Question

This lab experiment is on the preparation of benzoic acid: a Grignard reaction.

In the procedure: following the addition of carbon dioxide; then 6M HCL(aq); diethyl ether was added and the aqueous layer discarded; then 6M NaOH(aq) was added and the ether layer discarded; Finally, concentrated HCL(aq) was added and a precepitate formed, which was filtered and rinsed with cold deionized H₂O. The purpose of these steps was to isolate, and thus purify, benzoic acid from both water-soluble and organic-soluble impurities.

a) Provide a balanced equation for each step after the addition of CO₂.

b) State the impurities being removed in each step after the addition of CO₂.

In questions 1a and 1b; identify the phase (i.e. organic or aqueous) that each compound is in

Answer 1

The addition of carbon dioxide to Gringard reagents forms a carboxylic acid that forms due to the attack of the nucleophilic carbon on the Grignard reagent on the electron-deficient carbon in carbon dioxide. This results in the formation of a magnesium carbonxylate complex. Thus the magnesium center serves a dual purpose as a nucleophile generator and a strong base that reacts with protons. Therefore, after the addition of carbon dioxide, a bromomagnesium salt is formed. This reaction can be written as: C₆H₅MgBr + CO₂ -----> C₆H₅COO^-+MgBr

After this salt is formed, the reaction mixture is dissolved in a double layer of ether after acidification. Acidification results in the protonation of the benzoate to give benzoic acid. Benzoic acid is relatively non-polar due to its non-polar phenyl chain and so is only sparingly soluble in water. Upon addition of ether however, the product also goes into the organic layer. The reaction of bromomagnesium benzoate upon acidification to give benzoic acid can be written as: C₆H₅COO^-+MgBr + HCl -----> C₆H₅COOH + MgBrCl. Apart from benzoic acid, other possible impurities are unreacted Grignard reagent and the product biphenyl and benzene that can be formed by self-destruction and dimerization of the reagent respectively. When the aqueous layer is discarded after acidification, the mangesium salt with bromide and chloride is removed.

Next, upon addition of NaOH, a strong base, the acid gets deprotonated and forms a polar sodium benzoate salt which is highly-soluble in water. Now, the aqueous layer contains the product while the organic layer contains the impurities mentioned before. By discarding the organic layer, these are removed. The balanced equation of the formation of the salt is C₆H₅COOH + NaOH -----> C₆H₅COO^- Na⁺ + H₂O.

Finally, when the aqueous layer is again acidified, the water-insoluble benzoic acid refroms after neutralization of the sodium salt to given sodium chloride as a side product. As only the desired product is water-insoluble here, it can now be filtered as in its pure form. C₆H₅COO^- Na⁺ + HCl -----> C₆H₅COOH + NaCl.

Summarizing, the impurities removed in the first step, upon acidification are the inorganic impurities like bromomagnesium chloride. In the next step, organic impurities benzene, phenylmagnesium bromide and biphenyl are removed. Finally, in the final step, the water-soluble sodium chloride, formed by neutralization is removed from the benzoic acid precipitated.

Also, in the first step after carbon dioxide additon, the organic layer contains the compound. In the next step however, the compound goes to the aqueous layer as sodium benozate. Finally, at the last, it precipitates out and the only layer - aqueous layer present does not contain the compound.