Question

In this experiment we will synthesize dibenzalacetone form benzaldehyde and acetone, taking advantage of the acidity of the hydrogen atoms located on the a carbons in acetone. There are a possibility of three products, the trans/trans, the trans/cis, and the cis/cis. Only one is isolated. 1) Draw all three products that could form in this reaction. 2) How will we determine which products have been formed? One way will be to check the melting points of the products. The other will be to use 'H NMR to distinguish between the possible products. Since the coupling between adjacent hydrogen atoms in a molecule is strongly dependent on the dihedral angle (H-C-C-H), we will determine the coupling constants. Spin-spin coupling occurs between neighboring nonequivalent protons. This gives rise to signal splitting in the spectrum. A signal is split into (n + 1) peaks, where n is the number of magnetically coupled protons. The distance between the peaks in a signal is the coupling constant (1) and is measured in Hertz (Hz). (Refer to Chapter 13 in your textbook for a more thorough explanation of spin-spin coupling.) The magnitude of the coupling constant is important as it can provide stereochemical information. For three bond coupling (between neighboring protons H2-Hb) the size of the coupling constant Jab is related to the dihedral angle. The Karplus relationship states that the maximum value of Jab will occur when the dihedral angle is 0° or 180°, with the 180° giving the larger value of Jab. The minimum value of occurs at 90° J = A-BCOS + C Cos20 2 0 8 III 6 0 4 2 0 20 40 60 80 100 120 140 160 180 Why does the coupling constant depend on the dihedral angle? The spin information is transferred from one proton to another through the bonding electrons and their spin. The best overlap occurs when the orbitals are parallel to each other, and no overlap occurs when they are perpendicular.