Glaser's "Chemistry is in the News"
To Accompany Wade Organic Chemistry 4/e.
Chapter 18. Ketones and Aldehydes.
In the struggle against bugs people have many weapons, most of which are pesticides. These chemicals are used to irritate and/or kill the bugs directly or interrupt a part of their lifecycle rendering the bugs unable to produce future generations. Pesticides come in the form of insecticides and repellents. Repellents only irritate them in order to keep them away from a desired area, while insecticides are used mainly to kill the pests. Insecticides and repellents can be derived from nature or synthesized. Naturally derived insecticides and herbal repellents are less effective than their synthetic counterparts. Both natural and synthetic insecticides and repellents have the ability to combat bugs. It is the functional groups of these chemicals that give the pesticides their potency. For example, the repelling odors of pyrethrin and citronella make them effective repellents. These odors are due to their ketone and aldehyde functional groups. Since these chemicals are meant to harm the insects they also have characteristics that can be generally detrimental to other organisms.
Naturally derived pesticides can have toxic and irritant effects on people, but the harmful effects of these natural occurring pesticides are less than those of the synthetic pesticides. Since natural pesticides are less toxic they can be used in other ways. For example, citronella can be used as an aid to train dogs. The chemical has an unpleasant odor for dogs, which trains them not to bark. There are also medicinal qualities to these natural pesticides. For example, some further applications of citronella include use as an antiseptic and as a therapeutic massage oil. Even with these healing abilities citronella, along with other natural pesticides, can be very irritating to sensitive skin.
Synthetic pesticides are usually far more harmful to people than naturally derived pesticides. However, not all synthetic pesticides follow the highly toxic trend. For example, Amdro is only a mild irritant to people causing slight skin and eye irritation. Some synthetic pesticides contain heterocyclic organophosphate groups. These groups make insecticides like Dursban and Diazinon extremely toxic to both people and insects. Both chemicals can cause blurred vision, memory loss, and headaches if used improperly. Synthetic counterparts to natural pesticides are far more toxic than the natural form. Allethrin, the first generation pyrethroid, is a synthetic form of pyrethrin that can cause rashes and coughing. Allethrin is so complex that it takes 22 synthesis reactions to make. Second generation pyrethroids, such as resmethrin, are more toxic, longer lasting, and more effective, than their first generation predecessor.
Another synthetic pesticide is Ficam, which is a carbamate. This pesticide is for indoor use on a wide variety of pests. Consequently, it is highly toxic, especially in high temperature and humidity. These conditions increase the absorption into the skin, creating symptoms of blurred vision, headaches, and muscle spasms.
Since these chemicals possess toxic properties they provide a diversity of negative biological effects on insects. Amdro is a synthetic pesticide often used as a slow (3-8 weeks), but effective, pesticide against fire ants. The chemical is usually applied to grits or pieces of corn which, in turn, is then picked up by the foragers in the ant colony. Eventually, the tainted food is regurgitated to the queen ant of the colony. After ingestion, she metabolizes the toxic meal, which eventually kills her. When a queen ant dies, so does the entire colony.
Another effective pesticide against fire ants is Dursban. It works in a similar fashion to Amdro, except Dursban is even more toxic (thus it must be used in relatively moderate quantities outside to prevent poisoning of the ecosystem). Dursban, when ingested by the queen fire ant, effectively acts as a cholinesterase inhibitor, an enzyme affecting the ant's nervous system. When these enzymes are inhibited, the queen ant's nervous system becomes paralyzed, resulting in twitching, paralysis, and, ultimately, respiratory failure. In addition, Dursban works on fleas by breaking down their egg-hatch cycles. Diazinon and Ficam are organophosphates, which create effects similar to Durban.
An effective pesticide against spiders and mosquitoes is pyrethrin, a naturally occurring substance produced by chrysanthemum flowers. This makes it a relatively cheap and aesthetically pleasing pesticide when used on garden-variety insects. Pyrethrin extracted from the chrysanthemum plants can have an adverse effect on any spiders or mosquitoes that have direct contact with the chemical. The pesticide works quickly as a toxin, paralyzing the nervous system of the spider or mosquito, much like Dursban, followed shortly by death. Pyrethrin can be applied directly to spider webs guaranteeing death either by consumption or contact. For mosquitoes, the chemical is usually administered in spray form.
