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THE CONCEPT OF RESONANCE ENERGY I N ELEMENTARY ORGANIC CHEMISTRY' ALEXANDER GERO Villanova College, Villanova, Pennsylvania
THE teacher of a first-year mganic chemistry course,
sible even to beginners and gives some scope to their who desires to impart a knowledge of the elements of the reasoning ability. It is therefore quite properly electronic theory of organic chemistry to his students, stressed by all textbooks for first-year organic chernisis faced with the problem of doing so in a way which try. offers some scope to their reasoning ability rather than But it is a different story when it comes to the concept requiring them t o memorize an accumulation of data. of resonance energy. This is simply stated as a fact Since elementary organic chemistry is generally taught to be memorized. Let me quote from a few standard in the second or third year of college, textbooks: - . the question is how to present the electronic theory nondogmatically HbRRoW, AND APPELBAUM ( I ) : ,The possibility of on a level adequate for sophomores or juniors. fievernl electronic arrangements for the same atomic layout is There would be no problem in teaching electronic connected with increased stability." CONANT A N D ~ A T (T8 ) : "Whenever there is s possibility of theory if we could rely on quantum mechanics, but we cannottake it for granted that the average firsbyearfesonance between two electronic structures . . . t.hr oompound organic chemistry student has quantum mechanics suffi- IR ~ ~ S ~ ~ , " , " ' ~ o l c Dare uIe9 ciently at his command. Therefore we inevitably have than would he expected from an examination of to make certain dogmatic statements and ask our stu- their structural formulas as ordinarilv writt,en." FIESERAND FIESER(4): "A . . . consequence of resonance is dents to take our word for them and accept and absorb dissipation of energy, or thermodynamic atdili5ation." them like experimental facts, ~h~ question is: WEETAEIM(5): "The molecule is more stable than would he we go to extremes and treat the entire electronic theory , , + , , , , ,v..y,, as something that has to be memorized, or shall we, and C stmcture may he r* W~,A,S AND HITcA (6): l f ~ h true ns . . . a resonating.hybrid which is more stahlz than can we. t w to k e e ~ doematic statements to a minimum aarded . and derive as man; facts as possible by reasoning. The either." (7): compound^ that resonate are characterized latter course is obviously desirable; it is my contention byMULDOON stability." that it is also possible. More examples could be quoted but the foregoing is Resonance is one of the more difficult concepts for the beginner in organic chemistry t o absorb, and in view representative of the dogmatic statements found in alof what was just said of his acquaintance with quantum most all textbooks. Those texts which do not present mechanics it will help him little to be told that a wave resonance energy dogmatically either do not present it function which is a linear combination of two or more a t all (8) or use the concept without explaining it (9). other wave functions may correspond to a lower energy Yet it would be possible to derive the entire concept of level than any of the component wave functions. But resonance energy by reasoning from experimental data, no knowledge of quantum mechanics is required in order just as the concept of resonance can be so derived. In the following I would like to give a presentation to understand that the C--C .distance in benzene is intermediate between the single bond distance and the of how I attempt to go about arriving with my students double bond distance for carbon. That all C-C bonds in a t all the facts concerning resonance energy by a procbenzene are identical is simply a matter of deduction ess of reasoning which is perfectly within reach of from the chemical properties of benzene, particularly beginners of organic chemistry. We base our discussion on two fundamentals. One is from the absence of ortho isomerism. It is likewise no strain on understanding to learn that amides have acidic the chemical approach to resonance which defines resoproperties while amines are basic, or that chlorine in nance in terms of intermediate structures as an expresvinyl chloride or chlorohenzene is something very differ- sion of experimental results; the other is the concept of ent from chlorine in ethyl chloride, and that the mole- energy levels and the greater stability of a lower energy cules of amides, of vinyl chloride; of benzene, and of level.!~Tochemists and chemical engineers who learned many other substances can he best described by a physical chemistry a t the same time as organic chemissimultaneous combination of two or more structures try, such ideas as free energy, entropy, and probability distribution are quite familiar. But even premedical rather than bv a uniaue stmcture. This emDirkal a&-oach t o resonance is quite acces- students know how t o drive and know that their car -will roll downhill on an incline if they shift into neutral Presented hofore the Division of Chemical Education a t and release the brake. It is easy t o generalim from the 119th Meeting of the American Chemical society, B O S ~ , this commonplace experience to the statement that all April, 1951.
