last updated Wednesday, February 25, 2015

Meeting 14 (Condensed Answer Key)

1. a. The electrophile in this reaction is the acylium ion.

The acylium ion is a linear because the resonance structure with the C-O triple bond is dominant (CH₃C≡O⁺) which means that the central carbon is sp-hybridized.

b. The bulk of the positive charge is located on the oxygen atom (see resonance structures above). Thus, carbon atom is not very electrophilic making it a weak electrophile.

c. The infrared spectrum of the [CH₃CO]SbF₆ displays a peak at n=2302 cm^-1, which is even higher than in the free CO molecule (2143 cm^-1).

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. The reaction mixture foams very heavily and an increases amount of precipitate is observed due to the increase of polarity of the solvent.

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 exhibits six signals in the ¹³C-NMR spectrum. The methyl group of the acetyl function is located at d=27 ppm, while the four signals due to the ferrocene part are located between d=69-79 ppm. The carbonyl group is located at d=202 ppm.While the methyl and the carbonyl group is located in the normal range, the ferrocene carbons are much lower than other aromatic carbons because of the high p-electrondensity in the cyclopentadienyl ring.

h. The infrared spectrum should be acquired from v=450-4000 cm^-1, which is a little bit lower than normal in order to observe the ferrocene modes around 480 cm^-1.