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Aspirin (acetylsalicylic acid)

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)

Aspirin (acetylsalicylic acid) contains three groups:

  • carboxylic acid functional group (R-COOH)

  • ester functional group (R-O-CO-R')

  • aromatic group (benzene ring)

Properties of Aspirin (acetylsalicylic acid)

Acidity

Aspirin is a monoprotic weak acid, Ka = 2.8 x 10-4 at 25oC, so very little of the molecular aspirin (acetylsalicylic acid) dissociates to form acetylsalicylate ions.
For the equilibrium dissociation reaction:
aspirin (acetylsalicylic acid)acetylsalicylate ion+H+
+H+

the equilibrium position lies well to the left, favouring molecular aspirin.

Solubility

Aspirin is only slightly soluble in water and acidic solutions such as is present in the stomach. Aspirin contains polar functional groups which can form hydrogen bonds with polar water molecules.
Aspirin is more soluble in basic (alkaline) solutions, so it readily dissolves in the duodenum which is the first part of the intestine.

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 acetylsalicylateacetylsalicylate ions+sodium ions
+Na+
C9H7O4-Na+C9H7O4-+Na+

In the acidic stomach, molecular aspirin crystallizes out:
acetylsalicylate ions+H+aspirin
+H+
C9H7O4-+H+C9H8O4

Preparation of Aspirin (acetylsalicylic acid)

Salicylic acid will rapidly react with acetic anhydride in the presence of an acid catalyst to produce aspirin (acetylsalicylic acid) and acetic acid (ethanoic acid).
Sulfuric acid or phosphoric acid are often used to catalyse the reaction.

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).

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