Salicylic acid

 

Salicylic acid

Chemical formula	C7H6O3
Molar Mass(g/mol)	138.12
Density(g/cc)	1.443
Melting Point(°C)	158.6
Boiling Point(°C)	200
Solubility in water(g/L)	2.2-2.48
Solubility in ethanol(g/L)	275 (35%w/w)
NFPA 704	1 2 0

Uses

• Precursor to acetylsalicylic acid
• Precursor to bismuth subsalicylate
• Topical treatment for warts and dandruff
• Potential precursor for salbutamol

Natural occurence

• Does occur natually in several plants. See methyl salicylate

Hazards

• LD₅₀ (rat, oral): 0.4g/kg

Production

Synthesis

From methyl salicylate

• Methyl salicylate combines with sodium hydroxide leaving sodium salicylate and methanol.

  C₈H₈O₃ + NaOH → NaC₇H₅O₃ + CH₃OH

• Combine sodium salicylate and sulfuric acid forming salicylic acid and sodium sulfate.

  2 NaC₇H₅O₃ + H₂SO₄ → 2 C₇H₆O₃(s) + Na₂SO₄(aq)

From phenol

carbonylation

• React phenol with sodium hydroxide, producing sodium phenoxide

  C₆H₅OH + NaOH → NaC₆H₅O + H₂O

• React the sodium phenoxide with carbon dioxide at 180°C, producing sodium salicylate

  NaC₆H₅O + CO₂

  {

  →

  180°C}

  NaC₇H₅O₃

• React sodium salicylate with sulfuric acid producing salicylic acid and sodium sulfate

  NaC₇H₅O₃ + H₂SO₄ → C₇H₆O₃ + Na₂SO₄

carbon tetrachloride

React phenol with carbon tetrachloride and sodium hydroxide giving salicylic acid

C₆H₅OH + CCl₄ + 4 NaOH(aq)

{

→

strong stirring}

C₆H₄OHCOOH + 4 NaCl + 2 H₂O

The reaction must be heated to begin reaction, then cooled as it progresses.

Reaction must be stirred thoroughly or some additional solvent (dioxane) added to get the three materials to contact one another

trichloromethane

1. React Phenol with trichloromethane and sodium hydroxide giving salicylaldehyde

   C₆H₅OH + CHCl₃ + 3 NaOH(aq)

   {

   →

   strong stirring}

   C₆H₄OHCOH + 3 NaCl + 2 H₂O

   The reaction must be heated to begin reaction, then cooled as it progresses.

   Reaction must be stirred thoroughly or some additional solvent (dioxane) added to get the three materials to contact one another

2. react salicylaldehyde with potassium dichromate to give salicylic acid

   C₆H₄OHCOH + 2 K₂Cr₂O₇ + H₂O

   {H₂SO₄

   →

   }

   C₆H₄OHCOOH + 2 K₂CrO₄ + 2 HCrO₃

Purification

1. REPEAT
   1. Dissolve acid in diethyl ether (333g/L) or ethanol (275g/L)
   2. Add water
   3. Neutralize to sodium salicylate (via sodium carbonate or sodium bicarbonate) to make it water soluble
   4. Extract aq layer
   5. Acidify w/ dilute sulfuric acid precipitates it from the aqueous layer
   6. Filter, discard filtrate
   7. The residue is pure(er) salicylic acid.
2. UNTIL pure enough

See Also

• methyl salicylate
• acetylsalicylic acid

Web

• Preparation of salicylic acid from methyl salicilate, I am Stephen Bahl