exp 5

Liquid-Liquid Extractions and Acid-Base Chemistry



As seen in previous experiments, compounds can be separated on the basis of polarity (“like dissolves like”).  The analgesics used in this experiment (acetylsalicylic acid, acetaminophen and caffeine) all have low solubility in water, and so when placed in a typical separatory funnel extraction, they are in the organic layer.  


Weak acid and bases can also be separated on the basis of their pKa, since neutral weak acids and bases form ions, and for most organic weak acids and bases, the neutral conjugate will be soluble in the organic layer, and the ionic conjugate will be soluble in the aqueous layer.  A stronger weak acid will deprotonate more readily than a weaker weak acid, so by carefully choosing the base, one of the acids can be selectively deprotonated.

Figure 8.1 Weak Acid and Weak Base Reactions


In this experiment, acetylsalicylic acid and acetaminophen are both weak acids, the former having a carboxylic acid and the latter having a phenol.  Acetylsalicylic acid is a stronger acid than acetaminophen (pKa of 2.97 vs 9.5, respectively), so it can be separated from the mixture through the use of a weak base (K2HPO4, pKb = 6.8).  Acetaminophen requires a stronger base (KOH) to react and be extracted.  Caffeine, being a weak base, is left behind in the organic layer.  For better visualization of the separation, refer to the following flow chart. 

Figure 8.2 Flow Chart Illustrating the Migration of the Solutes in Each Extraction



After each acid is deprotonated and separated, it is reacted again to return it to its neutral form.  Then, excess solvent is removed from all three using the Rotavapor®.  Refer to the Background in Exp. 4 regarding the use of a separatory funnel and drying agents. 


Learning Goals


  • Perform multiple acid-base reactions.

  • Conduct a separation of multiple substances with similar polarity on the basis of their pKa.

  • Manage a multi-part experiment and be attentive to the time.