Chapter 1.
Introduction
The Diels-Alder reaction is extremely important in organic synthesis as it provides a way to form carbon–carbon bonds with very high regio- and stereoselectivity. The reaction involves two starting materials, a conjugated diene and a dienophile, which react together to form a six-membered ring. The reaction is an example of a class of reactions called thermal pericyclic reactions in which the transition state involves six electrons moving in a cyclic manner to form two new sigma bonds and a pi bond.
Diels-Alder reactions work best when the diene is an electron-rich molecule (high energy HOMO) and the dienophile has electron-withdrawing substituents (low energy LUMO) because this arrangement maximizes orbital overlap in the transition state. In this experiment you will study the Diels-Alder reaction of cyclopentadiene with maleic anhydride to give cis-norbornene-5,6-endo-dicarboxylic acid anhydride. This reaction and the transition state geometry that leads to the “endo” isomer is shown below.
Techniques
Recrystallization
Gravity & Vacuum Filtration
Thin Layer Chromatography
Infrared Spectroscopy
Lab Objective
To use the Diels-Alder reaction to synthesize cis-norbornene-5,6-endo-dicarboxylic acid anhydride and to characterize it using TLC, m.p., and Infrared spectroscopy.
Procedure
Dissolve 0.20 g of powdered maleic anhydride in 1 mL of ethyl acetate in a tared 10 × 100 mm reaction tube and then add 1 mL of hexanes (bp 60 °C–80 °C). This solvent mixture is used because the product is too soluble in ethyl acetate, but not soluble enough in pure hexanes. To the solution of maleic anhydride add 0.20 mL (0.160 g) of dry, freshly prepared cyclopentadiene, mix the reactants and observe the reaction. Allow the tube to cool to room temperature, during which time crystallization of the products should occur.
If crystallization does not take place, scratch the inside of the tube with a glass rod to initiate nucleation of crystals. If crystallization occurs too rapidly, the crystals formed will be too small. If this happens, remove a seed crystal and redissolve the crystals by heating the mixture. Add the seed crystal and allow the solution to cool slowly to room temperature; large plate-like crystals should grow.
Carefully remove the solvents from the crystals with a Pasteur pipet and wash the crystals with one portion of cold ligroin, and remove the solvent. Scrape the crystals onto a filter paper and allow them to air-dry. Determine their weight and calculate the yield. Characterize your product by melting point, TLC, and IR spectrum.
Reagents
| Compound | CAS Number | mol. wt. (g mol-1) | Concentration or Density | m.p. or b.p. (°C) | SAFETY |
|---|---|---|---|---|---|
| Maleic anhydride | 108-31-6 | 98.06 | -- | 51–56 (m.p.) | Toxic, corrosive, irritant, health hazard |
| Ethyl acetate | 141-78-6 | 88.11 | 0.902 g/mL | 76.5–77.5 (b.p.) | Flammable, toxic |
| Hexanes | 110-54-3 | 86.18 | 0.659 g/mL | 69 (b.p.) | Flammable, acute and chronic toxicity, environmental hazard |
| *Dicyclopentadiene | 77-73-6 | 132.20 | 0.986 g/mL | 35 (m.p.) 170 (b.p.) | Flammable, acute toxicity, environmental hazard |
| **Ligroin | 232-453-7 | -- | 0.656 g/mL | 60-80 (b.p.) | Flammable, chronic toxicity |
*Dicyclopentadiene is heated to generate "cylcopentadiene," which is unstable and not commercially available. For this experiment, cyclopentadiene will be generated for you.
**Ligroin, also called petroleum ether, is composed of a mixture of hydrocarbons (hexanes, pentane, heptane, etc.) and is generally used as a nonpolar solvent.