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Environmental Chemistry Chapter 7: Pesticides Copyright © 2007 by DBS.
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Transcript of Environmental Chemistry Chapter 7: Pesticides Copyright © 2007 by DBS.
Environmental Chemistry
Chapter 7:Pesticides
Copyright © 2007 by DBS
Contents
• Organochlorine Insecticides• Toxic Effects – Dose-response relationship• The Distribution of Environmental Pollutants• Organophosphate and Carbamate Insecticides• Herbicide, triazines, 2,4-D, Agent Orange
Background
• Birth defects due to exposure to synthetic chemicals
• Toxicity to mammals and birds• Carcinogenic effects• Xenoestrogenic effects
Gt. Lakes cormorant with crossed bill
Pesticides- Insecticides- Herbicides- Fungicides
Major use is agriculture
80-90% of US households have at least one synthetic pesticide
Insecticides – growing cotton
Herbicides – growing corn and soybeans
Pesticide regulation to date has not paid enough attention to the protection of health
ES&T Article
Organochlorine Insecticides
• Stable:
Against degradation (inert to both hydrolysis and oxidation)
• Hydrophobic:
Very low solubility in water, non-polar
• Lipophillic:
High solubility in hydrocarbon-like environments (fatty materials, organic matter)
• Toxicity:Relatively high to insects but low to humans
1 of the ‘dirty dozen’ listed by UNEP as POP’s…
• Known carcinogen• HCB used as a fungicide for cereal crops,
now being phased out• 99% of Americans have detectable levels
of HCB
POP’s
Phase out of POP’s by the 2004 Stockholm Convention
Organochlorine InsecticidesOrganochlorines In the Environment
• Over 217,000 contaminated sites have been identified by EPA. - An estimated 500,000 tons of dioxin contaminated soil alone is in need of treatment.
• Chlorinated phenols and organochlorines enter the environment as emissions from manufacture, incineration, use as a wood and leather preservative (Creosote) and biocide. - find their way into soil, sludges and sediments)
• The scale and complexity of remediation sites pose problems in applying common treatment processes such as bioremediation
• Incineration is effective, but can produce harmful chemicals during incomplete combustion. Projected cost is also high ($1200/ton).
Concentration Units
• ppm, ppb, etc. (assumes ρ = 1.00 g mL-1)
1 mg L-1 = 1 ppm
1 μg L-1 = 1 ppb
1 ng L-1 = ppt
ppm 1000 10 / 10 x g/g 0.001 = OH g 10
pollutant g 1000 =
OH g 1000
OH L 1.00
mg 1000
pollutant g 1.00 OH of /Lpollutant mg 1000 66
26
2
22
Question
Convert 0.04 μg L-1 to ppm and ppb
0.04 μg L-1 = 0.04 ppb
0.04 ppb x 1 ppm = 4.0 x 10-5 ppm 1000 ppb
Organochlorine InsecticidesDDT
• Prepared by Zeidler in 1874, insecticidal activity discovered by Müller in 1939
• Hailed as miraculous during its use in WWII• Found to be effective against malaria (carried
by mosquitoes) and typhus (carried by lice)• Saved lives of millions of people• Its effectiveness led to overuse in agriculture
– resistance• Now banned in most countries for agricultural
use
DDT
2 planar rings
2 tetrahedral carbons
Para (1,4)
Ortho (1,3)
Meta (1,1)
para-dichlorodiphenyltrichloroethane
p, p’-isomer
Organochlorine InsecticidesAdvantages of DDT
Advantages:• Stability• Persistence of insecticidal action - Low vapor pressure and low solubility in
water gave protection for weeks to months• Cheap to manufacture
• Moderate mammalian toxicity (Rat LD50 100 mg kg-1)
• Broad spectrum of activity
His
tory
of D
DT
‘elixir of death’
Organochlorine InsecticidesMode of Action of DDT
• Pesticides block vital metabolic processes
Other compounds with similar shape may be activeSimilar compounds with different shape are less likely to be active
‘Molecular wedges’ - DDT works by virtue of its shape - wedges into the insect nerve channel
Organochlorine InsecticidesMode of Action of DDT
• The size of the apex (-CCl3 group) corresponds to the size of a hydrated sodium atom
• Affects nervous system• DDT dissolves in lipid membrane of nerve cells• Interferes with normal AXONAL CONDUCTION – apex opens ion channel• Prolonged influx of sodium ions causes the nerves to fire repeatedly and this
causes death of the insect
Dichlorodiphenyldichloroethene, DDE
• DDE is a metabolite of DDT. Also produced slowly during the degradation in the environment
• Greatly affected bird population
• DDE interferes with the enzyme that regulates the distribution of calcium….leads to thin eggshells
DDT
- HCl
Endocrine disrupter (‘gender bender’ agent)
DDT dehydrochlorinase
DDE in Body Fat
Solubility in fat and animal tissue has caused accumulation of DDT and DDE in birds, fish and humans
DDT is now banned in most industrialized countries (circa 1970)
BUT 106 tonnes dispersed world-wide
Lake Ontario Salmon
Tissue levels in N. American wildlife began to decline 10 yrs after ban
Species at risk have recovered
DDE in Body Fat
DDT and DDE in breast milk
Which is more persistent: DDT or DDE?
