin produced in the synthesis of Agent Orange is the infamous molecule; 2,3,7,8-TCDD.

Dioxins are distinguished from other molecules only by the position of the chlorine atoms on the

molecule.  For example, consider the following dioxin; hexachlorophene in figure three below:
 

Figure 3 Hexachlorophene

topical antibacterial agent.  When administered in large doses, however, hexachlorophene has been

shown to cause neurotoxic symptoms in laboratory animals by affecting the myelin (an insulating layer

around axons which aids in the conduction of nerve impulses) of the brain and spinal cord.  Scientists

have now banned hexachlorophene from most uses due to the possible risks associated with its long-

term exposure.

     Hexachlorophene is prepared by the condensation of two (2) moles of 2,4,5-Trichlorophenol with

one (1) mole of formaldehyde in the presence of concentrated sulfuric acid.  Let's take a closer look at

this reaction:
 
 

Reaction 2 Mechanistic Synthesis of Hexachlorophene

In the above reaction, two moles of 2,4,5-Trichlorophenol react stoichiometrically with one mole of

formaldehyde to produce hexachlorophene.  The important step of this reaction is the addition of

concentrated sulfuric acid (H₂SO₄).  In many organic reactions, sulfuric acid aids in directing the

nucleophile in its location of attack.  In this case, sulfuric acid most likely produces a carbocation that

locates itself at the secondary carbon atom denoted by the arrow above.  An intermediate that may

explain this perhaps non-concerted reaction is displayed in the following figure:
 

Figure 4 Proposed Intermediate of 2,4,5-Trichlorophenol

     Dioxins are created unintentionally during the manufacture of chlorine containing products like the

Polychlorinated Byphenal (PCB) oils used for years in the utility transformers that supply power to

homes.  Dioxins are created by burning chlorine containing wastes such as Polyvinyl Chloride (PVC).

Because of this widespread use, dioxins are present, in trace amounts, in the body fat of nearly

everyone in the civilized world.

     The particular dioxin in agent orange (2,3,7,8-TCDD), has been described by some scientists as

"perhaps the most toxic molecule ever synthesized by man."  The structure of TCDD appears in figure

five below.  Note the characteristics of TCDD and compare them to that of hexachlorophene, figure

three above.  What do you see?
 

Figure 5 2,3,7,8 - Tetrachlorodibenzo-p-dioxin (TCDD)

chlorine and are, in fact, tetra-substituted chlorodioxins (contain four substituted chlorine atoms).

Obviously, the dioxins contain two oxygen atoms and contain benzene rings.  Remarkably, TCDD has

a very high molecular mass, 321.97 grams/mole.

     An interesting question now arises.  This question is:  What does the mechanism look like that

describes the formation of 2,4,5-T and the dioxin; 2,3,7,8-TCDD, from reaction with

2,4,5-Trichlorophenol (refer to Reaction 1)?  This is a somewhat difficult question because organic

chemistry can sometimes be very tricky.  For example, all organic reactions produce at least two

products because reactants tend to isomerize and some products tend to rearrange.  An excellent

example of two products in a reaction is the production of (+)-Carvone and

(-)-Carvone.  Let's take a look at their structures:
 

                                

     The molecules (+)-Carvone and (-)-Carvone are called enantiomers.  If you recall, enantiomers

are nonsuperimposable mirror images.  The term nonsuperimposable means that one molecule cannot

be superimposed onto the other molecule without the movement of bond(s) and/or substituent(s).  For

these molecules to be mirror images, imagine a mirror between the two molecules.  Now look at the

other molecule.  For it to be a mirror image, the corresponding substituents will be "reflected."

Imposing these definitions for the two molecules of Carvone, they are indeed nonsuperimposable and

they are also mirror images; therefore, they are enantiomers.  Organic chemists recognized

enantiomers and immediately discovered that they must design some way of naming these enantiomers

as their properties are most times distinctly different.  Chemists came up with the method called the

Cahn-Ignold-Prelog rules that are used to assign absolute stereochemistry.  The rules are very

simple and are explained briefly in the link.  The most interesting characteristic concerning the

enantiomers of Carvone is that they are so different.  Although their structures differ by their

stereochemical rotation, each molecule possesses its own characteristic very different from its

corresponding enantiomeric partner.

     As was stated earlier in this text, it can be very difficult to assign mechanisms to some reactions,

especially those that produce such toxic molecules (such as TCDD).  Many organic reactions proceed

so mysteriously that conceiving a mechanism can be a very trivial process.  However, it is sometimes

possible to look to other undisputed mechanisms when approaching a more difficult reaction.  Referring

to the reaction in question (Reaction 1), the solvent can be the most important piece of information that

can be used to determine how the mechanism proceeds.  In this reaction, 2,4,5-Trichlorophenol is

reacted with chloroacetic acid in the presence of sodium hydroxide (NaOH).  Using basic reaction

knowledge, a mechanism for the production of 2,4,5-Trichlorophenoxyacetic acid can be proposed.
 
