A New Approach to the Generation of Sigma Complex Structures Joseph G. Young Chicago State University. Chicago, IL 60626
The subject of electrophilic aromatic substitution is covered in all introductory organic texts. This topic is the source of much difficulty for many students. For the most part students simply memorized a list of directors, activators, and deactivators. Armed with this, they hope to be able to answer questions by regurgitation. This approach by students seems regrettable but is their natural response to a situation where understanding is lacking. We have mechanistic models of these reactions tbat are quite powerful and should aid the student in understanding these reactions. The central argument of these are the cationic sigma complex for electrophilic aromatic substitution and a similar anionic complex for the non-benzyne nucleophilic aromatic substitution. The resonance delocalization of charge in these intermediates as represented by the drawing of resonance structures is the key to understanding the pattern of substitution. However, most students are unable to arrive at these intermediate resonance structures through c h is traditionallv. -Drethe "electron ~ushine"a-~- ~ r o a that sented to them. I have develo~edand taueht a different a ~ ~ r o a to c hthe generation of thksestructures that most stud&ts seem to be able to use. The basic idea is for the student to draw all the possible ways in which the electrons in the intermediate can hedistributed in the available p atomicorbitals and note the bonding that can result. ~ h e nbased i on their own sense of what structures are "reasonable", the student can come up with the correct resonance structures for each substitution. At this point the student can usually discern which set of "reasonable" structures will give more stable intermediate by applying the principles of carbon ionic stability that they already know. Figure 1 is an example of this approach applied to the electrophilic substitution ortho to R-substituted benzene. A generic E+ is used to stress the fact tbat the reaction is independent of the particular electrophile. The first step is the localization of two r electrons at a ring carbon and the donation of these to form the sigma bond between a ring carbon and the electrophile. This leads to the intermediate
sigma complex, which can be viewed as having five carbon atoms each with a p atomic orbital and only four electrons to be distributed among them. In step 1they are then asked to draw a representation of the atomic p orbitals on the ring of the sigma complex and make five copies of it. They must take care to draw the hydrogen on the carbon to which the E+ has bonded in hopes that this will prevent them from drawing a p orbital on this carbon. With this in step 2 they can then show all five ways in which the four electrons can be distributed in five p atomic orbitals and denote the empty orbital with a
+.
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550
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Figure 1. Steps used to generate the resonance structures of the sigma complex for ortho substitution.
Figure 2. Steps used to generate complex for meta substitution.
me
resonance structures of the slgma
In step 3 they are asked to show what bonding is possible in each of these distributions and count the number of bonds, radicals, and the type of cation that are present in each. For structure A the count is two bonds and one secondarv cation. For structure B the count is one bond. two radicais, and a secondary cation. In structure C the count is the same as A. and in D it is the same as R. In structure E the count is two bonds and a carbon cation that is next to the R group. Based on this count the students can usually tell on their own that structures B and D are not "reasonable" and the normal-looking structures are A, C, and E, with E being different. They can then recopy these reasonable structures using the line format for step 4. If they repeat this procedure for meta, shown in Figure 2, and para attack of E+,they will have generated a complex set of all the siema comnlex structures that are "reasonable". They can tcen look at the structures that place the cation next to the R group and make the judgement concerning the increased or decreased stability of these as influenced by the
identity of the R group. The important idea of having nonbonding electrons on R donated to stabilize a carbocation can be more graphically demonstrated using this approach. This approach does not make any judgement as to the activating or deactivating nature of the R group, which can be approached as a separate topic. These two unrelated concepts, one dealingwith the electronic density of the starting material, and the other with the stability of the sigma complex intermediate, have become intertwined in the current "memorize it" approach. This is the source of some confusion. The same approach can be used with nucleophilic aromatic substitutions that proceed through an anionic intermediate. The structures will have six electrons in five p atomic orbitals. Here factors that stabilize anions become the governing principles in selecting the more stable intermediates in the compounds that can undergo this reaction. This can also show whichgroup can be substituted if several potential leaving groups are present. Thismay seem like a lot of work, so why notjust memorize it and slog on with the lecture? Certainly this does require more work on the part of, the student t o draw all these structures. However, i t is work that is very straightforward and can be done even by weak students who will follow its simplicity. With a little practice, students can do the drawing required very quickly and obtain correct information with some understanding of the way in which it was obtained. This method is valuable for several reasons. First, it gives the student the same information as the line drawings using arrows to "push" electrons. However, i t is presented in a much more graphical manner. To most students at this point, lines on a page are still not electrons. Second, mastery of thia approach seems to help students beable tomoveon to the electron pushing approach. The electron pushing formalism is more efficient. Third, i t allows the student to understand a concept that previously has, for the most part, just been memorized. There is sufficient material in organic that must be memorized, a t least a t first encounter. Lastly the concept of resonance structures must obviously be reemphasized in this approach, and students seem to grasp and keep the concept better from this intensive contact. Finally, let me point out that for this method to work the instructor must convince the students that it is worth their time. When students see that by using this method they can think through the problem, most will take the time. Presenting this as a serious method, I have been successful in ~ u r s u ing understanding over rote.
FACSS Annual Meeting The 17thAnnualMeeting ofthe Federation of Analytical Chemists and Spectroscopy Societies will be held October 112,1990, in Cleveland, Ohio. In addition to the technical papers, exhibits, and society meetings, a variety of social and cultural events will be available. Further information may he obtained from Charles J. Belle, Lucaa Aerospace, PEC, 4259 W. 192 Street, F a i ~ e wPark, OH 44126; Carol Paxton, BP Research, 4440 Warrensville Center Road, Warrensville Heights, OH 44128; or Jeanette Graaelli, 151 Greentree Road, Chagrin Falls, OH 44022.
Volume 67
Number 7
July 1990
551