Estimate the half lives of each
Concentrations have also declined in humans…
Noren and Meironyte, 2000
Question
The fat content of breast milk is ~ 4.2 g / 100mL
Calculate the mass of DDE ingested by a typical breast fed infant in 1972 upon consuming 250 mL.
From Fig. 7-2 concentration of DDE in breast milk was 2500 ng g-1 fat
Mass DDE in 250 mL milk = 250 mL x 4.2 g fat / 100 mL x 2500 ng DDE / g fat
Mass DDE in 250 mL milk = 26250 ng = 26 g
Accumulation of Organochlorines in Biological Systems
Measure of lipophilicity:
Partition Coefficient = concentration o solute in octanol
concentration of solute in water
Kow = [S]octanol/[S]water
e.g. KOW > 106
Strong tendency to partition into lipids
Accumulation of Organochlorines in Biological Systems
Bioconcentration factor, BCF
BCF = concentration of solute in organism
concentration of solute in water
Taking octanol as model for fat:
BCF KOW x % by weight of fat
(assumes fatty tissues have reached equilibrium)
For DDT
log KOW = 6 orKw= 1000000
BCF for DDT lies20000 - 400000
Hence KOw can be used to predict BCF
Higher the KOw morelikely chemical is boundto organic matter in soiland fatty materials
Biomagnification
A chemical whose concentration increases along a food chain is said to be biomagnified
DDT concentration in Lake Ontario Trout
Biomagnification results from asequence of bioaccumulation steps
Biomagnification
25 ppm of fat in the fatof the Cormorants
2 ppm of DDT inNeedlefish
0.5 ppm of DDT in thefat of minnows
0.04 ppm of DDT inPlankton
0.003 ppb of DDT inLong Island seawater
Question
Fish (5.0 % body fat) taken from a particular lake were tested and found to contain 200 ppm DDT in their tissues.
Determine the concentration of DDT in this lake.
From Table 7-2 Log KOW = 6.2, KOW = 106.2
BCF = KOW x (% body fat/100)
BCF = 1.6 x 106 x (5/100) = 7.9 x 104
BCF = concentration in fish / concentration in lake
Concentration in lake = 200 / 7.9 x 104 = 2.5 x 10-3 ppm = 2.5 ppb
DDT Analog(molecular shape)
1. Non-persistent 2. Doesn’t accumulate3. Forms water soluble compounds4. Toxic to insects, non-toxic to mammals – birds + mammals have
enzymatic detoxification pathway
LD50 = 6000 mg kg-1
Other Organochlorine Insecticides
• Toxaphene:
Mixture of hundreds of similar substances (produced from chlorinated camphene- pine tree product) was widely used after DDT ban in 70s
• Toxaphene is extremely toxic to fish• Restrictions were placed in 1982
and a total ban in 1990
Spread of Toxaphene in N. America from SE cotton states
Chlorinated Cyclohexane
(Lindane - 1 of 8 isomers are active)
Agricultural and health care uses (c.f. warfarin)
Used to treat scabies and head lice
Cyclodienes (Banned)
Series of fully chlorinated cyclic hydrocarbons containing a chlorinated endomethylene bridge
Chlordaneb. 1988
Heptachlorb. 1988
Aldrinb. 1974 Dieldrin
b. 1974
Endrin(isomer)b. 1974
[O]
Mirexb. 1978
Extremely effective against fire ant
Other cyclodienes
Endosulfan
Banned in EU, regulated in US
(Not on UN list due to reactivity)
LD50 <100 mg kg-1
One of the more toxic pesticides in use
Confirmed xenoestrogen
Doses as low as 35 mg kg-1 have been documented to cause death in humans
Under consideration for ban
Principles of Toxicology
• Toxicology – study of the harmful effects of chemicals on living organisms
– Animal studies
– Epidemiology – health history of selected groups
• Toxicology studies the relationship between dose and effect, almost all substances can be toxic under the right conditions
