Aromatic carboxylic acids, such as benzoic acid, in which -COOH is directly attached to a benzene ring, and their esters are important raw materials for pharmaceuticals, dyes, and polymers.
Basic Knowledge
Main aromatic carboxylic acids:
・Benzoic acid C6H5-COOH (melting point 122℃, white crystals, sodium salt used as a preservative) - obtained by oxidation of toluene
・Salicylic acid (o-hydroxybenzoic acid)
・Phthalic acid (o-benzenedicarboxylic acid) - on heating, forms phthalic anhydride by intramolecular cyclization; raw material for polyester (PET) and dyes
・Isophthalic acid (m-) and terephthalic acid (p-) - raw materials for PET (terephthalic acid + ethylene glycol)
Synthesis methods:
① Oxidation of toluene: C6H5-CH3 + 3[O] → C6H5-COOH + H2O (KMnO4, etc.; the side chain is oxidized)
② Oxidation of xylene (dimethylbenzene): p-xylene → terephthalic acid
Esterification and pharmaceuticals:
・Aspirin (acetylsalicylic acid): salicylic acid + acetic anhydride → the OH group of salicylic acid is acetylated; antipyretic, analgesic, and anti-inflammatory effects.
・Methyl salicylate: salicylic acid + methanol → the COOH group is methyl-esterified; used in topical analgesics.
・Parabens (p-hydroxybenzoic acid esters) - preservatives in cosmetics and foods (methyl/ethyl/propyl/butyl).
Benzoic acid (C6H5-COOH, preservative and raw material for dyes)
Phthalic acid/phthalic anhydride (o-benzenedicarboxylic acid, raw material for polyester)
Terephthalic acid (p-, raw material for PET = plastic bottles)
Aspirin (acetylsalicylic acid, the world's first synthetic pharmaceutical)
Methyl salicylate (topical analgesic, wintergreen odor)
PET (polyethylene terephthalate, plastic bottles and fibers)
Further Study (Principles and Applications)
PET (polyethylene terephthalate): Condensation polymerization of terephthalic acid and ethylene glycol HO-CH2-CH2-OH. It is a key material in modern life, used for plastic bottles, polyester fibers (clothing and insulation), films (magnetic tape and X-ray film), and engineering plastics. It can be recycled.
History of aspirin: Willow bark (Salix) was used as an antipyretic in ancient times, and salicylic acid, its active ingredient, was isolated in the 19th century. Salicylic acid caused severe gastrointestinal problems, but in 1897 Hoffmann of Bayer acetylated it to reduce side effects, and it was launched as aspirin in 1899. It became one of the best-selling drugs of the 20th century. Today, low-dose aspirin is also used to prevent cardiovascular disease because of its antiplatelet effect.
Synthetic dyes: Since Perkin's discovery of mauve in 1856 (the first synthetic dye, derived from aniline), aromatic compounds have been central to dye chemistry. Examples include azo dyes (-N=N- bond, discussed in a later lesson), anthraquinone dyes, and indigo.
- Benzoic acid C6H5COOH, salicylic acid, and phthalic acid are representative examples.
- Benzoic acid is obtained by oxidation of the side chain of toluene.
- Phthalic acid forms phthalic anhydride (a five-membered cyclic anhydride) on heating.
- Terephthalic acid + ethylene glycol = PET.
- Aspirin is made by acetylating the OH group of salicylic acid with acetic anhydride.
- Methyl salicylate is made by methyl-esterifying the COOH group and is used in topical analgesics.
- Parabens are preservatives for foods and cosmetics.
Points of Caution (Common Confusions and Frequent Errors)
① Benzoic acid has -COOH attached to the benzene ring. It is more crystalline than aliphatic carboxylic acids (melting point 122℃). ② Conversion of salicylic acid to aspirin is acetylation of -OH (ester formation), whereas conversion to methyl salicylate is esterification of -COOH. Different ester positions produce completely different medicines from the same salicylic acid. ③ In phthalic anhydride, two -COOH groups at the ortho positions undergo dehydration to form a five-membered cyclic anhydride. Terephthalic acid cannot undergo intramolecular dehydration because its groups are at the para positions (its linear structure is advantageous for a rigid polymer backbone).
Practice
- Write the reaction equation and product formed when toluene is oxidized with an acidic potassium permanganate solution.
- Write the reaction equations for synthesizing aspirin and methyl salicylate from salicylic acid.
- Show the synthesis equation for PET as condensation polymerization of two monomers using structural formulas.