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Meeting 14 (Condensed Answer Key)
1. a. The electrophile in this reaction is the acylium ion.
The acylium ion is a primarily linear because the resonance structure with the C-O triple bond is dominant (CH3C≡O+).
b. The electrophile is obtained by the reaction of acetic acid anhydride and phosphoric acid.
c. The isotropic shift of the carbon atom is d=154 ppm which is indicative of a sp-hybridization. Carbon atoms of nitrile functions appear at 117 ppm. The carbon in the acetyl ion is shifted more because of the higher electronegativity of the oxygen and the partial positive charge on the oxygen that causes further deshielding.
d. The target temperature for the reaction is 80-85 oC. At lower temperatures, the reaction proceeds too slowly, at higher temperature an increased amount of oxidation is observed. In both cases, the yield deteriorates significantly.
e. The neutralization is performed in two steps in a large beaker. First, the reaction mixture is poured into a 1 M sodium acetate solution to raise the pH-value and neutralize some of the acids. Then, solid sodium bicarbonate is added to adjust the pH-value to pH=5-7.
f. In the beginning, a mixture of ethyl acetate:petroleum ether (2:98) is used to apply the sample to the column and also elute the ferrocene from the column. Afterwards, the polarity (eluting power) is increased by increasing the fraction of ethyl acetate in the mobile phase (ethyl acetate:petroleum ether (10:90)) to elute the mono-acylation product. Under these conditions, the diacylation product moves relatively slow down the column and would therefore elute much later.
g. Acetyl ferrocene is not sensitive towards weak acids. Thus, there is no need to neutralize the acidic residues in the silica by pretreating the column with 1% triethylamine solution. One has to keep in mind that the pretreatment also deactivates the stationary phase.
h. Acetyl ferrocene exhibits four signals in the 1H-NMR spectrum. The methyl group of the acetyl function is located at d=2.4 ppm(s, 3 H), while the three signals due to the ferrocene part are located between d=4.2-4.8 ppm (d=4.2 ppm (s, 5 H), 4.5 ppm ("s", 2 H), 4.77 ppm ("s", 2 H)). The While the methyl group is located in a normal range, the ferrocene protons are much lower than other aromatic protons because of the high p-electrondensity in the cyclopentadienyl ring.