‘The dose makes the poison’
Principles of Toxicology
• Acute toxicity:
– Sudden and severe exposure
– Rapid onset of symptoms
• Chronic toxicity:
– Continuous, long-term exposure
– Relatively low dose
– Cancer, birth defects, neurological damage
Same chemical may show both effects
e.g. skin irritation vs. cancer
Toxic EffectsDose-Response Relationships
• Animal tests are acute exposures at high doses due to time constraints
• Results are extrapolated down to environmental levels
Typical dose-response curve
Response is usually death
Toxic EffectsDose-Response Relationships
• Effects at the low end cannot be seen clearly on a linear scale
Typical logarithmic dose-response curve
NOEL – no observable effects level
Toxic EffectsDose-Response
Relationships
The dose of the substanceadministered in toxicity testsis usually expressed as themass of the chemical usuallyin milligrams per unit of testanimal’s body weight(usually kg)
LD50 – does that is lethal to 50 % of the population
The smaller the LD50 value,the more potent the chemical
Question
If a dose of 0.1 μg is sufficient to kill a mouse, what mass would be fatal to you?
What average level of substance would have to be present in drinking water for you to receive a fatal dose in one week?
Ratio mass human : mouse = 200 : 1Mass that would kill you = 200 x 0.1 μg = 20 μg
2 L water d-1 x 7 d w-1 = 14LFor fatal dose 20 μg / 14 L = 1 μg L-1 = 1 ppb
The Dirty DozenOCP US
pesticide use ban
Approx. LD50 rat oral
(mg kg-1)
IARC (W.H.O.)Category
Aldrin 1974 50 3
Dieldrin 1974 100 3
DDT 1970 100 2B
Endrin 1974 3 3
Chlordane 1988 100 2B
Heptachlor 1988 100 2B
HCB 1966 10,000 2B
Mirex 1978 1000 2B
Toxaphene 1990 50 2B
International Agency for Research on Cancer - 1: carcinogenic, 2A: Probably, 2B: possibly, 3: insufficient data
Environmental testing
• Experiments with test animals are used to determine how carcinogenic a compound is, take many years
• The simple Ames test can be used fairly rapidly to distinguish compounds likely to be human carcinogens
Environmental testing
• Development of new pesticides:
– Low Acute toxicity towards non-target species
– Metabolism studies to establish breakdown pathways
– Physiochemical testing to determine distribution in environment
– Residue studies to determine levels in treated crops
– Ecological testing to determine impact on non-target species
Risk AssessmentTo perform risk assessment it is important to know:
Hazard evaluation information (acute, cancer ???)Quantitative dose-response informationAn estimate of the potential human exposure to the chemical
The highest dose at which no observable effects level is called NOEL(expressed in terms of mg kg-1 body weight day-1)
To determine the threshold level for the most sensitive members of the human population, EPA uses Toxicity reference dose or RfD.(RfD is also referred as Acceptable Daily Intake or ADI)
RfD (or ADI) = NOEL/100 (divide by safety factor of 100)
If NOEL for a chemical is 0.01 mg/kg/day, the ADI or RfD for a 80 kg manwould be (0.01 mg/kg/day /100) x 80 kg = 0.008 mg
Environmental Fate
Over 95 % of sprayed pesticides reach a destination other than target species
Decreases biodiversity of the soil
Environmental Fate
Fate in soil is determined by:
1. Adsorption - Binding by soil is critical
2. Leaching
3. Volatilization
4. Uptake
5. Run off
6. Degradation
Environmental Fate
ADSORBED
DISSOLVED
MO DECOMPOSITION
LEACHING
CHEMICAL DECOMPOSITION
VOLATILIZATIONWORMS ETC.
PLA
NT
S
WATER TABLE
PHOTO DECOMP.