 

The most difficult mechanism for this reaction is the production of the by-product: 2,3,7,8 TCDD.  One

must remember that this product is a minuscule portion of the product obtained.  Nevertheless, the

mechanism can describe how the dioxins can be synthesized and may lay a foundation on preventing

further synthesis of unwanted dioxins.  My proposed mechanism for TCDD follows:
 

     According to the Center for Disease Control and the National Cancer Institute, out of the

approximate 75 chemicals in the dioxin family, TCDD is the most toxic.  The Twelfth Edition of the

Merck Index (1996) states the following toxic effects in animals:  wasting syndrome, gastric ulcers,

immunotoxicity, hepatotoxicity, hepatoporphyria, vascular lesions, chloracne, teratogenicity,

fetotoxicity, impaired reproductive performance, endometriosis and delayed death.  Industrial workers

who have came in contact with the concentrated forms of TCDD have developed chloracne,

porphyrinuria, and porphyria cutanea tarda.

     In May of 1996, President Clinton added prostate cancer to the list of seven diseases for which

Agent Orange victims can receive disability payments.  These actions came on the heel of a recent

report by the National Academy of Sciences that reconfirmed a limited association between Agent

Orange and prostate cancer. Academy scientists also raised the possibility of a link between the

herbicide and spina bifida.

     One may ask just how toxic is 2,3,7,8-TCDD?  According to the National Cancer Institute, the

TCDD level in Agent Orange varied from 0.02 to 54 micrograms per gram of 2,4,5-T.  To aid in the

understanding of the toxicity of 2,3,7,8-TCDD, consider the research of biologist Arthur Galston.

Galston was a professor at Yale University specializing in herbicide research during the late 1970s

when the cases of agent orange related illnesses were filed.  Dr. Galston's findings were astonishing,

not only to other scientists but to the general public who learned about the on-going research.  In

Galston's initial data, he reported that dioxin concentrations as low as five (5) parts per trillion (ppt),

"can, when supplied on a daily basis, induce a cancerous condition in rats."  To put the concentration of

five parts per trillion into perspective, imagine four (4) million gallons of water and you dropping one (1)

drop of a substance (e.g. food coloring) into that volume.  That is an extremely small concentration!

     Galston's report continues in saying, "Concentrations about 1 part per billion (ppb) result in a

premature death from more acute causes, and concentrations above 50 ppb produce rapid signs of

acute toxicity and early death...[Researchers] have found that lower concentrations of 2,3,7,8-TCDD

produce the same effects as higher concentrations, but merely take longer to do so...Even the purest

agent orange (2,4,5-T) currently available commercially contains about 0.05 ppm (mg/kg) of

2,3,7,8-TCDD."

     In this text, the only dioxins that has been mentioned are 2,3,7,8-TCDD and hexachlorophene.

However, there are many dioxins.  Focus has been on the tetra-chloro-substituted di-benzo dioxin, but

there are also hexa- (6); hepta- (7); and octa- (8) chloro-substituted dioxins.  Of the aforementioned

poly-substituted dioxins; 1,2,3,4,6,7,8,9-octachloro dibenzo-p-dioxin (OCDD) is the most stable and

therefore more thoroughly researched.  The structure for OCDD can be seen by following this link.

     Now that the most common dioxins have been identified, some focus must be shed on the current

research that is being performed in the determination of toxicity of these molecules.  Most research

today involving dioxins utilizes the superb technology of separations, namely that of Gas

Chromatography.  Please refer to the link for a brief explanation on how a Gas Chromatographer (GC)

works.  Some analytical methods used by the EPA use GC-MS (Gas Chromatography-Mass

Spectrometry).  Research has provided the world of organic chemistry many answers relating to the

toxicity of the dioxins and how the toxicity is dependent on the degree of chlorine substitution.

     Take, for example, the research performed at the Center for Environmental Science and

Technology, Department of Chemistry (University of Missouri-Rolla), and Environmental Affairs

Division (Southern California Edison Company) conducted by analysts:  L.D. Sivils, S. Kapila, Q. Yan

and A.A. Elseewi. Their 1995 study examined phototransformation of chlorinated dioxins in the vapor

phase and on aerosol particles.  The gas phase studies were carried out with a two-dimensional gas

chromatographic (GC) system. Studies on dioxin-bearing aerosol were carried out in a photoreaction

chamber coupled to an electrostatic classifier and particle counter. These arrangements permitted

isolation and irradiation of selected chlorinated dioxins in the photoreactor for varied periods and under

different atmospheres. The experiments conducted revealed that degradation rates in both the gas

phase and on aerosol particles are dependent on dioxin structure.  For example, approximately 80% of

2,3,7-trichlorodibenzo-p-dioxin was transformed after a 20-minute irradiation while less than 30% of

TCDD was transformed over the same exposure period.  Photodegradation rates decreased with an

increase in the number of chlorines. Moreover, degradation rates were also influenced by the position

of chlorine substitutions.  The results showed that dioxins with para chlorines photodegrade more

rapidly than dioxins with ortho or meta substituted chlorines.  This would definitely explain why the

degredation of 2,3,7-trichlorodibenzo-p-dioxin was nearly three times that of TCDD.  An explanation

for this behavior likely relates to the directing effects of the substituents and their relative reactivitys.