CH 2020 Diels-Alder Reaction: Preparation of a Butadiene Adduct from 3-Sulfolene (adapted from Organic Chemistry: A Short Course, H. Hart, L. E. Craine, D. J. Hart, and T.K. Vinod 13th ed. Houghton-Mifflin, Boston, 2012.) Reference: Bruice, P.Y.,Seventh Edition, Chapter 8.

Materials From the Chemicals Hood: From the Stockroom (Blue Bin): 3-Sulfolene ½” “pea-sized” stirbar Reflux condenser Maleic anhydride Stirbar retriever Xylenes Toluene Petroleum ether (60 – 90 °C) 25 mL round-bottom flask

Calcium chloride drying Cork ring tube with adapter (2) Wire clips Small metal clamp

Introduction The Diels-Alder reaction is a one-step reaction of a conjugated diene and a dienophile, which is reversible. The general reaction and mechanism is depicted in Figure 1. The reaction is the 1,4- addition of an electrophile and a nucleophile to a conjugated diene. However, unlike other 1,4- addition reactions you may have seen, the Diels-Alder reaction is a concerted reaction: the addition of the electrophile and the nucleophile occurs in a single step. This works well because both of the organic substrates are symmetrical.

Figure 1. The Diels-Alder reaction to form cis-4-cyclohexene-1,2-dicarboxylic anhydride

We will be using 3-sulfolene in our Diels-Alder reaction. 3-Sulfolene thermally decomposes to give the 1,3-butadiene reactant as shown in Figure 2:

Figure 2. Thermal decomposition of 3-sulfolene to 1,3-butadiene

Procedure Into a 25 mL round-bottom flask, combine 2.0 g 3-sulfolene, 1.2 g powdered maleic anhydride, and a ½” stirbar. Using a pipet, add 1.0 mL xylenes. Attach a water-cooled reflux condenser with a calcium chloride drying tube on top, as shown in Figure 2. Heat the mixture on a sand bath with stirring, setting the hot plate to 250 °C, and reflux for 30 minutes. Start timing from when the condensate begins dripping back into the round-bottom flask from the condenser.

Figure 2. Reflux apparatus with drying tube

After refluxing, lower the lab jack and allow the reaction mixture to cool for five minutes. With 10 mL of toluene ready to pour, quickly remove the drying tube, pour the toluene down the condenser and into the round-bottom flask, and reattach the drying tube. Raise the lab jack to reapply heat and bring the reaction mixture back to a boil. Then, remove the reaction mixture from heat and pour the contents of the round-bottom flask, without the stirbar, into a dry 125 mL Erlenmeyer flask. Add a wooden stick and reheat the solution on a steam bath. Then, add petroleum ether (60 – 90 °C) by the pipet squirt to the solution until slight turbidity persists. Rewarm the mixture on the steam bath until the solution becomes clear and then place the flask in an ice-water bath to crystallize the product. Collect the product by vacuum filtration and allow it to dry.

Next period: weigh the dry product, calculate the percent yield, and determine the melting point.

Waste Disposal The filtrate may be poured into the Flammable Organic Waste container.