UPTAKE
RUN OFF
-
Environmental FateAdsorption
P
P
P
P
P
P
P -
1. Adsorption: To clay or organic matter:
Pesticide is bound to or adsorbed onto surface of clay, can be released as free or solvated pesticide
Equilibrium is influenced by:1) Nature of pesticide2) Water content3) Soil pH and temperature
--
--
- P
P
P
P
P
P
P
Environmental Fate
2. Leaching:
Lower solubility = slower leaching = greater persistence
3. Volatilization:
Influenced by chemical structure (V.P.) and atmospheric conditions
4. Uptake:
By plants etc.
Warm, moist conditions, acidic soil, high solubility = greater uptake
5. Run off / soil erosion:
Effect is greater with least adsorption, erosion caries particle away
6. Degradation:
Microbial, chemical and light induced
Distribution of Environmental Pollutants
• Chemicals distribute according to their physiochemical properties• ‘Fugacity’ = escaping tendency of a substance from a phase, it is
proportional to concentration in a given phase:
f = C Z
(Where f = fugacity, C = concentration, Z = fugacity capacity)
• To calculate fugacity, Z must be known (derived from physiochemical Properties)
• At equilibrium the fugacities of all phases are equal:
fair = fwater = fsediments = fbiota = fsoil …
(no net diffusion between phases)
Distribution of Environmental Pollutants
IMPORTANT!
Fugacities are equal, concentrations are not!!!
Distribution of Environmental Pollutants
Z is a fugacity capacity constant for the substance and the phase. Generally higher the Z value, the greater the tendency to accumulate in that phase…..analogous to equilibrium constant
For a given phase (x) fx = Cx/Zx Or Cx= fx .Zx
If total number of moles preset is is ntotal, and in phase x is nx
Cx = nx/Vx or nx =Cx.Vx ∴ nx= f.Zx.Vx
ntotal = f ΣZxVx
Or f = ntotal
ΣZxVx
Dimensions for a new world
Area 1 km x 1 kmAtmosphere: 6 km highWater column: 80 m deepBiota = 1 ppm by volume (density = 1000 kg m-3)Suspended sediment = 5 ppm by volumeBottom Sediment: 3 cm deep (active layer)
Z(Atmos) = ZA = 4.04 x 10-4 mol m-3 Pa-1
Z(Water) = ZW = 1.3 x 10-2 mol m-3 Pa-1
Z(Biota) = ZB = 374 mol m-3 Pa-1
Z(Sus. Solids) = ZSS = 1856 mol m-3 Pa-1
Z(Bot. Seds) = ZBS = 186 mol m-3 Pa-1
Question
Consider the distribution of 1 mole of trichlorobiphenyl in the six compartment world mentioned above,
Calculate:
(a) volume of each compartment
(b the fugacity (Pa) at equilibrium
(c) the concentration of tri CBP in each compartment
(d) the concentration of tri CBP in biota (mg/kg) (M = 257 g mol-1)
(e) the total amount (moles) of tri CBP in each compartment
(f) the distribution (%) among the compartments
(a) Volume
V(Atmosphere) = 6 x 103 x 1 x 103 x 1 x 103 = 6 x 109 m3
V(Water column) = 80 x 1000 x 1000 = 8 x 107 m3
V(Sus. Solids) = 5/106 x 8 x 107 m3 = 4 x 102 m3
V(Biota) = 1/106 x 8 x 107 m3 = 80 m3
V(Bottom Sediments) = 0.03 x 1000 x 1000 = 3 x 104 m3
(b) feq
Fugacity = n / ΣZxVx = 1 / ΣZxVx
ΣZxVx = ZAVA + ZWVW + ZSVS + ZBVB + ZBSVBS
= 2424000 + 1040000 + 742400 + 29920 + 5580000
= 9816320
f = 1 / 9816320 = 1.02 x 10-7 Pa
(c) C in each compartment
CA = f ZA = 1.02 x 10-7 x 4.04 x 10-4 = 4.12 x 10-11 mol m-3
CW = f ZW = 1.02 x 10-7 x 1.3 x 10-2 = 1.33 x 10-9 mol m-3
CSS = f ZSS = 1.02 x 10-7 x 1856 = 1.89 x 10-4 mol m-3
CB = f ZB = 1.02 x 10-7 x 374 = 3.81 x 10-5 mol m-3
CBS = f ZBS = 1.02 x 10-7 x 186 = 1.90 x 10-5 mol m-3
(d) Concentration (mg kg-1)
C(Biota) = 3.81 x 10-5 mol m-3
= 3.81 x 10-5 mol m-3 x 257 g mol-1
= 0.0098 g m-3 x 1 m3 /1000 kg (density biota)
= 9.8 x 10-6 g kg-1
= 9.8 x 10-3 mg kg-1
(e) Moles tri CBP
Atmosphere, nA = CAVA = 4.12 x 10-11 x 6 x 109 = 0.247 moles
Water, nW = CWVW = 1.33 x 10-9 x 8 x 107 = 0.106 moles
Sus. Solids, nSS = CSSVSS = 1.89 x 10-4 x 4 x 102 = 0.076 moles
Biota, nB = CBVB = 3.81 x 10-5 x 8 x 101 = 0.003 moles
Bottom Seds, nBS = CBSVBS = 1.90 x 10-5 x 3 x 104 = 0.570 moles
1.000 mols
(f) Distribution (%)
Atmosphere, 24.7
Water, 10.6
Sus. Solids, 7.6
Biota, 0.3
Bottom Seds, 57.0
100 %
Organophosphates and Carbamates
Organophosphate and carbamate insecticides are commonly used for small animals as flea and tick powders, sprays, foggers, shampoos, dips, and flea collars
They are also frequently used as household, garden, and farminsecticides.
