Identification of Conjugated Diene

Identification of a Conjugated Diene from Eucalyptus Oil

Purpose:

The purpose of this experiment was to prepare the Diels-Alder adduct of the unknown

Conjugated diene in eucalyptus oil, to separate the adduct, and identify the unknown diene from the melting point of itЎЇs adduct.

Introduction:

This experiment was performed using techniques of heating under reflux, vacuum filtration, recrystallization, and melting point measurements. These techniques were used to separate the Diels-Alder adduct of the unknown conjugated diene in eucalyptus oil and identify the diene. Dienes contribute to the characteristic flavors and aromas that are found in the essential oils of many plants. Some of these dienes are conjugated and have the ability to make Diels-Alder adducts with maleic anhydride. The Diels-Alder reaction is the conjugate addition of an alkene to a diene. In order for a Diels-Alder reaction to occur the diene must exist as an s-cis conformation and must be conjugated. One reactant (the diene) contributes four carbons and the other reactant (the dienophile, in this case the maleic anhydride) contributes two carbons to the six-membered ring of the resulting cyclic compound (the adduct). This reaction proceeds in one step with no intermediates. This reaction is also stereoselective and yields either of two more stable adduct, an exo or endo. (Lehman, 131).

Reaction Mechanism:

Procedure:

This experiment was preformed according to the methods described in the laboratory manual (Lehman, 134-135). To carry out the Diels-Alder reaction 1.00g of eucalyptus oil was dissolved with a 2.00ml ethyl ether. Assuming that 50% of the mass of the eucalyptus oil was the unknown diene, the mass of maleic anhydride needed to react with .50g of diene was calculated and added to the reaction mixture. The reaction mixture was than heated under gentle reflux, using a hot water bath for 45 minutes. After heating the reaction mixture, it was cooled to room temperature allowing crystals to form.

To separate the adduct, vacuum filtration was preformed using 1.5 ml of cold low-boiling petroleum ether. For purification the adduct was recrystalized from 4.00ml of dry methanol. To find the identity of the diene the melting point obtained was compared to the list of melting points of maleic anhydride adducts of the dienes.

Calculation of maleic anhydride mass needed for reaction