Two Working Models for the SN2Mechanism Martin M. Anderson University of Portland, Portland, OR 97203 An organic reaction mechanisnl that usually occurs early in the organic chemistry course and serves as an introduction to steric effects in reactions is the attack of strong nucleophiles on simple organic substrates. This bimolecular nucleophilic substitution (S$) can easily be demonstrated in three dimensions by the models described below. By these models, one can clearly show the process of inversion of configuration that occurs when a nucleophile directly displaces a leaving group. This inversion is often compared to the "turning inside out" of an umbrella in a strong wind. In fact, for one of these models, theumbrellaanalogy was taken astep further by the use of old umbrella parts for construction of the prototype. The models are designed to illustrate the reaction CH3Br + OH-
-
+
CHBOH Br-
The mechanism of this reaction can he represented as
The first model consists solely of Styrofoam balls and wooden dowels and can he made in a few minutes. A wooden dowel is passed through the center of the hall thatrepresents the carbon atom and serves to hold the oncoming nucleophile and leaving group. Three triangular slots are made in the ball to accommodate dowels holding halls that represent
the hydrogen atoms that are bonded to carhon. These slots can be made using a hot nail. The hot nail is pushed in to the center perpendicular to the axial dowel. While it is inserted, and with the end of the dowel positioned a t the center of the hall, the nail is rocked back and forth in the plane of the axis, with arcs of about 20" to each side of the perpendicular line. A dowel bearing a ball representing a hydrogen atom is then placed in this slot. It is able to move from an initial position on the nucleophile side of the axis, in which it is about l l O o from the leaving group, to a final position on the leaving group side, in which it is about 110' from the nucleonhile. ' I ' a u more sloti for the other two hydrogen atoms are made in similar iashion. Thev are parallel to the first and seDarated from it by 120°. This model can be used to show relative positions of nucleophile, leaving group, and hydrogen atoms around the carhon atom before reaction, a t the transition state and after reaction. The dowels representing the bonds must be moved individually to achieve the three different structures. An articulated model for the S N reaction ~ has also been designed. I t is constructed from Styrofoam balls, wooden dowels, metal rods cut from a coat hanger, thin wire, and Velcro. This model is called "articulated" because the halls representing the leaving group and the hydrogens move as the nucleophile ball approaches the carbon hall. T o demonstrate the reaction, the central hall (carbon) is held steady with one hand while the nucleophile (OH) is pushed toward it. As the nucleophile approaches carbon, the leaving group (Br) is forced away from carbon and simultaneously the three hydrogens on carhon arc to their inverted positions. Action can be interrupted to show the pentavalent transition state. Figure 1shows the relative positions of the atoms immediately before reaction, a t the transition state, and immediately after reaction.
Figure 1. Views of articulated model before reaction. at bansition stale, and after reaction
Volume 84
Number 12 December 1987
1023
318 Inch dowel
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Figure 2. Detailed view of articulating assembly.
Construction Details for t h e Altlculated Model Materials Styrofoam balls of various diameters: for hromine, 6 in. for carbon, 5 in. for oxygen, 4 in. for hydrogen, 2 in., 4 halls Woad dowels l-in. diameter, % in. '&in. diameter, 9 in. Y-in. diameter, 9 in. Wire coat hanger Florist's wire Adhesive-hacked Velcro Tools Hammer Vise (also serves as anvil) Electric drill and hits Pliers Wire cutters String or fish line Protractor Ruler Hacksaw Rasp Pencil sharpener Exacto or other sharp knife Preparation of Parts 1. Axial dowel. Cut %-in.dowel t o 9 in. length. Three inehesfrom an end. cut a circular eraove around it about 1mm deem At intervals
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Journal of Chemical Education
sharpened after i t has been inserted through the Styrofoam ball.) 2. C-H bond dculels. Cut the V8-in. dowel t o make three 3-in. lengths. Drill a small bole 'I8in. from an end of each piece, and round uff this end with a rasp. One inch from the rounded end, drill another small hole parallel t o the first hole. 3. I-in. dowel. Cut t o % in. length. Drill alla-in. hale from one end uf the dowel to the other. This hole must he wide enough toallow the 'I4-in.dowel t o pass through freely. Cut a circular groove around the middle of the dowel about 1mm deep. At intervals of 120' along the groove, use a :'is-in. hit to drill indentations about 3 mm deep for seating the rounded ends of the 318-in. dowels. 4. Metal rods. Cut the clothes hanger to make three 2-in. lengths. Pound eachend of eachrod to flatten it, then drillasmall holeabout 2 mm from each end. 5. Carbon boll. With a hacksaw, cut the 5-in. ball into three equivalent segments. To do this, first wrap a length of slender, nonstretchine strine or fish line around the hall in a ereat circle. hlsrk thrre ps~intsmthe ballequidiitant frcmenrh othwalmg rhir linp'rhrn idmtify t w pointson ~ the axiiperpendirulur 11, the grmt cinlr. lnsrrr the axrnl dt,wcl thnmgh the t d l along this axi* Beeinning a t each point marked on the great circle, saw to the central axis so that each saw cut is in aplane 120' from each of the others. Carve each segment an its internal faces to provide space for the l-in. dowel a t the center, for theL14-in.dowel along the central axis, and to allow the C-H bond dowels to arc from amsition 70' from the axial dwrcl c m the nurlpophilr side to "0' on the lem,ing grvup side. .\l,rut in. depth need3 lo be cut away toalluw fcrmotion of th? CH bond dowels.
')
Assembly ( S e e Fig. 2) 1. Position the three C-H hond dowels a t the l-in dowel, inserting the rounded ends of the dowels into the indentations drilled in the l-in. dowel. Wire the three dowels in position, with one piece of florist's wire inserted through the hales of thedowels. Makesure the wire lies in the groove cut in the l-in. dowel. Then twist the ends of the wire to tighten i t around the 1-in. dowel. 2. Wire eaeh metal rod to a C-H hond dowel, w that all three rods extend in the same direction relative to the axial dowel. 3. Insert the axial dowel in the 1-in. dowel, and wire the metal rods to it a t the indentations made t o accommodate them. Make sure the wire lies in the groove cut in the dowel. Then twist the ends of the wire t o tighten it around the dowel. Sharpen the remaining end of the axial dowel. 4. Place the three segments of the carbon ball in position around the articulating assembly. Cut six complementary pairs of Velcro, eaeh about 1 in. square. Paste these in pairs on adjacent faces of the Styrofoam segments so that they hold the segments together. 5. Test the action of the model as the axial dowel is pushed in either direction through t h e carbon hall. If there is too much binding of moving parts against the Styrofoam, judiciously cut away a little more Styrofoam. Clip and tuck in the ends of the wires as necessary 1 0 avoid g( ~lxin:!thr S ~ ? r o f o d m . 6 . Plwr the nuclrc,philc, lea$,ing droup, and h\,drogen hnlls in rmi~tiln on the ends < f th?d