Chlorpyrifos, parathion, diazinon, famphur, phorate, terbufos, and malathion are examples of organophosphates while carbofuran, aldicarb, and carbaryl, are carbamates. They are all marketed under a wide variety of trade names.
All OP/Carbamate insecticides are fat soluble and therefore are easily absorbed through the skin and then transported throughout the body. These chemicals kill insects and cause poisoning in animals by inhibiting the enzyme, acetylcholinesterase (AChE) which normally functions to degrade acetylcholine in nerve synapses. Inhibition of AChE in the nerves results in a buildup of acetylcholine (ACh) and overstimulation of ACh receptors.http://www.addl.purdue.edu/newsletters/1998/summer/organos.html
Organophosphate Insecticides
Consumption of organophosphatepesticides by various crops in the US
Non persistant:Decompose (hydrolyse) within days-weeks after the accumulation
Unlike organochlorins, organophosphates do not bioaccumulate
Not carcinogenic or teratogenic BUT have higher acute toxicity than organochlorine pesticides
Structurally all organophosphatepesticides contain a central pentavalent P atom, methoxy/ethoxy groups and R- group connected through oxygen or sulfur atom
Organophosphate Insecticides
• The organophosphates, e.g., parathion, are related to the nerve gases developed during World War II (sarin, VX etc.)
• They react irreversibly with the enzyme acetylcholinesterase, which is responsible for inactivating the neurotransmitter acetylcholine
• Some other examples:malathiondiazinonphosmet (Imidan®)chlorpyrifos (Lorsban®)
• Some of the organophosphates are very acutely toxic. Parathion, for example, is 30 times more toxic than DDT
• Each year organophosphates poison thousands of humans throughout the world, causing hundreds of deaths.
Carbamate Insecticides
Carbamate insecticides are inhibitors of acetylcholinesterase, but their action is reversible
Some examples:
carbaryl (Sevin®)aldicarb (Temik®)methomyl (Lannate®)
Features:These compounds are rapidly detoxified and excreted so their risk to warmblooded animals is less than the other agents we have looked at.They are degraded rapidly in the environment so persistence is not a problem. They are, however, a danger to many useful insects, especially honeybees.
Pesticides from Natural Sources
- Obtained from flowers, chrysanthemum- Paralyze insects but not strong enough to kill them- Synthetic pyretrins are called pyrethroids- Common ingredients in domestic insecticides
Mechanism of Action
Oganochlorines & Pyrethroids - Enzymes, axonal membranes (Na+, K+, Ca++, Cl-)
Organophosphates & Carbamates - excess acetylcholine
Herbicides
• Selective herbicide, effective in killing many broadleaf plants
• Replaces weeding
• Most widely used pesticide in USA
• Marketed under many different names: Weed-Bgon (Chevron), Fernoxone (ICI), and Weedone (Union Carbide), to name a few
• Over 30,000 tons of 2,4-D are utilized in the US each year
• Natural degradation in water is slow (6-170 days)
Pesticides in the Hydrologic System
Pesticide movement in the hydrologic cycle
About 1 billion pounds of pesticides areused each year in the United States tocontrol weeds, insects, and otherorganisms. About 80 percent of thisquantity is used in agriculture. Although theuse of pesticides has resulted in increasedcrop production and other benefits, it hasraised concerns about potential adverseeffects on the environment and humanhealth.
