Question

1. Interpret the infrared and NMR spectra of benzocaine.

2.What is the structure of the precipitate that forms after the sulfuric acid has been added?

3. When 10% sodium carbonate solution is added, a gas evolves. What is the gas? Give a balanced equation for this reaction.

4. Explain why benzocaine precipitates during the neutralization.

5. Refer to the structure of procaine in the table in the essay “Local Anesthetics.” Using p-aminobenzoic acid, give equations showing how procaine and procaine monohydrochloride could be prepared. Which of the two possible amino functional groups in procaine will be protonated first? Defend your choice. (Hint: Consider resonance.)

Answer 1

2. The precipitate that forms after acidification is a zwitterion of p-aminobenzoic acid. The nitrogen in an amine has a lone pair of electrons it can act as a Lewis base. p-aminobenzoic acid is contains both an amine and an acid group, so its -COOH donates H+ and its -NH₂ portion accepts it and forms a zwitterion.

3. The product you obtain is a HSO₄^-1 salt of benzocaine. The -NH₂ group is protonated in the acidic solution. When Na₂CO₃ is added, CO₃^2- reacts with the H⁺ from the protonated -NH₂ group and evolvs CO₂ gas.

2 H₃N⁺-PHE-COOEt + CO₃^2- = 2H₂N-PHE-COOEt + H2O + CO₂

4. The product you obtain is a HSO₄^-1 salt of benzocaine. This salt is very polar and soluble in the solvent ethanol. After neutralization free base benzocaine forms. Free benzocaine in not polar and precipitates in the polar ethanol solvent.

5.

The lone pair of the aniline nitrogen is delocalized into the aromatic ring and stabilized by resonance, so it is a weaker base than the other nitrogen atom. So the side chain N atom will protonate first.