Teaching the Concept of Resonance with Transparent Overlays W. S. Riiardson Auburn University at Montgomery. Montgomery, AL 36193 The concept of resonance plays a fundamental role in understanding the electronic siructure and reaction properties of many molecules and ions. Most chemistry teachers have had the occasion to introduce the conceot to studenta or to reintroduce it to those who have previou&y studied the tonic. The concent is included in manv eeneral or introductoiy chemistry c&rsesl as a necessar; model for the structure of molecules or ions with properties, such as bond length, charge density, and reactivity, that cannot be adeauatelv. explained with a single Lewis structure. The reso. nance concept is reintroduced in organic chemistry2 and used extensivelv throunhout the course to clarify physical and chemical p;operties of molecules and reaction & m e diates, becoming one of the basic themes in the study of organic chemistry. There are several difficulties inherent in the introduction of the resonance concept. The first is immediately encountered when one explains the need for resonance theory in the first nlace. Students are iust becomine comfortable with ~ e w i structures s when weboint out that the structures do not satisfactorily represent all molecules or ions. Once students see the need for an alternate representation of these molecules and their faith in Lewis structures has heensubsequently undermined, we present several Lewis structures, related to each other bv the double-headed arrow, as a representation for a molecule or ion. After ntudents have 1)ecumefamiliar and reasonably competent with the use of the resonance concept, the relationship of the various, arrow-connected Lewis structures to actual molecular structures remains a considerable conceptual problem t o most students. The use of the term "resonance" has contributed to this difficultv. Too manv students think in terms of "resonating" struciures, whici, in turn, leads to the concept that the realsubstance is an equilibrium mixture of molecules represented hy the Lewis structures, although this idea may include the insidious misconception of rapidly changing structures that can he neither separated nor independently observed. Students may be reasonahly satisfiedwith this misconception since, in their minds, thk results are virtually the same, and they can still use the resonance principle to answer many questions and explain observations in a manner similar to those students who have a correct understanding of the actual meaning of the Lewis structures. The analogy of the rhinoceros as a cross between
Presented, in part, at the 34th Southeast Regional Meeting of the American Chemical Society. Birmingham. AL. November 3-5,1982. '(a) Mortimer, Charles E. "Chemistry". 5th ed.: Wadsworth. Belmont. CA, 1983:pp 144-146. (b)Ebbing,Darrell D. "General Chemis try," 1st ed.; Houghton-Mifflin: Boston, 1984; pp 218-220. (c)Cotton. F. Albert; Darlington. C. Leroy: Lynch. Lawrence D. "Chemistry: An Investigative Approach", rev. ed.; Houghlon-Miffiin:Boston, 1976: pp 41 1, 412. (d) Metcalfe, H. Clark; Williams, John E.; Castka, Joseph F. "Modern Chemistry", teacher's ed.; 1978; pp 114-115. %(a) Morriston. Robert T.: J. Boyd, Robert N. "Organic Chemistry", 4th ed.: Allyn and Bacon: Boston. 1983; pp 395-434. (b)Streitwieser, Andrew, Jr.; Heathcock. Clayton H. "Introduction to Organic Chemistry", 2nd ed.; Macmiilan: New York. 1981; p 1014. (c) Solomons. T. W. Graham "Organic Chemistry". 3rd ed.;Wiley: New York. 1982;pp 14. 15.412-416.
