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How to off the bugs, or maybe how not to off the bugs !

OK for a while (100s of years) people have been looking for methods of killing off insects which trouble them. The methods used for insect control over the years have included some rather novel ones.   

One I like is “mosquito finito”, it is my 2000 volt electric insect whacker (it leaves no chemical residue in my house). I have found that it works very well on flying pests. But for protecting a farm field we can not use it as I doubt if you can afford to pay a mob of people to rush about whacking each troublesome insect. I also suspect that “mosquito finito” would not work on large insects such as locusts. I can think of a way to modify “mosquito finito” into a weapon which would blast even the most hardy insects but I think in the interests of public safety I will not share this idea with you.   

A more interesting one was reported in “Chemistry in the Marketplace” by Ben Selinger, many years ago he wrote that Pliny sent a young lady to dance in an insect infested orchard to entice caterpillars from the trees. Later on some horrible inorganic poisons such as lead arsenate (PbHAsO4) were introduced, quite rightly these horrible toxins were identified quickly as targets for replacement.   

Below is the unit cell of PbHAsO4, I have left out the hydrogens because they are disordered. Disorder is when in different unit cells the atoms are either different or are arranged in different ways. For example cobalt and nickel have the same crystal structure, and an alloy has the two different elements randomly arranged in the solid. In the solid unit cells are arranged like bricks, next to each other and on top of each other to create the solid.   

Unit cell of PbHAsO4


It is interesting that DDT was brought into service as a non-toxic replacement for the lead and arsenic. I hold a view that sadly it is often the case that todays wonder substance will turn out in twenty years to become public enemy number one. Maybe it might become public enemy number two if some other ‘worthy substance’ beats it to the number one slot. Just so you know what DDT looks like here is a picture of a molecule of a close relative (methoxychlor). The difference between methoxychlor and DDT is that methoxychlor has methoxy (CH3O) groups rather than chlorine atoms on the benzene (six membered rings). These methoxy groups make methoxychlor degrade more quickly than DDT. Before you ask methoxychlor has been banned in both the EU and the USA.   

Here is a picture of methoxychlor, I have used the crystal structure of the compound to create a picture of the molecule. There are some things to note about this molecule. Firstly look at the arrangement of the green chlorine atoms on the carbon at the back. Note that these atoms are placed in sites (staggered arrangement) such that they are as far as possible from the atoms or groups on the next carbon. This is due to something called a steric effect. I will cover steric effects another day on the blog for those who do not know about them yet.   

A molecule of methoxychlor (1,1-di-(4-methoxyphenyl)-2,2,2-trichloroethane)


The above diagram was drawn to allow you to see where the atoms and bonds are, in the following diagram the shape of the molecule as seen by another molecule is shown. In the next diagram the radius of the atoms have been set to about their real size so you can see how the shape of the molecule looks to another molecule.   

Picture of methoxychlor using van der waals sizes for the atoms


When DDT and the other chlorine containing insecticides were brought out they were viewed as wonder substances which would make the world a better place. They were viewed as being cheap, easy to use and non toxic to humans and other higher life forms. While chemicals such as DDT are low in acute (short term) human toxicity when compared with the lead and arsenic, they are clearly not harmless. The problem which the chemical industry did not know about was that the chronic (long term) effects of some of the chlorine containing insecticides on humans are very bad. Some of the organochlorine containing insecticides have been linked to cancer. For example the classic breakdown product of DDT (DDE) has been linked to cancer in humans. A recent paper on this link was written by Mark P. Purdue et. al. (Environmental Health Perspectives, 2009, volume 117, issue 10, pages 1514-1519). Also some of the long lived organochlorine insecticides have been found to harm other higher life forms, a classic example is the shell thickness of bird eggs. It is known that DDE causes the shells of bird eggs to become too thin, this then leads to a reproductive failure (eggs break and chicks die). A recent paper which discusses this matter was published by Eric Mellnk et. al. in Environmental Pollution, 2009, volume 157, pages 2184 to 2188.   

