Three Easy Puzzles Based on the Diels-Alder Reaction Miles Pickering Princeton University, Princeton. NJ 08544 T o answer a real mechanistic question in organic chemistry, i t is almost always necessary to have the results of more than one ex~eriment.This is impossibly tedious for the teaching lab &less the reactions can be run simultaneously. Since most labs have only enough apparatus to set up one reflux apparatus per student, the puzzle-oriented mecbanistic lab is only possible for reactions that can he run in test tubes, preferably a t room temperature. Avoiding reflux also saves time, not only thereflux time itself, but also time spent setting up, warming, cooling and washing apparatus. Frequently, reactions a t room temperature lead to purer products. The Diels-Alder reaction is well represented in the student literature (1-8). But the systems used mostly require reflux and are purely synthetic (as opposed t o mechanistic) experiments. This paper recasts some classic systems (and offers a new one) so that they can be done a t room temperature on asmallscale in alarge lab course without sophisticated instrumentation. Furthermore, they start the student using experimental results to solve mechanistic problems, the real business of oreanic chemistm, instead of cooking .up . a batch of "white stufp following a ieeipe. The cis stereochemistry of the Diels-Alder reaction is hard to illustrate with material tractable enough to be safe for students. The most accessible Diels-Alder puzzle is the question of exo or endo addition. For example, if 1,3-cyclohexadiene is allowed t o react with maleic anhydride, there are two possible products.
endo
exo
A similar reaction can he run with any cyclic diene. For several common cases, both exo and endo isomers are known, and their melting points measured. S o the student can he asked to make the adduct and determine which comnound was formed. T h e puzzle of whether cyclopentadiene gives exo or endo product with maleic anhydride has been published as a student experiment (Z), but nobody seems to have reported the use of 1,3-cyclohexadiene in student work. This is unfortunate because i t is much easier to work with. There is no need to crack the dimer, and the reaction is not as exothermic. T h e same exo or endo question can he asked about the reaction of maleic anhydride and furan. T h e procedure for the maleic anhydride and furan reaction is in a texthook (9) but not in puzzle form. Another svstem which works well a t room temoerature is the reactionbf a-phellandrene and maleic anhydride. Reflux or warmine of this svstem is usuallv recommended (9, 10) but is unnecessary. The endo isomer is formed:
524
Journal
of Chemical Education
The exo isomer does not appear t o have been made. T h e ouzzle auestion oosed to the students is therefore based on ihe ve j large dkference in melting point between the upticallv oure (-)-n-ohellandreneadduct and theadduct of (*). . a-
