Question

Organic lab.

Experiment: Acetylsalicy acid (aspirin)

I had 3 tubes with with 1g of salicylic acid and 2 mL of acetic anhydride. I added to tube 1 0.2g of anhydrous sodium. To tube 2 I added 5 drops of pyridine. To tube 3 I added 5 drops of concentrated sulfuric acid.

I put all of them in a hot water bath. In tube 1 and 2 salicylic acid dissolved. Why 3 did not dissolve?

Answer 1

Ans. The reaction for synthesis of acetylsalicylic acid (aspirin) is as follow-

            C₇H₆O₃ + C₄H₆O₃ ------> C₉H₈O₄ + CH₃COOH - reaction 1

C₇H₆O₃ = salicylic acid        

C₄H₆O₃= acetic anhydride

C₉H₈O₄ = aspirin

CH₃COOH = Acetic acid

# Note the formation of weak acid, acetic acid, during the reaction.

CH₃COOH dissociates into acetate ion and H⁺ in aqueous solution as soon as it forms during the reaction as follow-

                        CH₃COOH <------> CH₃COO^- + H⁺         - reaction 2

In simple aqueous solution, the dissociation of acetic acid is considerably significant.

Le Chatelier’s principle states “if a dynamic equilibrium is disturbed by changing the conditions (Concentration, Volume, Pressure, temperature, etc.), the position of equilibrium shifts to counteract the change to reestablish an equilibrium”.

Dissociation of acetic acid in reaction 2 lowers the [acetic acid] in reaction 1. That is, the product of reaction 1, acetic acid, is constantly consumed up. So, the dissociation of product of reaction 1 does not let that product accumulate in reaction mixture. This drives the equilibrium to the right as long as acetic acid keeps donating its proton.

So, under normal condition, the reaction goes to right.

Forward reaction consumes the reactants salicylic acid and acetic anhydride. So, constant consumption of the insoluble acetic acid and poorly soluble salicylic acid. Consumption of salicylic acid during the reaction gives the perception of its solubility because no salicylic acid crystals would be visible when reaction is complete.

# Addition of anhydrous sodium: Sodium metal or Na⁺ does not affect the dissociation of acetic acid. So, the reaction goes to the right as usual, the reactants are consumed up, and the final reaction mixture is clear when hot – the salicylic acid is said to be solvated.

# Addition of Pyridine: Pyridine is a weak base. It accepts the proton donated by acetic acid, so it speeds up the rate of forwards reaction. So, the salicylic acid crystals are also solvated when pyridine is added.

# Addition of H₂SO₄: Sulfuric is a strong acid. In presence of sulfuric acid, acetic acid can’t donate its proton to the aqueous medium. That is, in presence of sulfuric acid, [CH₃COOH] builds up constantly because it can’t donate its proton.

So, the reaction 1 reaches its equilibrium relatively sooner than in absence of sulfuric acid.

Therefore, very few salicylic acid crystals would actually be consumed up during the whole reaction time.

Therefore, salicylic acid does not dissolve in presence of sulfuric acid because the reaction does not proceed to the right.