Question

In a lab, we seperated Excedrin (paracetamol, acetylsalicylic acid, caffeine, and binder). First we dissolved the powder in diethyl ether. This seperated the mixture into caffeine/acetylsalicylc acid and binder/paracetamol. K₂HPO₄ was added to the caffeine/acetylsalicylic acid. K₂HPO₄ + caffeine was washed with 1) NaCl and then dried with 2) anhydrous sodium sulphate. Give the reaction mechanism for 1) and 2).

Answer 1

• Excedrin is an analgesic drug which contains the following components- paracetamol, acetylsalicylic acid, caffeine and binder.
• Separation of organic compounds can be done based on their solubility in certain compounds. We use the rule of 'like dissolves in like'. This depends on relative polarity of different compounds in mixture. Such as, polar compounds dissolve in polar compounds and non polar compounds dissolve in non polar conmpounds.
• We can say that, organic compounds of very low polarity are insolube in water, but soluble in organic media like ether. Organic compounds with high polarity, like alcohols, are soluble in water.
• For such separations, we expoit acid-base chemistry. An acid reacts with a base to form a conjugate base and conjugate acid. One of the products becomes ionic or charged, so it tends to be soluble in water than its non charged counter parts.
• The functional groups usually involved in acid-base separation are strongly acidic carboxylic acid, weakly acidic phenols and basic amines.
• Carboxylic acid gets deprotonated with both strong and weak bases like sodium hydroxide and sodium bicarbonate to for sodium carboxylate salt, which is soluble in water. Phenols only react with strong bases like sodium hydroxide to form sodium phenolate, soluble in water. Amines react with strong acids to form water soluble ammonium chloride salts.

Coming to the above experiment:

• After adding diethy ether, we obtain caffeine/ acetlysalicylic acid. To this, a mild base is added, K2HPO4 in the separating funnel. The funnel contains an organic phase, diethyl ether, and an aqueous phase, K2HPO4.
• A chemical reaction takes place, where the acetylsalicylic acid gets deprotonated and come into the into the mildly basic K2HPO4 layer. This can be separated out, leaving behind the organic layer containing the caffeine.
• Now this organic layer is washed with 1) NaCl. It is also called the brine solution. NaCl renders proper separation in the organic layer. The salt solution make sures the water in the organic layer is pulled into the aqueous layer. Then NaCl solution is separated out.
• 2) sodium sulphate is added to the organic layer. It absorbs all the water in the organic layer and dries it. It is added in salt form. When it absorbs water, its crystals form larger clumps. They settle down, leaving the organic layer on top which can be easily separated by rota evaporation.