updated last Sunday, April 24, 2016

Answer Key Extra Credit Project (DHPM)

1. The planarity of the central six-membered ring is due to the resonance of the alkene function and the amide function (only the center ring is shown below). Most of the atoms in the ring display formally a sp²-hybridization.

2. The amide functions, which are very polar, form very strong hydrogen bonds with the carbonyl oxygen atoms of the neighboring molecule that leads to a double-strand of molecules (four hydrogen bonds per molecule, see X-ray structure in the reader). These strong intermolecular forces lead to a high melting point.

3. The p-toluenesulfonic acid (p-CH₃C₆H₄SO₃H, TSA) is used as catalyst in the reaction. Initially, urea and 4-nitrobenzaldehyde undergo an acid-catalyzed formation of a N-acyliminium ion. The cation reacts with a CH-acidic compound (via its enol form) to yield the ureide, which undergoes an acid-catalyzed cyclization and dehydration to form a racemic mixture of 4-(4-nitrophenyl)–substitited 3,4-dihydropyrimidin-2-one.

4. The mixture has to be mixed thoroughly before it is stored in the sunlight to overcome the barriers associated with solvent-free reactions. Only part of the urea, the 4-nitrobenzaldehyde and the catalyst dissolve in the liquids (ethyl acetoacetate). The vial has to be closed well to prevent the oxidation of the benzaldehyde.

5. A 6-dram vial is used for the reaction. The regular black cap is replaced by a compression cap and a flat Teflon septum that is pushed inwards a little (as shown on the right side below).

 6. A mixture of acetone and ethyl acetate is used for recrystallization. The crude is first dissolved in acetone. Ethyl acetate is added to lower the polarity of the solvent causing the precipitation of the product.

7. The ¹H-NMR spectrum of the compound displays eight signals: two amide signals (7.89 ppm (s, 1H), 9.35 ppm (s, 1H)), two signals in the aromatic range (7.48 ppm (d, 2H), 8.17 ppm(d, 2H) and four signals in the aliphatic range (5.25 ppm (s, 1H), 3.95 ppm (q, 2H), 2.25 ppm (s, 3 H) and 1.05 ppm (t, 3H)).