TOPIC dominating the pesticide industry, and were used

TOPIC SUMMARY 2

PESTICIDE CHEMISTRY

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Kimaya Rajwade  

 

Abstract: In this paper, definitions and
classification as well as history of pesticides were looked at. Detailed study
into carbamate pesticides was done. This included method of carbamate
synthesis, mechanism of toxicity (neurotransmitter inhibition), fate of
carbamate pesticides after being consumed, some drawbacks and benefits. 

 

WHAT ARE PESTICIDES?

According to the EPA, Pesticides are any substance or mixture
of substances meant to destroy, prevent or mitigate any pest. Pesticides can
also be defined as any substance or mixture of substances synthesized for use
as plant defoliant, desiccant or regulator ( Environment Protection
Agency , 2013).

In a broader sense, pesticides are chemical compounds which
are meant to control pest (Environmental Protection
Agency, 2007).

There are several types of pesticides available in the market
today, with various methods of classification present to help determine which
pesticide is to be used for a particular case.

Based on target organisms, pesticides can be classified into
insecticides, herbicides, rodenticide, fungicide, bactericide, piscicide,
molluscicide, nematicide, avicide, animal repellant, insect repellant,
antibacterial and sanitizer (Randall, Hock, Crow,
Hudak-Wise, & Kasai, 2013).

On the basis of the chemical composition, pesticides can be
classified into organic and inorganic pesticides. Inorganic pesticides can be
further classified as mineral or non-mineral based pesticides. Organic
pesticides are classified into natural organics and synthetic organics. Natural
organics can be plant based or animal based. Synthetic organics are further
classified into organophosphates, organochlorides and carbamates.

Based on the way pesticides function, they are classified
into contact toxicants, systemic toxicant, fumigants and chemical repellents.

The use of pesticides is not a new phenomenon. Humans have
been using pesticides to control pests since before 2000 B. C. The first known
pesticide used was sulfur dusting, in the ancient Mesopotamia (this dates back
to 4500 years ago) (Rao, Rupela, Rao, &
Reddy, 2007).
In the 15th century, poisonous chemicals like lead, arsenic, mercury
etc. were applied on soil to kill insects and rodents. This trend did not
change for a long time. Until the mid-20th century, arsenic based
pesticides were predominantly used in the industry. In the 1950s, Paul Muller
discovered that DDT was a very effective pesticide. From this point on,
organochlorides started dominating the pesticide industry, and were used
extensively in agriculture and landscape pest control (Ritter, 2009). However, the
harmful effects of DDT and such soon started coming to the forefront, after
Rachel Carson’s Silent Springs was
published. The U. S. has since banned the use of organochlorides (effective
since 1975), with the pesticide industry making a transition to
organophosphates and carbamates.

Due to the ever-growing population, a tremendous pressure on
food supply and agriculture industries has been created. In such a scenario,
pesticide use has become a necessity. There wasn’t much information available
on the consequences of use of pesticides when their use first began, but in the
current pesticide industry, efforts are being made to create pesticides that do
not harm the ecology in the way previously used pesticides have. Despite of the
negative effects, the use pf pesticides has been profitable for the crop
industry. Every dollar spent on pesticides leads to 4 dollars saved on crop
yields (Pimental, Acquay, Biltonen,
Rice, & Silva, 1992).

 

CARBAMATES

Carbamates are synthetic organic pesticides used to control
pests in agricultural, landscape as well as household environments. Carbamates
are derived from carbamic acid. The first carbamate to be used as a pesticide
was carbaryl, which was introduced in 1956. To this day, carbaryl is the most
used carbamate in the pesticide industry (Fishel, 2005).

                                                                              Fig:
Structure of Carbaryl.

 

Carbamates are effective on a broad spectrum of pests. Three
types of carbamate pesticides are used in industry. Carbamate ester derivatives
are used as insecticides, carbamates with aromatic groups are used as herbicides
and benzimidazoles are used as fungicides (World Health Organization ,
1986)

The figure above represents a general carbamate compound. The
R and R’ represent two different alkyl groups.

Carbamates are low vapor pressure compounds, which makes them
highly volatile. They are sparingly soluble in water; however, oxidation
products of carbamates are soluble in water (World Health Organization ,
1986)

Temik, Fudaran and Sevin are a few commercially used
carbamates. 

 

SYNTHESIS

Various methods for the synthesis of carbamates are
available. Listed below are few such methods.

1)     
 Synthesis by Carbonylimidazolide.

 

In this method, Carbonylimidazolide
reacts with a nucleophile to give a carbamate. Urea and thiocarbamate are also
released as byproducts. The initial reaction takes place at 0o C
with pressure equal to 1.2 equilibrium. This method of synthesis is economic
since it does not require an inert atmosphere. It also liberates commercially
useful byproducts, which can be sold for profit. The three end products of this
reaction are separated by the means of filtration (Padiya, et al., 2012).

Fig: (Padiya, et al., 2012)

 

2)     
Synthesis
by Hydroxamic acid

 

Hydroxamic
acids are converted to isocyanates through the Lossen rearrangement in this
method (in the presence of Carbonyl diimidazole). The reaction takes place at
room temperature, with 3 equilibrium pressure (with a reaction time of about 4
hours). This process is quite mild and simple, with carbon dioxide and
imidazole being the only by-products. This method avoids the use of any
hazardous reagents, hence proving to be a green alternative to some of the more
traditional routes for synthesising carbamates (Dubé, et al., 2009).