Now the saddest part, I know that many women hold the view that breast is best. This is not a bad idea, I know that in many ways that breast milk is a safer and better way to feed a baby in the third world. One classic 1980s nuclear warfare textbook indicates that a woman with a baby should breast feed her baby under the conditions of nuclear warfare. I would say that while breast feeding is normally a good and healthy way to feed a baby it is not perfect. I can think off the top of my head of three health problems. The HIV positive woman who has a HIV negative baby (solution use formula milk), the woman who has just had a thyroid test using radioactive iodine (I-131) (Solution is to bottle feed the baby on formula milk for a few days while the woman uses a breast pump so that she continues to make milk) and the last and saddest one is organochlorine compounds in milk. In many places in the world organochlorine insecticides have been detected in human breast milk, the fact that these substances can get into the breast milk which a woman feeds to her baby is a good reason for society to regulate the use of long lived organic pollutants.  S.M. Waliszewski et. al. in the Bulletin of Environmental Contamination and Toxicology, 1996, volume 57, pages 22-28 showed (yet again) that DDT and lindane could be detected in Mexican human milk. If the use of DDT is reduced or stopped then the concentration in human milk will go down. I suspect that for most woman their DDT level in their milk is not at a dangerous level, but if you are a woman who has a fear of passing on DDT to your baby then I would suggest that you discuss it with your doctor. A wise precaution would be for a pregnant or breast feeding woman to avoid using DDT and to avoid areas where it is being sprayed.   

In recent times a movement towards using short lived biocides has occurred. The reasoning is that the biocide only needs to be present in the environment for a short time, and that it will decay away into harmless substances quickly rather than lurking about to assail plants and animals. The advantages of these short lived are that they do not linger in the environment and harm non targeted lifeforms long after they were needed but the disadvantage is that they may need to be applied more often than longer lasting biocides.   

For example for keeping the weeds under control in a substation a long lasting herbicide such as sodium chlorate might be a better choice than paraquat. Paraquat becomes inactive as soon as it touches soil while sodium chlorate remains active in soil for years. On the other hand a non chemical method of pest control can be used, I once saw a substation in the Czech Republic where a flock of sheep were used to keep the grass short in one part of the compound.   

One thing in favour of the sheep is that sodium chlorate and paraquat are being banned for environmental and safety reasons but it is very unlikely that sheep will be outlawed. Before we get any further I think it is important to make one thing clear the nastiness or niceness of a compound is not related to if a substance is natural or artificial.   

Cocaine, ricin, alfatoxin, lead and radon are natural. Each is perfectly horrible. In case you wonder what an alfatoxin is, it is a nasty carcinogen which causes liver cancer and is formed by a mold which grows on nuts. This is why it is important for every batch of nuts imported into the UK to be checked for mold and/or the toxin. The structure of the alfatoxin is shown below.    

Alfatoxin, now you get to look at the monster !


 Salbutamol is artificial and is one of the nicest chemicals you can think of. It is a drug which helps to keep many people’s lungs working normally.   

Salbutamol (Nitrogen is shown in blue)


 One of the more nasty natural insecticides is nicotine, it is a pyridine and pyrrolidine compound which has the pyrrolidine ring bonded at the 2 position to the pyridine at the 3 position. In heterocycles it is normal to start counting atoms at the heteroatom.   



Nicotine is a nasty poison which can be absorbed through the skin, I think that nicotine is just as dangerous as some of the worst of the organophosphorus insecticides which are in current use. The LD50 of nicotine is about 50 mg per kilo in rats. (http://www.inchem.org/documents/pims/chemical/nicotine.htm)   

Tetraethyl pyrophosphate (TEPP) is a nasty insecticide (http://www.osha.gov/SLTC/healthguidelines/tepp/recognition.html) which I think is almost as bad as sarin. It is a very poisoning chemical and the LD50 in rats is about 1 mg per kilo. Thankfully TEPP has largely been withdrawn from use in favour of other chemicals. Currently parathion is banned in many parts of the world, or is strongly restricted in many places. With the dire toxicity of parathion I hold the view that these restrictions and bans are a good thing.   

