chemical vignettes ROBERT C. BRASTED University of Minnesota Minneapolis, 55455
Professor Robert Brasted's column, "Chemical Vignettes," begins with this issue. Planned as a companion column to "Chemical Principles Exemplified," "Vignettes" will attempt to illustrate applications of chemistry es~eciallvin eneineerine and other scientific areas. Reeders are encouraged t o send items appropriate t o this column td the aithor.
Understanding a Culprit before Eliminating It
and/or fabric deterioration). From this road map of arrows and dots there are many variations of our environmental story. Perhaps you can think of others but here are a few that have become part of our course.
An Application of lewis Acid-Base Principles to Atmospheric SOzas a Pollutant Mechanism of Simple Reactions, Air Polluting, and Our Environm e n t 4 e n e r a l Chemistry
Few of our chemical culprits in the atmosphere cause more immediate problems than the species SOz. Most of us in the teaching business spend time (I hope) not only on its preparation but cover as well something on the properties (a bit on bond angles, bond strengths, and perhaps dipole moment) of this molecule. We too often leave the subject shy of a state that may be of great concern to the student. Why not continue discussion of the subject of SO2 a bit farther under the topic of acids and bases. When the Lewis theory is treated a magnificent opportunity is open to us as teachers in helping the student understand why in an area laden (and here ''laden" may mean only a few parts per million) with SOz his skin and eyes become irritated, his clothing rots, his car goes to pot (maybe better the car than the student), and why her panty hose have such a short half-life. Metal corrosion and fabric deterioration are unbelievably rapid in an atmosphere of SO?. I t is understandable that corrosion is linked closely with oxygen and with p H . Thus, we can move rather sensibly from Cloud 9 to our own hack yard by looking a t what electron pair chemistry can do to explain the interaction of atmospheric moisture and our culprit. A few Lewis dot formulas can take the place of much verhage.
1) Electron configurat,ion including resonance structures. 2) Electraphilie and nucleophilic systems-introduction of the nomenclature of Lewis concepts. 3) Formal charge calculations. 4) Charge density on the central atom (sulfur in this system). 6) Proton re- and expulsion. 6 ) Bronsted theory and its compatibility with Lewis. 7) Inductive effects, proton migration and partial chmge (+6 and -6). 8) Corrosion as a function of hydrogen ion concentration as well as anions such as sulfite, sulfate, and chloride. 9) K dissocitltion (primary and secondary) and pH. 10) General application of solution process of metal. 11) Organic materials (polymers) in an atmosphere of H+.
A Weevil's Loss is Our Gain or Sex Strikes the Pages of the Journal Cis-trans Isomerism, Siructural Chemistry, Organic Chemistry and an Interface with Agricultural or Soil Chemistry and Entomology
I t has been estimated that as much as 70% of the DDT used in the agricultural insecticide and pesticide control in the United States is applied in the southern states in cotton growing. The application is largely for the control of the boll weevil. The compound 1-acetoxy 10-n propyltrideca-trans5,9 diene CH&H,CH,
.. :O: ....10: .. acid orelectrophile QS
H'"H
CH,CH&H2-C-CH-C&-CH~-CH=CH-C&-CHH-CHCH-CHH-acOcHa
...... 8.
+
2,.
I
formalcharge
fi..~Y+~
:WO'
-f
proton migration
base or nucleophile
sulfurous acid
Contrast K, = 10-" with Kt LZ 1 X 10W2 for HzSOsfollowed by 11 nH+ + M+" nH2 (metal
+
+
has been isolated using the extract from a/a million virgin female pink boll worm moths, pectinophora gossypiella (Saunders). The teacher can introduce to the student a host of relevant matters including the very elegant synthesis of the compound and the many instrumental techniques used to identify the factors which prove that the trans-isomer is a million times more active than the cis-isomer as a sex attractant. Volume 47, Number 6, June 1970
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447
As the name indicates the trans positioning (of biological interest) is about the double bond between 5 and 6 (not a t the 9-10). If the synthesis and identification are of concern the reader is referred to Pattenden.' To show that nature is willing to give "equal time" to a cis-isomer, we find that a stereospecific compound cis-2-isopropenyl-1-methylcyclobutaneethano12 H,C
is a sex attractant for the Anlhonomus grandis (Boheman) boll weevil. The synthetic chemist seems always willing to rise to the occasion when a specific problem is presented. There is no reason why the fundamental and (to most people) surprisingly important differences based upon stereoisomerism cannot be presented even to the general chemistry student. As.was the case in the previously mentioned "propylure" (versus the "grandisol" just described) the elegant synthetic and structural problems can he reserved for the upper division or graduate student.
448
/
journal o f Chemical
Education
What might he a relatively sterile subject, then, may be brought to the student a t any level (with appropriate simplification) showing the importance of understanding basic chemical concepts as they affect our environment and ecology. If the boll weevil could be brought under control by letting "Mohammed come to the Mountain" rather than bringing the mountain of DDT to the weevil, we stand to regain much lost ground in our wildlife preservation. We may even find that DDT is not necessary a t all if the insects attracted to the source of the trans-isomer just described are subjected ta a nontoxic material (to the environment) in small amounts and in a more restricted area. If the trans-isomer is "sex attractive," why not devote more of our scientific skills to isolating sex repellents. The literature is not without information on such compounds. It wouldn't take very many generations of weevils (or other pests) not on speaking terms to whittle them down to manageable numbers. Here is a cry (hopefully not in the wilderness) for more and wisely directed research rather than a sometime-voiced sentiment for a moratorium on all research. 1 PATTENDEN, G.,J . Chrm. Sac., 18(c) 2385 (1968). 1
C h m . and Eng. News, 48 [4] 40, (1970).
