Draw the resonance structure of benzene
Answers
Additional reading recommendation: You may find Chapter 2 of Pushing Electrons by Daniel P. Weeks (Saunders College Publishing; ISBN 0-03-0206936) to be a useful tool for mastering the fundamentals of resonance structures.
Discussion: Consider the Lewis structure of the carbonate ion, CO32-. The Lewis structure for this ion has a carbon-oxygen double bond, and two carbon-oxygen single bonds. Each of the singly bonded oxygen atoms bears a formal charge of 1-. (Review the formal charge tutorial if needed.) But which of the three oxygens forms the double bond? There are three possibilities:
These structures are similar in that the have the same types of bonds and electron positions, but they are not identical. The position of the carbon-oxygen double bond makes them different. In structure A the double bond is with the top oxygen atom, in B with the right hand oxygen atom, and C with the left hand oxygen atom. These oxygen atoms are at different places in space, so these are different structures. Consider this analogy: when the hands on a clock are at a 90o angle, the time could be 3 o'clock, or 6:15. The angle between the hands stays the same, but because they point to different places in space, they indicate a different time. The position of the carbon-oxygen bond is like the fixed angle of the clock hands, but pointing to different places on the clock face.
When more than one Lewis structure can be drawn, the molecule or ion is said to have resonance. The individual Lewis structures are termed contributing resonance structures. Resonance is a common feature of many molecules and ions of interest in organic chemistry.
Which one of these three structures is the correct one? How could we tell? If structure A was correct, laboratory measurements would show one shorter bond (the carbon-oxygen double bond) and two longer bonds (the carbon-oxygen single bonds). Measurement of structures B and C would give the same results as well. As it turns out, laboratory measurements show that all three bonds are equal and between single and double bond length. This suggests that none of the Lewis structures we have drawn are correct. It further suggests that the actual structure has three equal carbon-oxygen bonds that are intermediate between single and double bonds.
Perhaps the three Lewis structures for carbonate ion are in rapid equilibrium. The structures are changing so quickly that all we see can measure is an average blur (structure D), instead of being able to detect individual structures. By analogy, consider a camera with the shutter left open. The picture would be a blur that looks like A, B and C all at the same time. Structures A, B and C have the same bonds and electron distribution, the only difference is the position of the bond. Thus the three structures have equal stability, and the three structures would occur to the same extent at equilibrium. The blur we see would look like 1/3 A, 1/3 B and 1/3 C. The bond lengths would be a blur as well; we would perceive them as being something between single and double bonds. The charge would shift so rapidly that we would see it on all oxygen atoms at once. Since each oxygen atom has a 1- charge in two of the three equilibrium contributors, each oxygen atom would appear to have, on average, a charge of 2/3-.
All laboratory experiments have failed to detect structures A, B and C. No matter what experiments are performed, analysis has always concluded that D is the best description for the structure of the carbonate ion. This suggests that A, B and C do not exist, and are not adequate descriptions for carbonate ion at any time. The actual structure is D, and not equilibrium between A, B and C.
That D appears to be a combination of A, B and C still appears to be a useful way to determine the actual structure of carbonate ion. The only problem is that A, B, C and D are not in equilibrium. Can they be interacting in some other way? The answer is that the true structure of carbonate ion appears to be a simultaneous hybrid of the three resonance contributors A, B and C. Structure D has features derived from A, B, and C, but is never just A or just B or just C. For example, D has some double bond character, and