Question

In an experiment of synthesis of Bromoacetanilide - bromination.

Procedure

1. Dissolve 1g acetanilide and 1.8g of NaBr using 6mL 95% ethanol and 5 mL acetic acid.

2. Cool the resulting mixture in an ice bath to at least 5 C.

3. In fume hood add enough bleach to deliver 0.6g of NaClO.

4. Allow mixture to warm up to room temperature for over 15 minutes.

5. Cool the reaction down in an ice bath and then quench the untracked bromine using 1g of sodium thiosulfate and 1g of sodium hydroxide.

6. Use water for recrystallization of the crude product.

Question 1: why is 95% ethanol and acetic acid used to dissolve the acetanilide and NaBr?

Question 2: Why do you let the mixture warm up to room temp before putting in an ice bath?

Question 3: what does sodium thiosulfate and sodium hydroxide do to the unreacted bromine ?

Question: What is the purpose of recrystallization ?

Answer 1

Answer 1 :The ethanol solution is used as an solvent and the acetic acid is used to provide acidic condition for the reaction of NaClO and NaBr which liberates molecular bromine.

Answer 2 :The reaction is warmed to room temperature to faster the rate of reaction. the reaction is endothermic and would become very slow in the ice bath. The solution is thus kept for a while at room temp for the completion of reaction and then freezed in ice bath to cease the reaction .

Answer 3 Sodium thiosulfate and sodium hydroxide reacts with unreacted bromine to form salt of Sodium Bromide and quench the bromination

Na₂S₂O₃ + 4Br₂ + 10NaOH → 2Na₂SO₄ + 8NaBr + 5H₂O

Answer 4 ; Recrystallization is used as purification process.It purify the crude non volatile sample from any miscible impurities such as NaBr ,Na₂SO_4.