4-Hydroxycinnamic acid (C?H?O?), also known as p-Coumaric acid, is a phenolic derivative of cinnamic acid with a hydroxyl group at the para-position of the benzene ring. Here's its key chemical profile:

1. Chemical Structure

• Molecular Formula: C?H?O?

• IUPAC Name: (E)-3-(4-Hydroxyphenyl)prop-2-enoic acid

• Core Structure:

• Benzene ring with a para-hydroxy group (?OH).

  Trans-configured acrylic acid group (?CH=CH?COOH) attached to the ring.

• Isomers: Naturally occurs as the stable trans (E) isomer; cis (Z) form is rare.

2. Physical Properties

• Appearance: Pale yellow crystals.

• Melting Point: 214°C (decomposes).

• Solubility:

• Low in cold water.

  Soluble in ethanol, ether, and hot water.

• UV Absorption: Strong absorbance at ~310 nm due to extended conjugation.

3. Chemical Properties

• Acidity:

• Carboxylic acid (pKa ~4.5) and phenolic hydroxyl (pKa ~9.5).

  Can form zwitterions in solution.

• Reactivity:

• Decarboxylation: Forms *p*-vinylphenol when heated.

  Photodimerization: Undergoes [2+2] cycloaddition under UV light.

  Antioxidant Activity: Phenolic ?OH donates H-atoms to quench free radicals.

• Tautomerism: Can exist as keto-enol tautomers in certain conditions.

4. Natural Occurrence & Biosynthesis

• Ubiquitous in plants: Found in nuts, grains, tomatoes, and carrots.

• Biosynthesis: Derived from phenylalanine via enzymatic deamination/hydroxylation.

• Role: Precursor to lignin, flavonoids, and other polyphenols.

5. Significance & Applications

• Pharmaceuticals: Antioxidant, anti-inflammatory, and anticancer studies.

• Food Chemistry: Precursor for flavor compounds (e.g., in honey, wine).

• Biotechnology: Used in enzymatic synthesis of complex natural products.

• Material Science: Building block for UV-absorbing polymers.

Key Derivatives:

• Ferulic acid (3-methoxy-4-hydroxy derivative).

• Caffeic acid (3,4-dihydroxy derivative).

CAS Number: 501-98-4 (trans isomer) | 4501-31-9 (cis isomer).

Summary: A key plant phenolic acid with dual functional groups (?COOH/?OH), enabling diverse reactivity, biological activity, and roles in natural product biosynthesis. Its para-hydroxy substitution enhances UV absorption and antioxidant capacity compared to cinnamic acid.