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physical systems if left to themselves will seek out the lowest energy level accessible to them. We drive this home thoroughly before resonance is ever mentioned. Then, having made clear on a number of examples that resonance means that the properties of the actual mole cule are intermediate between the properties we would read off from any of the component structures, we raise the question of the energy content of the resonance hybrid. Using the diagram shown in the figure, in which energy levels are represented on the ordinate axis and El and Eaare the energy levels of t,wo contributing structures I and I1 (for the sake of simplicity we discuss only the case of a hybrid of two structures), we ask the
of structure I1 to the state of the system, i. e., its resonance with all its corollaries (its resonance energy, in particular) increases. The maximum resonance and the maximum resonance energy are attained when El and E2 exactly coincide: resonance and resonance energy are a t a maximum if the energy levels of the contributing structures are identical, a case which certainly arises when the two structures are identical. We now suppose that there is a third contributing structure. We repeat our earlier reasoning, substituting the hybrid for structure I and the third contributing structure for structure 11. ' We arrive by the same logical steps a t the conclusion that the contribution of a third structure necessarily requires an energy level even lower than E, and likewise for each contributing structure: the more structures contributing to the state of the molecule the greater its resonance energy. Thus we are able to achieve the gratifying result that we can derive the concept of resonance, as well as all the most important statements about its bearing on the energy of the hybrid, and the influences which make students: Where in the diagram should the energy level for maximum resonance energy, by plain reasoning E of the hybrid be represented? The first reaction of from experimental facts and from fundamental laws of most students, after having been indoctrinated on the physics and physical chemistry. Nobody claims, of properties of the hybrid as intermediate between those course, that such reasoning can be a substitute for quanof the component structures, is, of course, to say that E tum mechanics and the insight it affords. But if we is intermediate between El and E* (dotted line) ; but it are faced with the choice of either forcing our students is easy to show that this is a fallacy by calling on the to learn quantum mechanics when they are not ready second of our fundamental principles, which would re- for it or making them accept dogmatic statements quire the molecule to change from the hybrid state to handed down from esoteric regions which must forever state I I the energy level of the latter (El) were lower remain closed to them, it is pleasant to be able to find a than that of the hybrid (E). In this case obviously third alternative which gives the student the satisfacstructure I would represent the molecule exclusively tion of arriving a t all the elementary essentials by his and there would be no resonance. If there is resonance own reasoning powers. -as we are led to postulate from experimental evi- LITERATURE CITED d e n c e i t is also a necessarv loeical conclusion that E. the energy content of the iybhd, must be lower tha= (1) LOWY,A., B. H m w , AND P. M. APPELBAUM,"AU 1ntroduction to Organic Chemistry," 7th ed., Jahn Wiley & El; resonance energy, and the greater stability of the Sons, Inc., New York, 1951, p. 56. hybrid, follows as an inevitable corollary of the concept (2) CONANT. BLAIT. TI,^ , ~ ~OF, .T~B.. ~ . A. ,H. ~ --- c h e r n i a +09 --of resonance. gmic Compounds," 3rd ed., The Macmillan Co., New Ynrk, 1947, p. 70. Next we ~roceedto an inauiw into the imnortance "Organic Chemistry,"John of the contributions bf ke struc- (3) FUSON,R.C., AND H. R. SNYDER, Wiley & Sons, Ine., New York, 1942, p. 212. tures. To this end, we imagine that Ez is moved up on (4) FIESER,L. F., AND M. FIESER, "Textbook of Organic the y axis to ever higher energy levels. Always relyhg Chemistrr." D. C. Heath and Co.. Boston. 1950. D. 73. on our second fundamental principle, we conclude that (5) WERTHEIM,"E., "Textbook of 0rgank chemistry," i h d ed., The Blakiston Co., Philadelphia, 1945, pp. 398-9. concurrentlv structure I1 becomes more and more im(6) WILLIAMS, R. J., AND L. F. HATCH,L'IntroductiOnto Organic probable its contribution to the state the mole Chemistry," 5th ed., D. Van Nostrand Co., New York, cule less and less important. In the extreme case the l a--, x rn 1% F. contribution of structure 11is zero, the molecule is com- (7) MULDOON, H. C., "Organic Chemistry," 3rd ed., The Blakiston Co., Philadelphia, 1948, p. 164. pletely represented by structure I, and we are dealing C. C. PRICE,AND H. R. SNYDER, with a nonresonating molecule. Without resonance, (8)Fmow R.C., R. CONNOR, Brief Course in Organic Chemistry," Jahn Wiley & Sons, there is, of course, also no resonance energy. We now Inc., New York, 1947. reverse what we did before and imagine EZcoming down (9)B,,, R. Q., "organic ~ h ~ ~ i ~p rte n~t i~c e ,- ~,a ~, ~ , on they axis; as Ez comes closer to El, the contribution Inc., New York, 1949. \-,
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