Properties of p-Anisic Acid: A Versatile Fine Chemical

p-Methoxybenzoic Acid Introduction

Chemical Name: p-Methoxybenzoic Acid

Synonyms: p-Anisic Acid, 4-Anisic Acid

CAS Number: 100-09-4

Molecular Formula: C8H8O3

Molecular Weight: 152.15 g/mol

I. What is p-Anisic Acid? 

p-Anisic Acid, chemically known as 4-Methoxybenzoic acid, is an important organic fine chemical with the CAS registry number 100-09-4. Its molecular structure features a benzoic acid backbone with a methoxy group (-OCH₃) substituted at the para position. This unique architecture endows it with distinct physical and chemical properties, making it invaluable in pharmaceuticals, flavors & fragrances, and preservation.

II. Detailed Physical & Chemical Properties

Understanding the properties of p-Anisic acid is fundamental to appreciating its applications. Below is a detailed breakdown of its key characteristics:

1. Basic Physical Properties

  Appearance: A white to off-white crystalline solid, typically in the form of flakes, needles, or a crystalline powder, available in high purity.

  Odor: Exhibits a faint, characteristic aromatic odor, reminiscent of vanilla or almond, which allows for its direct use in fragrance applications.

  Melting Point: 182 - 185 °C. This is a key identifier and indicates its stability at room temperature.

  Boiling Point: Approximately 275 - 280 °C. The high boiling point signifies low volatility, facilitating safe storage and transport.

  Flash Point: > 150 °C. This classifies the substance as non-flammable, ensuring a high degree of safety during handling, storage, and transportation.

  Density: The relative density (water = 1) is approximately 1.38.

  Solubility:

      Slightly soluble in cold water.

      Freely soluble in various organic solvents, including ethanol, ether, chloroform, and hot benzene. This property makes it easy to handle and purify in organic synthesis.

2. Key Chemical Properties

  Acidity: As a derivative of benzoic acid, it displays typical carboxylic acid behavior. It can react with bases to form corresponding salts (e.g., Sodium p-Anisate), which often possess improved water solubility and application profiles.

  Esterification: This is one of its most important chemical reactions. The carboxylic acid group can undergo esterification with alcohols to produce a series of valuable ester-based fragrances, such as Methyl p-anisate (cherry-like scent) and Ethyl p-anisate.

  Stability: Chemically stable under normal temperatures and pressures. It should be stored away from strong oxidizing agents and strong bases. Decomposition at high temperatures may yield carbon monoxide and carbon dioxide.

  Substitution Reactions: The methoxy group on the benzene ring is an electron-donating group, which influences the electron cloud distribution of the ring. This alters its reactivity and positional selectivity compared to benzoic acid in certain electrophilic substitution reactions.

III. Primary Application Fields

Leveraging its unique properties, p-Anisic acid is primarily used in the following sectors:

  Pharmaceutical Intermediate: Serves as a key building block for synthesizing more complex drug molecules. It is a crucial starting material for various antibiotics, cardiovascular drugs, and local anesthetics.

  Flavor & Fragrance Intermediate and Fixative: Its esters, synthesized via esterification, are core components in high-grade food flavorings and cosmetic fragrances. The acid itself can also be used directly as a fixative to prolong the lasting power of fragrance compositions.

  Preservative: It and its sodium salt exhibit certain antimicrobial and preservative properties, making them suitable for use in cosmetics, personal care products, and some industrial applications.

IV. Safety and Storage Guidelines

p-Anisic Acid is generally regarded as safe under normal conditions of use, but good laboratory practices should be followed.

  Safety: This product is not classified as hazardous. However, it may cause mild irritation to the eyes and skin. Appropriate personal protective equipment (PPE) such as gloves and safety glasses is recommended during handling.

  Storage: Store in a cool, dry, well-ventilated place. Keep the container tightly sealed. Store separately from strong oxidizing agents.

Conclusion

p-Anisic acid is an exceptionally versatile fine chemical, thanks to its stable crystalline structure, moderate melting point, unique solubility profile, and reactive functional groups that enable esterification and salt formation. Its value—whether as a critical scaffold in pharmaceutical synthesis or a precursor for creating pleasing scents—is fundamentally rooted in its excellent and intrinsic physicochemical properties.