 

Fig: (Dubé, et al., 2009)

 

 

 

3)     
One
Pot Synthesis from Amides

 

In this process, aliphatic or
aromatic amides are converted to methyl and benzyl carbamates respectively, via
the Hofmann rearrangement. High yields are obtained in the presence of
catalysts such as bromoacetamide and lithium hydroxide/methoxide. It is an
efficient, one pot process. In the presence of Lithium methoxide, reaction
takes place at room temperature for 24-48 hours. In the presence of Lithium
hydroxide, reaction takes place at high pressure (2-9 equilibrium) for a
shorter duration of time (Jevti?, Došen-Mi?ovi?,
Ivanovi?, & Ivanovi?, 2016).

 

                                             
Fig: (Jevti?, Došen-Mi?ovi?,
Ivanovi?, & Ivanovi?, 2016)

 

MODE OF ACTION

Carbamates are effective as
pesticides due to their ability to inhibit the acetylcholinesterase enzyme
(AChE) in the nervous system. AChE acts a catalyst to the hydrolysis of the
neurotransmitter acetylcholine (ACh) to choline and acetic acid. (ACh is a
synaptic mediator that plays a part in the transmission of nerve impulses in
insects and mammals) (World Health Organization ,
1986)

“Like organophosphates, carbamates
can inhibit esterases which have serine in their enzymatic centers (known as
serine esterases or beta esterases). This has also been linked to the toxic
potential of carbamates”  (World Health Organization ,
1986)

When a muscle is innervated, a nerve
impulse travels down a neuron to reach nerve endings. ACh is a muscle stimulating
chemical present on the tips of nerve endings which are connected to muscles. When
the compound is in this state, muscles contract. When a nerve impulse is
registered by the nerve endings, AChE hydrolyses ACh, which in turn causes
muscles to relax (World Health Organization ,
1986)

Carbamates inhibit this hydrolysis
reaction which means that the muscles are in a stimulated state at all times.
This leads to muscle exhaustion and damage. (World Health Organization ,
1986).

This inhibition mechanism is lethal
to insects and smaller mammals like rodents, but not to greater mammals and
human beings (the effect of exposure for humans wears off in four hours).

Fig: (World Health Organization , 1986)

 

The reaction mechanism of inhibition is given below:

Fig : (World Health Organization ,
1986)

 

 

As demonstrated in the figure, the carbamate compound reacts
with the enzyme to form a carbamate-enzyme complex. This complex is then
hydrolyzed instead of the acetylcholine compound on the nerve endings, thus
inhibiting the muscle relaxation process.

 

FATE OF CARBAMATE PESTICIDES:

After being consumed by organisms,
carbamates are metabolically biotransformed. The transformation reactions take
place with the help of enzymes naturally present in animal tissues. Carbamate
esters are converted into water soluble compounds with increased polar
properties. The compounds thus formed are excreted via urine and feces. The
excreted compounds are usually non-toxic (World Health Organization ,
1986).

The main method of metabolism of
insecticidal carbamate esters is through oxidation and is generally associated
with the mixed-function oxidase (MFO) enzymes, which exist in many tissues.
There are various sites on carbamate molecules which lend themselves to
oxidation (World Health Organization ,
1986).

“Typical oxidative reactions include:
(a) hydroxylation of aromatic rings, or epoxidation; (b) O -dealkylation; (c) N
-methyl hydroxylation; (d) N -dealkylation; (e) hydroxylation and subsequent
oxidation of aliphatic side chains; and (f) thioether oxidation to sulfoxides
and sulfones” (World Health Organization ,
1986)

 

 

 

Shown in the figure are various sites
on a carbamate molecule where oxidation could hypothetically take place.

DRAWBACKS OF CARBAMATES.

Carbamate compounds, though effective
as pesticides, do have their own set of drawbacks. Low dosage exposure to
carbamates causes skin irritation, skin sensitization and eye irritation. It
can also cause headache, nausea and vomiting if exposure dose is high (it
should be noted that, though these symptoms occur, the effect of the pesticide
wears off within a matter of a few hours) (World Health Organization ,
1986).
 

Studies done on male and female rats showed
that oral exposure to carbaryls lead to decrease in fertility in male rats and embryonic
deaths in female rats. Exposure to carbaryls also lead to endocrinal system
defects in tested rats (World Health Organization ,
1986).

Certain types of commercially
available carbamates (Benomyl, Carbendazim) were found to be potentially
carcinogenic since their structures are like those of known carcinogens (World Health Organization ,
1986).

Some carbamate oxidation products
(5-hydroxycarbyl, 5-hydroxypropoxur) are known to be toxic, and to exhibit ACh
reaction inhibiting properties. This increases the toxic potential of the
pesticide group (World Health Organization ,
1986).

 

WHY CARBAMATES?

Rachel Carson brought to light the
very real threat the environment was faced with due to overuse of
organochloride pesticides, especially DDT. Since then, scientists have
conducted numerous studies to find efficient pesticides which do not take a
toll on the environment. Carbamates have proven to fulfill these criteria.

Carbamates are not as ecologically
intrusive as organophosphates and organochlorides. Since 1975, most
organophosphates and organochlorides are banned in the U.S., which promoted the
use of carbamate pesticides (McDaniel College, 1998)

Carbamate pesticides are more
biodegradable than other classes of pesticides available, which means they can
get easily biotransformed into non-threatening compounds in the soil, hence not
leaving any toxic residue (in most cases) (McDaniel College, 1998).

Even when carbamates are consumed by
living beings (animals and humans), the compounds do not penetrate the central
nervous system. Any damage that is caused by the chemicals is treatable and
short-lived. Hence, carbamates are benign compared to other pesticides (McDaniel College, 1998).