Aspirin (acetylsalicylic acid) Chemistry Tutorial

Key Concepts

• Aspirin is widely used as an analgesic (pain reliever) and an antipyretic (for reducing fever). It is also used to help prevent heart attacks, strokes, and blood clot formation in people at risk of developing blood clots.
• Aspirin (acetylsalicylic acid) is an aromatic compound containing both a carboxylic acid functional group and an ester functional group.
• Aspirin is a weak acid that is only slightly soluble in water.
• Aspirin can be prepared by reacting salicylic acid and acetic anhydride in the presence of an acid catalyst.

Structure of Aspirin (acetylsalicylic acid)

• carboxylic acid functional group (R-COOH)
• ester functional group (R-O-CO-R')
• aromatic group (benzene ring)

Properties of Aspirin (acetylsalicylic acid)

Acidity

aspirin (acetylsalicylic acid)		acetylsalicylate ion	+	H+
			+	H+

Solubility

Ionic salts of aspirin, such as sodium acetylsalicylate, are more soluble in water since they form stronger ion-dipole interactions with water.
These ionic salts of aspirin are sometimes marketed as "soluble aspirin". When you add water to the soluble aspirin, eg, sodium acetylsalicylate, it dissociates to form sodium ions and acetylsalicylate ions:

sodium acetylsalicylate	→	acetylsalicylate ions	+	sodium ions
	→		+	Na+
C9H7O4-Na+	→	C9H7O4-	+	Na+

acetylsalicylate ions	+	H+	→	aspirin
	+	H+	→
C9H7O4-	+	H+	→	C9H8O4

Synthesis of Aspirin (acetylsalicylic acid)

salicylic acid	+	acetic anhydride (ethanoic anhydride)	[H2SO4] →	aspirin (acetylsalicylic acid)	+	acetic acid (ethanoic acid)
	+		[H2SO4] →		+	CH3COOH
C7H6O3(s)	+	C4H6O3(l)	[H2SO4] →	C9H8O4(s)	+	C2H4O2(aq)

Salicylic acid can react with acetic (ethanoic) acid in an esterification reaction, but the reaction is very slow, taking days to reach equilibrium, and the yield is low:

salicylic acid	+	acetic acid (ethanoic acid)	[H2SO4] →	aspirin (acetylsalicylic acid)	+	water
	+	CH3COOH	[H2SO4] →		+	H2O
C7H6O3	+	C2H4O2	[H2SO4] →	C9H8O4(s)	+	H2O

For this reason, the commercial preparation of aspirin relies on the faster reaction between salicylic acid and the more reactive acetic anhydride which produces a greater yield of aspirin.

Reactions of Aspirin (acetylsalicylic acid)

1. Neutralization:

   acid + base → salt + water

   aspirin (acetylsalicylic acid)	+	sodium hydroxide	→	a salt (sodium acetylsalicylate)	+	water
   	+	NaOH(aq)	→		+	H2O(l)
   C9H8O4(s)	+	NaOH(aq)	→	C9H7O4(s)-Na+	+	H2O(l)

   The neutralization reaction can be used to determine the amount of aspirin (acetylsalicylic acid) present in commercially available aspirin tablets using a back (indirect) titration method.

2. Reaction with carbonate:

   acid + carbonate → salt + carbon dioxide + water

   aspirin (acetylsalicylic acid)	+	sodium hydrogen carbonate	→	a salt (sodium acetylsalicylate)	+	carbon dioxide gas	+	water
   	+	NaHCO3(aq)	→		+	CO2(g)	+	H2O(l)
   C9H8O4(s)	+	NaHCO3(aq)	→	C9H7O4(s)-Na+	+	CO2(g)	+	H2O(l)

   The reaction with bicarbonate (hydrogen carbonate ion) is commonly used to prepare the salt of aspirin which is more soluble in water than the molecular form of aspirin.

3. Hydrolysis:

   cleavage of a covalent bond in a molecule by reaction with water

   aspirin (acetylsalicylic acid)	+	water	→	salicylic acid	+	acetic acid (ethanoic acid)
   	+	H2O(l)	→		+	CH3COOH(aq)
   C9H8O4(s)	+	H2O(l)	→	C7H6O3(s)	+	C2H4O2(aq)

   Old aspirin tablets may have a smell like vinegar as a result of the hydrolysis reaction producing acetic acid (ethanoic acid).