Question

c. Why do you use HCl in the extraction of caffeine?

d. Look at the structure of caffeine and decide which one of the four nitrogens is most basic. Why?

e. Could you separate a mixture of salicylic acid and aspirin by acid-base extraction? Briefly justify your answer.

Answer 1

(c): Due to presence of amine group in caffeine, it is basic and insoluble in water. Hence we use a strong acid like HCl to react with basic caffeine to form a water sluble salt.

Caffeine(-N-, water insoluble) + HCl ------> -NH+Cl- (water soluble salt)

Now we can easily separate the water soluble salt containing caffeine by treating it with a strong base like KOH.

(d): The basic character in N atom is due to availability of lone pair and resonance decreases the availability of lone pair. Hence the nitrogen atom whose participation towards resonance is least will be the most basic.

Hence the N-atom represented by 4 in the below figure is the most basic nitrogen atom

  

(e): We cannot separate salicyclic acid and aspirin by acid-base extraction as both contain -COOH group and will react with a base to form soluble salt.