Chemistry of Moth Repellents

Chemistry for Everyone edited by

Products of Chemistry

George B. Kauffman California State University Fresno, CA 93740

Chemistry of Moth Repellents Gabriel Pinto Departamento de Ingeniería Química Industrial y del Medio Ambiente, E.T.S.I. Industriales, Universidad Politécnica de Madrid, 28006 Madrid, Spain; [email protected]

When I was a child, as summer approached, I saw my mother place several white balls among the winter clothes (coats, sweaters, blankets, etc.) in our wardrobes. These balls had a distinct and pleasant odor. My mother told me that the balls were “naphthaline” (an old common name of naphthalene) and protected the clothes from moths. This process was fascinating to me, because, after the summer, there were no trace of these balls in the wardrobes. Later, I recalled that moth balls were used by my science teacher to illustrate the sublimation of solids. In this article, I suggest a further example drawn from everyday experiences to encourage critical thinking by chemistry students. Specifically I pose several questions about moth repellents. In previous articles I discussed the influence of temperature on the speed of dissolution of fizzy tablets in water (1), calculations of stoichiometry applied to medicines (2–4), the chemistry of chloroisocyanurates for disinfecting water (5), the accidental formation of ammonium chloride in the laboratory (6), the influence of temperature and NaCl concentration on the kinetics of the osmotic hydration of chickpeas (7), and calculations of stoichiometry applied to a boron fertilizer and to the composition of mineral waters (8) as examples from daily life to intrigue chemistry students. It is acknowledged that an effective way to teach chemistry is to examine the substances used in our daily lives (9– 13) from a pedagogical point of view: from the overlap of science, technology, and society (STS). The STS movement represents an attempt to accomplish the goal of “liberating students from narrow utilities” through an interdisciplinary approach to those three content areas. STS fits into several categories, but one of the goals behind the STS program is to provide a real-world connection for the students between the classroom and society (14, 15).

As the cycle of clothes moths begins, adults (moths) lay 100 to 150 tiny eggs (oval, ivory, and about 0.01-cm long). After about five days when the eggs hatch, the larvae, which grow to ∼1.2-cm length, begin to feed on the fabric, frequently attacking first areas that have been stained with food or beverages (18). The larva stage lasts from 2 to 3 months. During this time they chew the cloth (digesting keratin), causing damage to the fibers, and search for a protected place to prepare for cocoon building. After the pupal stage adults live about 1–3 months. Clothes moths rarely fly to lights at night but prefer darkness. They are near the bottom of the food chain and are eaten by birds, fish, mammals, frogs, lizards, and turtles. The adult stage is not long as their main task is reproduction (where the chemistry of pheromones plays an important role). Clothes damaged by moths are difficult to repair because some of the fabric has been digested by clothes moths’ larvae. Most damage is done to articles left undisturbed for a long time, such as old military uniforms and blankets. The undisturbed time allows for many complete growth cycles.

Clothes Moths

Question 2 Justify, in accordance with the predominant intermolecular forces, why the commercially-available moth repellents are solids that sublime. Relate the presence of molecules in the vapor phase to the detection of odor.

Moths are much more common than butterflies. In fact, almost 95% of all the Lepidoptera are moths. The subdivision is based on differences in lifestyle; butterflies are active during the daytime while moths are active during the nighttime (16). Fortunately, of the more than several thousand species of moths, only a few are known to take up residence in clothes wardrobes. Clothes moths go through complete metamorphosis: egg, larva (grub or crawling stage), pupa (cocoon), and adult (moth). However, it is in the larval stage that moths may damage fabric. Adults (moths) are entirely harmless but during the larvae stage they can digest the keratin contained in organic materials such as wool, hide, fur, silk, feathers, and so forth. They are one of the few organisms that can digest keratin, an insoluble tough structural protein produced by animals (17). www.JCE.DivCHED.org

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Questions To engage the students in the topic of moth repellents and to encourage them to investigate the chemistry in this familiar product, a series of questions is proposed. Students can use any resources to answer the questions.

