Meeting 7 - Condensed Answer Key

1. Online Quiz (no key provided. Consult instructor with questions)

2. a. The reaction can be catalyst by a broad variety of Lewis or Brønsted acids. In the lab, Amberlyst A15 is used as catalyst. It protonates the carbonyl group of the ester function of the ethyl acetoacetate making it a better electrophile for the attack of the phenol. The acid is also needed to activate the other carbonyl group for the electrophilic substitution at a later stage. Finally, it is needed to eliminate the water to produce the double bond in the new six-membered ring (see mechanism in reader).

b. The reaction mixture is placed in a small Erlenmeyer flask in order to keep the reactants in close contact. The reaction mixture is more homogeneous and displays a lower tendency to overheat.

c. One of the largest problems is that there is only a very limited temperature control during the reaction. Unfortunately, the heat distribution in a conventional microwave is not homogeneous, there are locations of less heating and also hot spots, which results to overheating. This leads to decomposition of the catalyst (T>120 ^oC for A15), oxidation of the resorcinol and/or evaporation of the ethyl acetoacetate.

d. A hot mixture of methanol and water (5:1) is used to dissolve the product and separate it from the solid catalyst. While the target compound dissolves very well in cold methanol, its solubility in cold water is low. Thus, the product precipitates from the solvent mixture at low temperatures.

e. The long wavelength (λ=365 nm) of the UV-lamp is used as excitation source, which results in a fluorescence in the visible range. Under these conditions, the neutral and basic solution should display a strong bluish fluorescence (λ=445 nm), while the acidic solution diplays a light turquiose fluorescence. The excitation at λ=254 nm will not usually not lead to a visible emission because if a similar shift Stoke shift is observed the emission maximum would be around 330 nm, which is in the UV-range. In addition, the question is also how strong the compound absorbs at this wavelength all together.

f. The ¹³C{¹H}-NMR spectrum of hymecromone should display ten signals. One signal at δ=162 ppm for the carbonyl group of the ester, two signals for the alkene function and six aromatic carbon in the range from δ=102-161 ppm and one signal at δ=18 ppm for the methyl group.


2. a. A 4-dram vial (and not the 6-dram vial like in the other projects) is used as reaction vessel. This will reduce the headspace (air) in the vial.

b. Benzophenone does not dissolve well in isopropanol at room temperature. Warming the solution up (50-60 ^oC) speeds up this dissolution process. One has to keep in mind that the solution will heat up when it is placed in the sun.

c. The septum on the inside of the cap is needed to ensure that the vial is closed properly. Without this septum, the cap will leak and the solution will be pushed out of the vial upon storage in the sun.

d. The label of has to be placed on the cap or the upper part of the vial.

e. The peak at m/z=105 is due to a benzoyl ion (due to dehydration and subsequent rearrangement) while the peak at m/z=183 is due to the half of the molecule (Ph₂COH⁺).