last updated Friday, February 19, 2016

Meeting 14 (Condensed Answer Key)

1. a. The electrophile in this reaction is the acylium ion (CH₃C≡O⁺).

b. The acylium ion is obtained by the reaction of acetic acid anhydride with Amberlyst A15 (sulfonated polystyrene). While the catalyst is heterogeneous, which requires a little longer reaction time, it greatly reduces the quantity of polymerization and oxidation products. Since it can be filtered out after the reaction (in principle), less acid has to be neutralized upon completion of the reaction, reducing the amount of waste as well.

c. The infrared spectrum of the [CH₃CO]SbF₆ displays a peak at ν=2302 cm^-1, which is even higher than in the free CO molecule (2143 cm^-1). The C-O bond distance is very short (110.8 pm) and the chemical shift of the central carbon atom is δ=154 ppm, which indicative of a highly deshielded sp-hybridized carbon atom.

d. The addition of the reaction mixture to water causes the bulk of the organometallic compounds to precipitate while most of the acetic acid and the acetic acid anhydride remain in solution at this point.

e. The extraction of the combined organic layers with sodium bicarbonate solution aims to remove the remaining acetic acid from the solution.

CH₃COOH  +  HCO₃^-    ------ >    CH₃COO^-  +   H₂O  +  CO₂

f. Silica is used as stationary phase, which is polar. The mobile phase, ethyl acetate:petroleum ether (10:90), causes the ferrocene to move very far up the plate, while the 1,1'-diacetylferrocene lmoves the least.

g. Due to its high symmetry, 1,1'-diacetylferrocene exhibits four signals in the ¹³C-NMR spectrum. The methyl group of the acetyl function is located at δ= ~27 ppm, while the carbonyl group is located at δ=200 ppm. The ferrocene carbons are are located from δ=70-80 ppm (three signals), much lower than other aromatic carbon atoms because of the high π-electrondensity in the cyclopentadienyl ring.

h. The infrared spectrum should be acquired from ν=450-4000 cm^-1, which is a little bit lower than normal in order to observe the ferrocene modes around ν=480-500 cm^-1.