Question 1 Searching the Internet, chemistry texts, or information on leaflets and packaging of moth-repellent products, find the names and chemical formulas of the most familiar homeuse moth repellents.

Question 3 Find the vapor pressure at 25 ⬚C of the solids used as moth repellents and compare them with values for other solids and for water. Question 4 Weigh a moth ball just after opening its bag and after an hour. Estimate the time for the ball to completely disappear. Compare the calculated time with the data provided by the manufacturer on the package.

Vol. 82 No. 9 September 2005

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Journal of Chemical Education

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Chemistry for Everyone

Answers to Questions

Answer 1 At least four different chemical compounds are commonly used as moth repellents: naphthalene (sometimes referred to as naphthaline), p-dichlorobenzene (or 1,4-dichlorobenzene), camphor (or 1,7,7-trimethylbicyclo[2.2.1]heptan-2-one, a bicyclic terpeneketone), and hexachloroethane. The structural and chemical formulas of these compounds are shown in Table 1. Although camphor has a long history of usage in clothing storage to kill adult and larval forms of clothes moths, the predominant material, until recently, had been naphthalene sold as “moth balls”. Currently p-dichlorobenzene is the compound most widely used as moth repellent, as the students can learn by reading in the packaging of moth-repellent products in supermarkets or searching the Internet. Hexachloroethane has also been used as a moth repellent. Answer 2 The four compounds cited in the previous question are solids at room temperature. However, all of them sublime easily. When the packages containing them are opened, the compounds are transformed from a solid to a gas. This happens because the solid seeks to achieve a phase equilibrium with its vapor. If no vapor of the substance is present, then vapor forms via sublimation. The released vapor acts as an insect repellent because it is toxic to the moths. The predominant interactions in the solid are weak London dispersion forces formed among the nonpolar molecules. Camphor is an exception where the predominant attractive forces are dipole–dipole interactions. In every case both the melting and boiling points of the moth-repellent compounds are low compared with typical solids (Table 1). Smell depends on sensory receptors that respond to airborne chemicals. In humans, these chemoreceptors are located in the olfactory epithelium. The molecules we perceive as smells are called odorants. They stimulate sensory nerve cells (sensory neurons) at the top of the nasal cavity and these

respond by sending impulses to the brain. There are several theories to explain olfaction. Most evidence supports a stereochemistry theory of odor. In this theory, molecules that fit into a similar primary odor family have roughly the same molecular shape and size. The book by Theimer (19) is recommended for further readings on the science of the sense of smell. For the detection of a penetrating (as in the moth repellents), obnoxious, or sweet odor a substance needs to evaporate sufficiently for the organism to sense the gaseous compound. Owing to their characteristic odor, the compounds used as moth repellents are occasionally used as deodorizers.

Answer 3 The vapor pressures of several solids are given in the Handbook of Chemistry and Physics (20). The concept of vapor pressure is difficult for students to understand in the case of liquids and even more difficult in the case of solids. This exercise can help students better understand the concept. In addition, a good comprehension of the vapor pressure concept is essential for studying phase diagrams. The values for vapor pressure at 25 ⬚C for the four moth-repellent substances; three solids, benzoic acid (an organic solid), iron, and copper; and water are given in Table 2. As shown in this table, the vapor pressures of moth repellents are various orders of magnitude greater than those of common solids (50 to 62 orders of magnitude is beyond comprehension), such as copper or iron, and closer to the value for water and for the benzoic acid, which is another compound that sublimes easily. Answer 4 The fact that these substances volatilize appreciably at room temperature can be shown by weighing a commercial moth ball on a balance, for example, at the beginning of a class and an hour later. After placing the moth ball in the laboratory, at 22 ⬚C, it is possible to observe, for example, a change in mass from 4.788 g to 4.667 g in 1 h, that is, a decrease of 2.5% in the mass. Without considering the reduction of the external surface of the moth ball we can pre-

Table 1. Some Characteristic Substances Commonly Used as Moth Repellents Antimoth Substance

Structural Formula

Naphthalene

1,4-Dichlorobenzene

Cl

Cl

H 3C

Chemical Formula

CAS No.

