last updated Friday, February 13, 2015

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 97.3 % or 1.08 g of the ester for the anhydrous benzoic acid (w=0) and 96.5 % (or 1.02 g) of the ester for the "wet" benzoic acid (w=0.05 g/18.02 g/mol).

b. 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  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. Despite the addition of the catalyst, the reaction is still very slow at room temperature. 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. In addition, the benzoic acid is going to dissolve completely.

d. 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.

e. 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!

2. a. 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.

b. 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 crude contains all three stereoisomers of the nitro ester as well as the dinitro ester. These compounds can be detected using GC/MS or HPLC (as it is done in the lab).

e. The crude is recrystallized from a mixture of methanol and water (4:1). Its solubility is high at high and at low temperatures, which means that the compound would not crystallize from methanol alone. The addition of water increases the polarity of the solvent and therefore decreases the solubility of the compound.

f. The final product is a white solid that forms needles or cubic crystals.