518
Journal of Chemical Education
a dragon and a unicorn28 has been helpful with this particular problem. Use of the double-headed arrow contributes t o the confusion since i t is reminiscent of the two half-arrows used to represent chemical equilibrium. If, however, the individual Lewis structures that represent the resonance hybrid structures are introduced without the use of the douhle-headed arrow, and if the molecule or ion is represented by the overlay of one Lewis structure over another, the concept of a composite structure for the actual molecule or ion can be introduced without confusing it with equilibrating structures. In turn, the concept of a single structure for the real molecule is easier for the students to conceive. The separate Lewis structures connected by the douhle-headed arrow can he introduced after the concept of resonance is better understood. At the same time topics such as hond order, partial charge, and intermediate hond length can be introduced and their structural derivation can be illustrated. Benzene, for example, may he represented by the equal contribution of two Lewis structures of equal energy. Individual Lewis structures of benzene with alternate double honds are drawn in different colors on separate, colorless acetate sheets. The structures are carefully drawn to represent the double bond by a shorter side of the hexagon than the side that represents the single hond. Small cross marks are drawn in one corner of each acetate sheet so that one can he placed over the other in the same relative position each time they are used. The Lewis structures are drawn so that one structure is slightly offset relative to the others and the individual features of each structure can be distinguished when one is placed over the other matching the cross marks. Each structure is projected separately so that the subtle hut different features of the structures can be emphasized. The structures are then projected together with one over the other by the method described above. This projection represents the combination of the two Lewis structures that, in turn, represents real benzene molecule. Since the molecule is represented by two structures, a reasonahle representation of benzene is then drawn by dividing the overlay projection by two. Thus, the usual hexagon,2b,drawn with a solid outside line and a dashed line inside the solid line, represents the real benzene molecule. One can now indicate that the realcarbon-carbon bond order is 3 bonds divided by 2, or 1.5, and that the carbon-carbon bond is therefore intermediate in length between that of a carbon-carbon single hond and a carbon-carbon double hond as represented by the solid and dashed line. This illustration explains the symmetrical structure of benzene as a hexagon with equal bond lengths as opposed to each Lewis structure with unequal single and double honds. As a result, the overall equal distribution of pi electrons is represented by the composite structure drawn as a representation of the two overlapping structures. The overlap method is also useful for the development of the concept of a partial charge on the atoms of an ion. The ally1 cation, for example, is represented by two Lewis structures with a positive charge on one of the terminal carbon atoms. Since the structures are equal in energy and contrihute equally to the structure of the ion, the actual cation may he represented by the overlay of the two Lewis structures. The structural representation of the cation is then drawn from the projection by the division of the overlays by two as
described above for the benzene molecule. The resulting structure nicely indicates a partial positive charge (+W)on the terminal carbon atoms as well as the 1.5 bond order for each terminal carhm-cilrhon hond. The equal contribution of three 1.ewis structures can easily he illustrated for substanres surh as the cnrhonate anion. Division of the overlays by three presents the equal, partial negative charge (-213) on each oxygen atom &d the 1.33 bond order (413) of each carbon-oxygen bond as represented by a dotted line that is lighter than the dashed line used in the benzene and ally1 cation structures. Three overlavs of Lewis structures are also useful for illustrating the effeit of relative stnhilities and thus the relative contributions of these structures to the distribution of the ni electrons in an ion or molecule. Consider the enolate ion: The Lewis structure with the negative charge on the oxygen is lower in energy than the structure with the negative charge on the carbon atom and contributes more, therefore, to the actual structure of the anion. As an approximation of its contribution to the actual structure of the ion, this Lewis structure is represented twice while the Lewis structure with the negative charge on the carbon atom is represented only one time. The "&erage" structure of the actual ion illustrates the larger negative charge on the oxygen atom (-213) with less charne (-113) on one of the terminal carbon atoms. The difference detwekn pi electron density of the carboncarbon bond involved in ni-electron delocalization and that of the carbon-oxygen double bond is represented by bond
orders of 1.67 and 1.33. resnectivelv. The ni carbon-carbon bond is thus represented b; a dashed line'while the pi rarbon-oxvaen bond is renresented bv a dotted line. The use of more than three overiays is cumbersome, and the value of the illustration diminishes, but the concept of relative contribution of various Lewis structures may be extended if we ask students to imagine the resulting "average" structure if we used, say, nine overlays of the 'nolate Lewis structure with the negative charge on the oxygen atom with only one 'Lewis structure of the other. In that case, the oxygen would carry more of the negative charge than the terminal carbon atom and the carbon-oxveen double bond order would anproach one. This process can be extended to represent t i e actual relative contribution of Lewis structures if the contributions are known. After annrouriate examoles of resonance have been discussed wiih particular emphasis placed on the nonexistenre of the individual Lewis structures, the use of the douhleheaded arrow ran be introdured with the separatestructures drawn on the chalkhoard. Havinr heen introduced to the resonance concept without the double-headed arrow, the students are less likely to confuse a Lewis structure with the actual structure of a molecule or ion or to consider resonance to he an equilibrium phenomenon. Note that if the concent of resonance is introduced after tautomerism, the overlay method will emphasize one major difference in the two concepts-the location of atoms in the same place in the case of resonance structures while tautomeric structures differ in the location of one or more atoms.
Volume 63 Number 6 June 1986
519