Another effective way to combat mosquitoes is pyrethroids, a chemically modified synthetic form of pyrethrin. Like pyrethrins, pyrethroids cause paralysis in the stomachs and respiratory systems of the mosquitoes resulting in immediate death. The only visible difference between pyrethrins and pyrethroids is that pyrethroids are stronger and faster acting. Allethrin and resmethrin are examples of pyrethroids.
Another effective way to repel mosquitoes is the use of candles or sprays
containing citronella oil. Citronella oil is a mixture, primarily
containing citronellal. Citronellal is an extract from plant sources like
the lemon grass of Sri Lanka. Citronella
in small quantities as an irritant to mosquitoes. Citronella, a weak
pesticide, functions by repelling the mosquitoes away from the applied
area. Citronella makes up for its weakness by being less toxic and more
versatile than the other pesticides mentioned above.
Pertinent Text References
Chapter 18: Section 6. Industrial importance of Ketones and Aldehydes
Chapter 21: Section 1. Introduction to Carboxylic Acid Derivatives
Question 1: Identify the functional groups for citronella, Amdro, Ficam, Dursban, and Pyrethroids. Using your knowledge about functional groups and biological affects, explain why each is a good insecticide.
Question 2: What are the environmental and health hazards of Diazinon, Dursban, and resmethrin?
Question 3: What application methods are best for each type (those listed in Question 1) of insecticide, and should humans also avoid these types of contact? (i.e. for spray insecticides should humans avoid skin contact, etc.)
Question 4: This article is mostly an advice column on pest control, but what does it suggest/ indicate about the differences between natural and synthetic insecticides?
Question 5: What are other uses for citronella, and is it toxic at all?
In your opinion, after looking at various web sites and
weighing the pros and cons, what is the best type of insecticide, natural
or synthetic? Remember to review pros and cons.
Citronella: Aldehyde; produces an unpleasant odor to insects and
Amdro: Ketone and amine; toxic when metabolized
Ficam: Carbamate (ester) and amine; inhibits cholinesterase by carbamylating causing build up of acetylcholine in synapses resulting in muscle spasms and eventually death
Dursban: Esters of phosphorus (organophosphates) and amines; inhibits cholinesterase by phosphorylating it causing build up of acetylcholine in synapses resulting in muscle spasms and eventually death
Pyrethroids: Ketone and ester; keep open sodium channels in neurons resulting in repetitive axonial discharges, which eventually causes paralysis.
The unpleasant odor from pesticides is due to amines, ketones, and aldehydes.
Diazinon: Produce treated with Diazinon should not be ingested until the
chemical has time to break down to safe levels, somewhere between 1-21
days. Diazinon poisoning can occur by ingestion, inhalation, and
absorption through skin. Residue of Diazinon remains in the environment,
and may contaminate water supplies.
Dursban: Exposure to Dursban, whether through contact with skin, inhalation, or ingestion can be harmful to humans. Dursban contains an organophosphate, which was originally developed for nerve gas warfare.
Resmethrin: Resmethrin breaks down quickly, but it is still a man-made chemical that acts as a nervous system toxin. If sprayed in high concentrations, by mistake, it could have detrimental effects on humans and the environment. Resmethrin, for instance, is more toxic to birds than other pesticides. It may also cause cancer in humans according to animal tests.
Ficam: Applied in spray form. Humans should avoid contact, but mammalian
toxicity is very low.
Amdro: Ingested via bait. Moderate toxicity in humans when ingested and absorbed dermally.
Pyrethroids: Usually used in spray form. Mildly dermally toxicity to humans.
Dursban: Ingested via bait. Highly toxic to humans when ingested or absorbed into the skin.
Citronella: Applied in spray for to repel insecticides. Only toxic to human in mass quantities.
Question 4: Synthetic pesticides are more effective than natural pesticides and can be used in much smaller quantities. Additionally, synthetic pesticides are more toxic to humans and the environment.
Question 5: Citronella can be used for training dogs not to bark, as an antiseptic, as a massage oil, as an aromatherapy oil, and as an ingredient in some perfumes. It is toxic in mass quantities or to people who are allergic to it. It may cause swelling and skin irritation if over used.
Question 6: PSP question, answers will vary.