For example parathion has an oral LD50 of about 3 to 6 mg per kilo in many animals, while the less toxic malathion has a LD50 of about 1600 mg per kilo in mammals. When I was a teenager, malathion was supplied to the general public for dealing with nasty insects. So nicotine is more toxic than some of the organophosphorus poisons which used to be sold to the general public.   

Table of LD50 values

Substance Animal LD50
Sarin Rabbit 0.247 mg kg-1
TEPP Rat 1 mg kg-1
Parathion circa 4 mg kg-1
Nicotine Rat 50 mg kg-1
Malathion 1600 mg kg-1

A much nicer class of insecticides are the pyrethrins of which pyrethrin I is a classic example, it is extracted from Chrysanthemum cinerariaefolium which is a nice flower. This flower is grown in africa, the extracted pyrethrins are used to control insects. One use is dog shampoo, one of the weaknesses of pyrethrin I is the fact that it is not light stable.   

The pyrethrins from the flowers tend to be oils so I can not show you a crystal structure of them. But I can show you a salt of the carboxylic acid. The carboxylic acid (chrysanthemic acid) can be formed as an ethyl ester by the action of ethyl diazoacetate on a 1,3-diene. This in the past has been an undergraduate experiment at some university chemistry departments. The carboxylic acid ester is formed as a mixture of cis an trans forms, each of these forms has two stereoisomers so a total of four isomers are known of the carboxylic acid. A recent paper was published (Goffredo Rosini et. al., Green Chemistry, 2007, volume 9, pages 441 to 448) in which a simple method of separating the isomers was described. From the data in this paper here is a picture of the carboxylic acid on the right with the resolving (separation agent) on the left.   

A chrysanthemic acid salt


Even while the pyrethrins are safer than most insecticides they should still be treated with a great deal of respect. They are esters which are sensitive to light, I have not been able to find LD50 values for pyrethrin and I suspect that these values would be high. A high LD50 value means that something is not very toxic. The LD50 value is the dose required to kill haf the population. I am sure you can all imagine how in humans the LD50 for ethanol is likely to be very high, while the LD50 for sodium cyanide is likely to be much lower.    

The measurement of LD50 values is very controversial, maybe another day I might discuss this issue with you. But please understand that my mention of the LD50 values is not an attack on or endorsement of animal testing. One day you might get my view on the matter. One weakness of animal testing is that for some substances such as dioxin the LD50 value differs greatly from one species to another.    

One of the insecticides which is only weakly toxic to mammals which is still very toxic to insects is azadirachtin, this is a very oxygen rich and complex molecule. Now you may have noticed that we have been going from some simple insecticides such as DDT to the more complex ones such as chrysanthemic acid esters. Now we are going to go to a super complex molecule. This is one which is found in neem oil and is responsible for most of the antiinsect activity of the oil. It has a misleading name, normally the letters “aza” in chemistry mean “contains nitrogen” in this case the molecule has the aza prefix but contains no nitrogen. I would like to now present to you azadirachtin.   

Azadirachtin could be the test of tests for any chemist to make it, it has so many different functional groups in it. It has alcohols, ethers, esters, an acetal and an epoxide group. Here is a picture of what the molecule looks like as a flat drawing on paper, I sincerely hope I have not made an error with it.   

Azadirachtin as a flat drawing on paper


Now here is a picture of the real shape of the molecule, this is from the crystal structure. This is what we sometimes call a cluster of spiders.   

Azadirachtin ball and stick view


Now here is the space filling version of the same picture.   

Azadirachtin space filling version


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