The following are two parts of a question on the Fall Term Final Exam for 2001-2002
Draw the best alternative resonance
structure you can imagine for A, and the best for B. In each
case specify which OMO and which UMO are being mixed to
generate the resonance structure. (A) The HOMO
; the LUMO is p*C=O (B) The HOMO is
nO ; the LUMO is p*C=C Which molecule, A or B, should be more
stabilized by resonance? Explain your
thinking. B should be more
stabilized. It has both an unusually high OMO
(nO) and an unusually low UMO
(p*C=C). A has an even lower UMO
is a pretty poor excuse for a OMO.
Still such resonance structures explain stabilization due to "hyperconjugation".
Draw the best alternative resonance structure you can imagine for A, and the best for B. In each case specify which OMO and which UMO are being mixed to generate the resonance structure.
(A) The HOMO is sC-H ; the LUMO is p*C=O
(B) The HOMO is nO ; the LUMO is p*C=C
Which molecule, A or B, should be more stabilized by resonance? Explain your thinking.
B should be more stabilized. It has both an unusually high OMO (nO) and an unusually low UMO (p*C=C).
A has an even lower UMO
is a pretty poor excuse for a OMO.
Among 57 exam papers there were 39 different answers for part A (5 of them sensible) and 29 different answers for part B (6 of them sensible)! This suggests that some review of the use of resonance structures is in order before applying these ideas in the second semester. One way to review would to evaluate all of the resonance structures that were proposed, finding the correct ones, and identifying what is wrong with those that are not sensible. I suggest that you do this, individually or in groups, until you are satisfied that, faced with the same type of problem in the future, you will have no difficulties.
After initially being rocked back on my heels by the variety of incorrect resonance structures that were proposed, I decided I was not too disappointed for two reasons.
First because we did not really drill on the use of resonance structures after the first week of class, choosing to focus on an orbital view of structure and reactivity, and the exam question was one which required a rapid response and included an unfamiliar "hyperconjugated" structure.
Second, because in many cases correct use of resonance structures depends on having mastered lore, and we haven't dealt yet with the lore of aldehydes and enols. Having learned facts allows you to pretend to yourself that you are predicting things from first principles, when in fact you are just restating or generalizing empirically from facts you already know. There is nothing fundamentally wrong with this approach, and in fact a version of resonance theory known as "mesomerism" was developed by organic chemists before they knew anything of quantum mechanics. You will get better at this kind of theory as time goes on and you learn more facts.
You can get a long way without knowing all the lore by using quantum mechanical ideas you have already mastered - knowing what bonds are and how HOMOs mix with LUMOs. As you will learn second semester, organic chemistry cannot be mastered by using fundamental theory alone, lore is essential in dealing with complex reality. But chemical lore is easire to organize and masterif you have guidance from bonding theory.
Still, it is important, both for everyday convenience and for communicating with others who put more reliance on resonance structures, to be able to manipulate such structures properly. Many individuals would be scandalized by some of the incorrect structures that appeared on the exam papers.
One of the most common kinds of resonance structure involves replacing a single covalent bond (or one member of a double bond) between atoms of different electronegativity by complementary charges on the bonded atoms. This indicates bond polarity due to uneven sharing of the bonding electron pair (because of poor energy match). However this kind of resonance doesn't lend itself very well to answering the question above, which asks for identification of HOMO/LUMO interactions that give rise to the resonance structures. Substantial credit was given for such polar resonance forms, but better answers were available.
The following criteria allow you to evaluate proposed resonance structures:
1) Two resonance structures must involve the same set of atoms (don't add or lose atoms)
2) The structures must have the same net charge (don't add or lose electrons)
3) The atoms must be in the same position (the structures show different bonding schemes for a single set of atomic positions)
4) The number of electrons associated with an atom should not exceed the capacity of its valence orbitals (octet for 2nd-row elements)
5) Be sure that the formal charge on an atom reflects its nuclear charge, its half-interest in bonding pairs, and its possession of unshared pairs or odd electrons.
6) Preserve the maximum number of bonds. Relocate bonds, but don't lose bonds without having a very good reason for doing so (bond polarity maybe)
7) When polar forms are used to replace bonds, be sure that the charge distribution properly reflects relative electronegativity (energy mismatch).
8) If the structure reflects OMO/UMO mixing within the molecule, be sure the two orbitals overlap (s and p orbitals on the same atom are orthogonal).
Point 8 is highlighted in the above list, because it is the one question 7 was aimed at. The other criteria allow you to dismiss erroneous structures, but effective mixing between localized OMOs and UMOs is the main reason one must consider resonance structures altogether.
First we construct a preliminary bonding scheme for a molecule based on localized orbitals formed as bonding/antibonding pairs by overlapping of two AOs on adjacent atoms. The AOs may be hybridized, and the scheme may include unpaired AOs with one or two electrons (unshared pairs or radicals) or none at all, as necessary. The bonds may be polarized because of energy mismatch between the two AOs of a bond (a primitive type of resonance writes polar structures to denote this fact).
Now the idea is to identify the high OMOs and low UMOs of this preliminary structure. If they overlap, the preliminary structure is inadequate for predicting the molecule's properties. The true molecule will have different properties (lower energy especially) than predicted by the preliminary structure because of the mixing between these localized MOs. If there are no important OMO/UMO interactions, the preliminary structure should be adequate.
The higher the energy of the preliminary OMO, and the lower the energy of the preliminary UMO, the more drastic the alteration in properties due to their overlapping, and the more important the resonance structure. The resonance structure will show increased bonding in the overlap region between OMO and UMO. It will show decreased bonding where there was overlap within the original OMO, or a node within the original UMO. It will show increased bonding where there was a node within the original OMO, or overlap within the original UMO.
With practice applying criteria 1-7 becomes intuitively obvious. Applying criterion 8 typically requires some thought.
Working conscientiously through the checklists below should help build your skills and confidence for understanding and using resonance structures. Among all the resonance structures there is only one really great one, the one indicated in B above. This was the point of question 7d.
Click here for the table of proposed aldehyde resonance structures, shown at the right, as a pdf file that should be easy to print out.
Click here for a blank aldehyde checklist (pdf file) to use in evaluating the proposed resonance structures for the aldehyde.
Click here for a checklist aldehyde answer key (pdf file) to use in checking your work.
Click here for the table of proposed enol resonance structures, shown at the right, as a pdf file that should be easy to print out.
Click here for a blank enol checklist (pdf file) to use in evaluating the proposed resonance structures for the enol.
Click here for a checklist enol answer key (pdf file) to use in checking your work.