In many respects, the greatest potential forunintended adverse effects of pesticides isthrough contamination of the hydrologicsystem, which supports aquatic life andrelated food chains and is used forrecreation, drinking water, irrigation, andmany other purposes. Water is one of theprimary pathways by which pesticides aretransported from their application areas toother parts of the environment
HerbicidesTriazines
Atrazine is a selective triazine herbicide used to control broadleaf and grassy weeds in corn, sorghum, sugarcane, pineapple, christmas trees, and other crops, and in conifer reforestation plantings.
Most heavily used herbicide in US
Toxicity: Atrazine is slightly to moderately toxic tohumans and other animals. It can be absorbed orally,dermally, and by inhalation
Symptoms of poisoning: abdominal pain, diarrhea and vomiting, eye irritation, irritation of mucous membranes, and skin reactions
HerbicidesTriazines
• Mode of action– Blocks photosynthesis
• Side effects– Loss of sensitive plants– Suspected xenoestrogen (feminizes male frogs – EPA says insufficient data)– Suspected teratogen
• Underwent 10 year EPA review and concluded safe for use, currently banned in EU• Degradation
– Dechlorinated and detoxified by MO’s– Half-life is extended once it enters waterways
• Risk Assessment– BCF < 10– Low toxicity– Possible carcinogen
Pesticide Combinations Imperil Frogs2006
http://www.berkeley.edu/news/media/releases/2006/02/02_pesticides.shtml
UC BERKELEY – The pesticide brew in many ponds bordering Midwestern cornfields is not only affecting the sexual development of frogs, but is making them more prone to deadly bacterial meningitis, according to a new study by University of California, Berkeley, scientists.
These physiological effects combine with environmental disruptions to make the life of a frog seem like something out of a horror movie and are likely among the factors causing a decline in amphibian populations worldwide, the researchers said.
Herbicides
EPA Won't Restrict Toxic Herbicide Atrazine, 2004The EPA has decided not to limit one of the nation's most widely used weed killers, a chemical that, according to several recent studies, threatens human health and the environment. The October 2003 decision - which the EPA was required to make under a court-approved consent decree reached with NRDC in 2001 - will allow Syngenta, the main manufacturer of atrazine, and other companies to continue to sell the chemical in the United States with no significant restrictions.
Get to know the views of both sideshttp://www.nrdc.org/health/pesticides/natrazine.asp
http://www.epa.gov/oppsrrd1/reregistration/atrazine/
WASHINGTON, DC, February 21, 2005 (ENS) - The U.S. EnvironmentalProtection Agency (EPA) is illegally negotiating and brokering regulatoryagreements with pesticide manufacturers that are friendly to the industry,according to a lawsuit filed by the Natural Resources Defense Council (NRDC).
Herbicides Chloracetamides
Herbicides Chloracetamides
http://www.monsanto.com/monsanto/layout/products/
Monsanto remains the primary globalproducer of glyphosate, the activeingredient in Roundup® agriculturalherbicides.
Monsanto's herbicide productsinclude more than 90 glyphosatebasedherbicides globally, includingRoundup agricultural herbicides andRoundup branded turf and ornamentalproducts.
These products can be used to controlweeds on the farm, the golf courseand in home gardens, createsustainable agricultural systems thatpreserve top soil, help retain soilmoisture, and provide a valuable toolfor integrated pest managementprograms.
Herbicides Phenoxy Herbicides
Phenoxide ion is reactive towards R-Cl
C6H5O−Na+ + Cl-R → C6H5-O-R + NaCl
By employing R-Cl as Cl-CH2-COOH we can obtain phenoxy acetic acid
Commercial route to large scale production of herbicide 2,4,5-T
Acid-base reaction produces phenoxide ion
Herbicides Phenoxy Herbicides
Summary
The Precautionary Principle
Introduction of new chemicals should follow the precautionary principle
‘When an activity raises threats to the environment or human health, precautionary measures should be taken, even if some cause-effect relationships are not fully established scientifically’
How much contamination can be avoided?What are the alternatives?
How safe is safe?What level of risk is acceptable?