Mass/ (g/mol)

Melting Point/ºC

Boiling Point/ºC

C10H8

91-20-3

128.2

180

218

C6H4Cl2

106-46-7

147.0

153

175

C10H16O

464-49-3

152.2

178

204

C2Cl6

67-72-1

236.7

CH3

Camphor H 3C

O

Cl Cl

Hexachloroethane

Cl

C

C

Cl

0187 (sublimes)

Cl Cl

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dict that in less than two days the moth ball will disappear completely. Experimentally it is easily to prove that in only a few days the moth ball disappears under the exposed conditions. Nevertheless, the manufacturer usually indicates that the mothballs will provide protection up to several months. Obviously the difference is due to the fact that, when the moth ball is with the clothing in a wardrobe (closed container), the environment becomes saturated from the vapor. The concept embodied here is one of the diffusive and convective transports of the compound away from the surface of the mothball. The wardrobe does become saturated with the vapor and because there is no mass transport of vapor out of the wardrobe the moth ball lasts longer than a few days. More about the Moth Repellents The proposed questions are open. In fact, students (and instructors) can find additional interesting chemical data related to these topics. For example, on the packaging of one commercial antimoth product can be found the statement “Its formulation, without naphthalene or p-dichlorobenzene, is very useful against the moths, without persistent odors and without chemicals”. However, the composition indicated on the same packaging shows that the product contains 0.095% of permethrin. Is this not a chemical? The students could be challenged to explain what a chemical is. Moreover, the fact

O H C

Cl

O O

Cl H 3C

CH3

Figure 1. Structural formula of permethrin.

CH

O H C

H 3C

CH O

CH 3 H 3C

CH3

H 3C

CH H 2C

CH3

Figure 2. Structural formula of empenthrin.

that certain manufacturers claim their product is useful without chemicals could be the basis for a semester’s worth of chemistry and society discussions. These questions could serve to explain more, as a basis for further discussion, about the antimoth products to students. Today, apart from the compounds cited, there is an increasing use of insecticides such as permethrin or empenthrin as antimoth agents. Both compounds are pyrethroids (semisynthetic derivatives of the pyrethrins), practically odorless, with high efficiency in destroying insects, and with a low toxicity to mammals compared to other substances. They act against moths in all its phases (egg, grub, pupa, and adult insect). Pyrethrins are the natural insecticides produced by certain species of the chrysanthemum plant that act as a contact poison, which quickly penetrates the nerve system of the insect. Permethrin, with a chemical formula of C21H20Cl2O3, is the common name of the 3-phenoxybenzyl (1RS )-cis,trans-3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropanecarboxylate. Empenthrin, C18H26O2 , is the common name of the (E )-(RS )-1-ethynyl-2-methylpent-2-enyl (1RS,3RS;1RS,3SR )-2,2-dimethyl-3-(2-methylprop-1enyl)cyclopropanecarboxylate. The structural formulas of permethrin and empenthrin are shown in Figures 1 and 2, respectively. It could be noted and discussed that more than 5000 people, including nearly 4000 children under the age of six years, are annually poisoned by naphthalene and p-dichlorobenzene, according to the American Association of Poison Control (17). Other alternative moth repellents, but less effective, are several herbal mixtures (labeled as “products free of chemistry”), cedar chips, laurel leafs, and the use of ultrasounds. As a remark for general culture, perhaps this would be a good occasion to note to students that humanoids have been on earth for more than 3 million years, while insects have existed for at least 250 million years. There are several further connections to other general chemistry topics related to the moth-repellent compounds, such as solubility in different solvents, understanding their industrial manufacture (for example, naphthalene is the most important single constituent of coal-tar and camphor is obtained from the wood of the camphor tree by steam distillation), use and importance of insecticides, and so forth. Concluding Remarks

Table 2. Vapor Pressure of Solids Used as Moth Repellents and Additional Solids and Water for Comparison Vapor Pressure/ (mm Hg)

Substance Naphthalene

0.2

1,4-Dichlorobenzene

1.0

Camphor

0.3

Hexachloroethane

0.5

Copper

~1 x 10᎑50

Iron

~1 x 10᎑62 7 x 10᎑3

Benzoic acid Water

23.8

NOTE: Vapor pressure given at 25 ⬚C.

