Melvin 5. Newman Ohio State University Columbus, 43210
A Molecular Model for S,2 Reactions
For many years I have been using a homemade mechanical model to demonstrate many features of S N reac~ tions. As students and other faculty members have often praised this model, a description of its advantages for teaching are discussed herein. In addition, some working drawings to aid in construction are given. Consider an S N reaction ~ in which an ion Y- displaces ion X from Cnn,X. 1 reuresents the reactants be--- Fieure " fore reaction. Figure 2 represents a symmetrical transition state. and Fiaure 3 revresents the situation after the reaction has heencompleted. In connection with Figure 1 a number of points are noteworthy. Not shown in the working drawings is a dual trigger device in which the forward trigger A is released by tapping with a downward motion. The internal spring, which is attached internally a t D and anchored a s shown a t E, causes ion Y to attack carbon C and invert the molecule via the intermediate transition state (Fig. 2 shows this) to the final products in which X has been displaced (Fig. 3). Thus, a dynamic demonstration of the S N reaction ~ is provided. If the trigger mechanism is set with the alternate trigger, B, there is not enough tension in the spring to effect the inversion reaction. Thus an example of a low energy collision can he demonstrated.' A number of points of interest may now he discussed.
Figure 1. Reactants before reactran
Steric Effects of the Neopentyl Type The hall, a, is bored so that hall and stick models of appropriate size may be inserted to give tetrahedral angles with the main arm which attaches a to the central carbon atom, C. Furthermore, a can be rotated through 360". Thus, if three hydrogen atoms are attached, a represents a methyl group. If b and c are hydrogen the reaction involves an ethyl group. If one hydrogen on a is now replaced with a methyl group, the group a t a represents an ethyl group. By rotating the ethyl group it can he demonstrated that the methyl moiety can be arranged so that it sterically hinders the approach of X . However, further rotation of a easily reduces the steric effect (polar interactions of groups can also he discussed here). If a second hydrogen on a is replaced by a methyl group it can easily he shown that the steric effect is about the same for each methyl hut that now there are two methyl groups. Hence there is essentially only one orientation of the isopropyl group which allows a hydrogen to he directed a t the incomine., ion.. Y -. However. when the third methvl . erouo - . is substituted on a (a is now a t-hutyl group) there is no orientation which allows for a n unhindered approach. Hence the reason for the changes in SN2 reaction rate in the ethyl, propyl, isohutyl, neopentyl progression is readily 'This feature need not be incorporated in any duplication of this model. Actually, unless the model is cleaned and oiled, the inversion reaction will fail if only the spring tension is applied. However, in mast discussions the movements are best controlled hv hand.
thus produce complications. 462 / Journal of Chemical Education
Figure 2 A symmetrical lransition state
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Figure3. The situalion after reaction has been completed.
demonstrated. Tvnical sets of Sw2 reaction rate constants for this series are quoted in many organic texts. At this voint it is important to point out that the yield of products by S N reactions ~ can-he higher when a-neopentyl halide is a reactant than when other halides which possess a @-hydrogen(or hydrogens) are involved. Students often get the idea that because a reaction rate is low a poor yield will naturally result. Actually, the yield of S N prod~ uct is determined by the ratio of the rate of the S Nreaction ~ divided by the rate of the accompanying E2 reaction. With neopentyl halides there cannot be an E2 reaction and therefore yields are often very high. One needs only to run the reactions a t a higher temperature to allow S Nreactions ~ to occur with neopentyl type halides2.3 a t reasonable rates.
Figure 5. Diagram of the assembled model
cussions about whether the transition state is nearer the starting materials, i.e. bond x is shorter than bond y, or nearer the reaction products, i.e. bond x is longer than bond y, can be given along with a graphic presentation of reaction profiles. Effectof Variations of X in C,b,X with Constant Y The treatment here is similar to that directly above. Some mention should be made of the accelerating effect possible due to release of strain in the transition state when X- is large. Stereochemical Specificity The use of this mechanical model makes it clear and ~ proceeds with inversion of obvious why every S N reaction configuration. In addition, the conventions involving the asignment of R and S configurations can readily be illustrated with this model. Figure 4. Working diagram of the parts of the model
Effect of Variations in Y - with Constant C,bcX on Reaction Rate With the aid of this model one can readily discuss sterir effects in\.ol\,ing the attacking Y - ion including urlvali~m features). 'l'he polar efiects can he discussed in terms of the n~~deophilicity 111 Y and the geometn of the transir e the svmmetrici~ltransition state. In Fieuw 2 a r ~ ~ r t u of tion state, i.e., bonds x and y of' equal length, is given. DisThese working drawings and the model were made (in about 1950) by Mr. Gordon Laverack, our machinist, after I described the type of model desired. They have been touched up a bit to
improve visibility. His model is still in fine working condition after these many years of use.
Working Drawings4 Figure 4 represents the parts needed while Figure 5 indicates the assembly. Not shown are the short springs S (three needed-two are shown in Fig. 2) which were attached to the movable legs in order to ensure smoother operation. These springs were added after the model was first tried out and are not shown in the working drawinps. . Points of attachment to the movable legs are needed. The ends of each leg to which the balls are affixed are shown in Figure 4 as-being threaded. Later these were changed to simple pegs which fit snugly into the holes in the halls. The model is supported on a wooden stand as shown. Small metal clamps are placed a t the top of each upright in order to hold the model in place. The bottom of the wooden stand has a lead plate on the bottom in order to increase the stability of the setup.
Volume 52, Number 7, July 1975 / 463
