last updatedTuesday, November 11, 2014
Meeting 12 (Condensed Key)
1. a. Using the equilbrium expression
K=x*(x+w)/((y-x)*(z-x))
x=moles of ester or water formed in the reaction, y=moles of benzoic acid used in the reaction, z=moles of methanol used in the reaction, w=moles of water in benzoic acid
one obtains a quadratic equation. The yield is 94.5 % or 1.58 g of the ester for the anhydrous benzoic acid (w=0) and 91.2 % (or 1.52 g) of the ester for the "wet" benzoic acid (w=0.15 g/18.02 g/mol).
b. The concentrated sulfuric acid is used as catalyst in this reaction. The mineral acid protonates the carbonyl group of the carboxylic acid (not the OH group) as shown in the example below for benzoic acid. This increases the electrophilic character of the carboxylic acid slightly. Note that the effect is not as strong as expected because of the resonance with the benzene ring, which results in six resonance structures (four of them are shown below, two more for the charge resonating in the aromatic ring).
c. The reaction is slow at room temperature despite the addition of the catalyst. In order to establish the equilibrium faster, the reaction mixture has to be refluxed well because at higher temperatures the rate of the reaction increases. An increase from 20 oC to about 65 oC will increase of the reaction by a factor of about 20, the difference between a 5 % conversion or 100 % conversion (theoretically). The mixture has to boil and a reflux ring has to be observed in the lower third of the reflux condenser.
d. The appearance of a large amount of precipitate indicates a poor conversion of the benzoic acid, which precipitates upon chilling the mixture. The student should add some more methanol and a few more drops of concentrated sulfuric acid before refluxing for an additional 30 minutes.
e. The amount of solution (~6-8 mL) being handled here is fairly small and would get lost in a 125 mL separatory funnel. Thus, the use of a centrifuge tube would be best here.
f. The extractions of the organic layers with saturated sodium bicarbonate solution are meant to neutralize the unreacted carboxylic acid and the catalyst dissolved in the combined organic layers. The reaction mixture will foam heavily because carbon dioxide (CO2) will be formed as a byproduct!
HCO3- + PhCOOH ----> PhCOO- + H2O + CO2
2 HCO3- + H2SO4 ----> SO42- + 2 H2O + 2 CO2
The extraction is discontinued after the carbon dioxide formation ceased. Excessive extractions cause the loss of product because the ester is not entirely insoluble in aqueous media!
g. The infrared spectrum of the ester should display a peak at n=1724 cm-1 (n(C=O)), n=1279 cm-1 (nas(COC)) and n=1112 cm-1 (ns(COC)). Any hydroxyl peaks should be absent.
2. a. The nitronium ion is the electrophile. It is obtained in-situ in the acid mixture of concentrated sulfuric and concentrated nitric acid.
HNO3 + H2SO4 ----> NO2+ + H2O + HSO4-
b. The nitronium ion is a very powerful electrophile because there is only one "good resonance structure", in which the nitrogen atom bears the bulk of the positive charge (1.84). The temperature has to be properly controlled to reduce the formation of the other isomers and the dinitration product. The temperature should be between T=0-5 oC during the reaction. This is accomplished by placing the reaction mixture in an ice-bath and adding the nitration mixture slowly.
c. The reaction mixture is poured over ice to precipitate the crude product. Ice is used because the reaction of concentrated acids, especially concentrated sulfuric acid, with water is very exothermic. This would lead to the hydrolysis of the ester function during this step.
d. The solubility of the product is high in methanol at high and at low temperatures. The crude compound would not crystallize from methanol alone. Thus, a solvent mixture of methanol and water is used in the lab.
e. The infrared spectrum of the nitro ester displays two strong peaks at