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In conclusion, a set of questions about well-known household products, such as moth repellents, can be used to motivate students to understand everyday phenomena through their underlying of chemical concepts. I believe, according to my experience (21), that practical applications and everyday examples of the type described in this article and other sources (22) will pique the interest of students to greater understanding of general chemistry principles.

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Acknowledgments The author gratefully recognizes the support provided by the Escuela Técnica Superior de Ingenieros Industriales (Universidad Politécnica de Madrid) under the project 4A: Apoyo al Aprendizaje Activo de los Alumnos (Support for


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Students’ Active Learning). I would also like to thank the reviewers and editors for useful suggestions. This article is dedicated to my mother, Dolores Cañón, who encouraged my interest in basic concepts.

13.

14. 15.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

Pinto, G. Educ. Chem. 2000, 37, 71. Pinto, G. Educ. Chem. 2001, 38, 150. Pinto, G. Educ. Chem. 2003, 40, 11. Pinto, G. J. Chem. Educ. 2005, 82, 1509–1512. Pinto, G.; Rohrig, B. J. Chem. Educ. 2003, 80, 41. Pinto, G. Educ. Chem. 2003, 40, 80. Pinto, G.; Esin, A. J. Chem. Educ. 2004, 81, 532. Pinto, G. Educ. Chem., in press. DeLorenzo, R. J. Chem. Educ. 1999, 76, 503. Jones, M. B.; Miller, C. R. J. Chem. Educ. 2001, 78, 484. Snyder, C. H. The Extraordinary Chemistry of Ordinary Things, 3rd ed.; Wiley: New York, 2002. 12. Didáctica de la Química y Vida Cotidiana; Pinto, G., Ed.; Sección Publicaciones ETSII Universidad Politécnica de Madrid: Madrid, 2003. Also available in pdf format in the

16. 17. 18. 19. 20. 21. 22.

Web address: http://quim.iqi.etsii.upm.es/vidacotidiana/ Libro.htm (accessed May 2005). Karukstis, K. K.; Van Hecke, G. R. Chemistry Connections. The Chemical Basis of Everyday Phenomena, 2nd ed.; Academic Press: New York, 2003. Yager, R. E. Social Educ. 1991, 198, 54. Yager, R. E. Science/Technology/Society as Reform in Science Education; State University of New York Press: New York, 1996. Guide to the moths of Great Britain and Ireland. http:// www.ukmoths.force9.co.uk/ (accessed May 2005). Clothes moths. http://www.conservation.state.mo.us/conmag/ 2002/09/40.htm (accessed May 2005). The bug clinic. http://www.bugclinic.com/clothes_moths.htm (accessed May 2005). Theimer, E. T. Fragance Chemistry. The Science of the Sense of Smell; Academic Press: New York, 1982. Handbook of Chemistry and Physics, 72nd ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1991; Sections 5 and 6. Pinto, G. Anales de la Real Sociedad Española de Química, 2004, 100, 37. For an example, see Kelter, P. B.; Carr, J. D.; Scott, A. Chemistry, a World of Choices, 2nd ed.; McGraw-Hill: New York, 2003.

The structure of a number of the molecules discussed in this article are available in fully manipulable Chime format as JCE Featured Molecules in the September issue of JCE Online.

JCE Featured Molecules

an interactive modeling feature, Only@JCE Online

http://www.JCE.DivCHED.org/JCEWWW/Features/MonthlyMolecules

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Chemistry of Moth Repellents

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