3,5-Dihydroxybenzoic Acid: Properties, Uses & Safety

3,5-Dihydroxybenzoic acid is a bifunctional aromatic compound bearing two hydroxyl groups and one carboxylic acid group on a benzene ring, valued in research for its distinctive colorimetric reactions and broad solubility in common laboratory solvents.

CAS / SKU

SKU D104386 (CAS to be confirmed)

Water Solubility

84 g/L at 20 °C

Crystallization

Crystallizes from water with 1.5 molecules of water of crystallization

Reaction with Fuming/Hot Concentrated H₂SO₄

Produces a purple-red colored compound

Reaction with FeCl₃

Turns pink

Solvent Compatibility

Soluble in water, ethanol, diethyl ether, and acetone

Acute Toxicity (Mouse, IV)

LD₅₀ = 2000 mg/kg

Chemical Identity and Physical Properties

3,5-Dihydroxybenzoic acid is an aromatic carboxylic acid in which two hydroxyl groups are positioned at the 3- and 5-positions of the benzene ring, with a carboxylic acid group at the 1-position. This symmetric substitution pattern confers notable hydrogen-bonding capacity and influences both its solubility and reactivity.

The compound crystallizes from water as a hydrate containing 1.5 molecules of water of crystallization, a feature that should be accounted for when preparing stock solutions by mass. Its water solubility is 84 g/L at 20 °C, placing it among the more water-soluble members of the hydroxybenzoic acid family. It is also freely soluble in common polar organic solvents including ethanol, diethyl ether, and acetone, making it versatile for a wide range of formulation and reaction conditions.

Colorimetric Reactions and Analytical Utility

3,5-Dihydroxybenzoic acid displays characteristic colorimetric responses that make it identifiable and potentially useful in qualitative analytical chemistry. When treated with fuming sulfuric acid or hot concentrated sulfuric acid, the compound produces a purple-red colored product, a reaction attributable to electrophilic sulfonation and subsequent chromophore formation involving the electron-rich aromatic ring.

Exposure to ferric chloride (FeCl₃) solution causes the compound to turn pink, consistent with the well-known phenol–iron complexation reaction. This FeCl₃ color test is widely used as a presumptive identification method for phenolic hydroxyl groups and can serve as a rapid purity or identity check in a laboratory setting. Researchers should note that these reactions are qualitative indicators; quantitative analysis requires validated spectrophotometric or chromatographic methods.

Safety, Handling, and Storage

3,5-Dihydroxybenzoic acid is classified as an irritant. Direct contact with skin, eyes, or mucous membranes should be avoided. Standard personal protective equipment—including laboratory coat, nitrile gloves, and safety glasses—should be worn when handling the compound. Work in a well-ventilated area or fume hood is recommended to minimize inhalation of dust.

The acute intravenous LD₅₀ in mice has been reported as 2000 mg/kg, indicating relatively low acute systemic toxicity by this route; however, this value does not eliminate the need for appropriate precautions during routine laboratory use. The compound should be stored in a cool, dry location away from strong oxidizers and strong bases. Always consult the current Safety Data Sheet (SDS) provided by Aladdin Scientific (SKU D104386) before use.

Applications in Research

As a polyphenolic building block, 3,5-dihydroxybenzoic acid serves multiple roles in chemical and biological research. Its symmetrically placed hydroxyl groups make it a useful starting material or intermediate in the synthesis of more complex phenolic natural products, dendrimers, and metal-organic frameworks, where phenolic coordination sites are desirable.

In biochemical studies, dihydroxybenzoic acid isomers have been investigated as antioxidants, chelating agents, and as markers of oxidative stress. The 3,5-isomer's specific reactivity profile—including FeCl₃ complexation and sulfonation—may be exploited in the development of colorimetric assays or as a reference standard for HPLC method validation. Researchers in food science and plant biochemistry may also use it as a reference compound when profiling phenolic acid content in plant extracts.

Frequently asked questions

What solvents can be used to dissolve 3,5-dihydroxybenzoic acid?

3,5-Dihydroxybenzoic acid is soluble in water (84 g/L at 20 °C), ethanol, diethyl ether, and acetone. For aqueous stock solutions, note that the crystalline form contains 1.5 molecules of water of crystallization, which should be factored into accurate mass-based calculations.

How does 3,5-dihydroxybenzoic acid react with ferric chloride?

Upon exposure to ferric chloride (FeCl₃) solution, 3,5-dihydroxybenzoic acid forms a pink-colored complex. This is characteristic of phenolic compounds and results from coordination between the phenolic hydroxyl groups and Fe³⁺ ions. The reaction is useful as a qualitative identity test.

What are the key safety precautions when handling 3,5-dihydroxybenzoic acid?

The compound is classified as an irritant and should be handled with appropriate PPE including gloves, safety glasses, and a lab coat. Work in a well-ventilated area or fume hood. The reported mouse IV LD₅₀ is 2000 mg/kg; always consult the current SDS (Aladdin Scientific SKU D104386) for complete hazard information before use.

Does the hydrated crystal form affect how I weigh out 3,5-dihydroxybenzoic acid?

Yes. The compound crystallizes from water with 1.5 molecules of water of crystallization, meaning the actual molecular weight of the solid as supplied may differ from the anhydrous free acid. When preparing solutions of a precise molar concentration, confirm the hydration state of your specific lot and adjust calculations accordingly.

References

1. Aladdin Scientific product description for 3,5-Dihydroxybenzoic acid, SKU D104386.
2. General physicochemical and toxicological properties derived from supplier product data; LD₅₀ value (mouse, IV, 2000 mg/kg) as stated in source material.
3. Ferric chloride color test for phenolic compounds: standard qualitative organic chemistry procedure (textbook-level knowledge).

Products are supplied for research and development use only. Not for use in humans, animals, diagnosis, or therapy.