4-Fluoro-3,5-Dimethylbenzoic Acid: Properties & Applications

4-Fluoro-3,5-dimethylbenzoic acid is a fluorinated aromatic carboxylic acid that serves as a versatile building block in medicinal chemistry, agrochemical synthesis, and materials research owing to the combined electronic and steric effects of its para-fluoro and meta-dimethyl substitution pattern.

IUPAC Name

4-Fluoro-3,5-dimethylbenzoic acid

Molecular Formula

C₉H₉FO₂

Molecular Weight

168.17 g/mol

CAS Number

145742-34-3

Key Functional Groups

Carboxylic acid (–COOH), aromatic C–F bond, two methyl substituents

Aladdin SKU

F729030

Primary Use

Synthetic intermediate and building block in pharmaceutical and agrochemical research

Chemical Structure and Physicochemical Properties

4-Fluoro-3,5-dimethylbenzoic acid (molecular formula C₉H₉FO₂, MW 168.17 g/mol) is a substituted benzoic acid featuring a fluorine atom at the para position and methyl groups at both meta positions (3 and 5) of the benzene ring. This trisubstituted aromatic framework creates a distinctive combination of electronic and steric influences on the ring system.

The carboxylic acid group confers acidity and hydrogen-bonding capability, enabling salt formation, esterification, and amide coupling reactions. The para-fluoro substituent is strongly electron-withdrawing by induction, modulating the electron density of the ring and the acidity of the carboxylic acid relative to unsubstituted benzoic acid. The two flanking methyl groups provide steric shielding around the fluorine atom and contribute weak electron-donating resonance effects, creating a nuanced electronic environment that is valuable in structure–activity relationship (SAR) studies.

The compound is typically a white to off-white crystalline solid at room temperature. It should be stored in a cool, dry place away from strong bases and oxidizing agents to maintain purity.

Synthesis and Reactivity

4-Fluoro-3,5-dimethylbenzoic acid can be accessed through several synthetic routes common to fluorinated aromatic chemistry. One general approach involves the oxidation of a suitably substituted fluoromethylbenzene precursor (e.g., 4-fluoro-3,5-dimethylbenzaldehyde or the corresponding methyl compound) under controlled conditions. Alternatively, directed metalation strategies or Balz–Schiemann-type fluorination of a diazonium intermediate derived from the corresponding aniline can introduce the fluorine at the para position.

As a carboxylic acid building block, 4-fluoro-3,5-dimethylbenzoic acid participates in a broad range of transformations:

• Esterification: Reaction with alcohols under acid catalysis or via activated intermediates (e.g., acyl chlorides) to form esters.
• Amide coupling: Use of standard peptide coupling reagents (e.g., EDC, HATU) to generate amides for pharmaceutical candidates.
• Reduction: Reduction of the carboxylic acid to the corresponding benzyl alcohol or aldehyde using appropriate hydride reagents.
• Nucleophilic Aromatic Substitution (SNAr): The activated para-fluoro group, flanked by the electron-withdrawing carboxylic acid, can participate in SNAr reactions with suitable nucleophiles under appropriate conditions, enabling further derivatization of the aromatic ring.

The steric bulk of the 3,5-dimethyl groups can influence regioselectivity and reaction rates, a consideration that researchers should account for when designing synthetic sequences.

Applications in Research

The unique substitution pattern of 4-fluoro-3,5-dimethylbenzoic acid makes it a valued reagent across several research domains:

Medicinal Chemistry and Drug Discovery: Fluorinated aromatic acids are frequently employed as fragments and intermediates in the synthesis of drug candidates. The introduction of fluorine into a bioactive molecule can enhance metabolic stability, improve membrane permeability, and fine-tune binding affinity to a target protein. The 3,5-dimethyl substitution provides steric directionality that can be exploited to optimize interactions within a binding pocket. This compound serves as a carboxylic acid handle for constructing amide or ester bonds to pharmacophores.

Agrochemical Research: Fluorinated benzoic acid derivatives are well-represented in the agrochemical literature as intermediates for herbicides, fungicides, and insecticides, where fluorine substitution often improves biological activity and environmental persistence profiles.

Materials Science and Organic Electronics: Fluorinated aromatic building blocks are used in the construction of liquid crystals, organic semiconductors, and functional polymers, where the C–F bond contributes to desired dielectric, thermal, and self-assembly properties.

Structure–Activity Relationship (SAR) Studies: As part of a compound library, 4-fluoro-3,5-dimethylbenzoic acid enables systematic exploration of how fluorine and methyl substitution patterns affect biological or physicochemical outcomes, supporting lead optimization programs.

Handling, Safety, and Storage

As with all laboratory reagents, appropriate safety precautions should be observed when handling 4-fluoro-3,5-dimethylbenzoic acid. Researchers should consult the Safety Data Sheet (SDS) provided by Aladdin Scientific before use.

General handling guidelines:

• Wear appropriate personal protective equipment (PPE) including laboratory coat, chemical-resistant gloves, and safety glasses.
• Handle in a well-ventilated area or fume hood to avoid inhalation of dust.
• Avoid contact with skin, eyes, and mucous membranes.
• Keep away from strong bases, strong oxidizing agents, and reactive metals.

Storage: Store in a tightly sealed container in a cool, dry, well-ventilated area away from incompatible materials. Protect from moisture and light to preserve purity over time.

Disposal: Dispose of in accordance with local, regional, and national regulations governing chemical waste. Do not dispose of via drain or municipal waste streams.

For specific hazard classifications, pictograms, first-aid measures, and regulatory information, always refer to the current Aladdin Scientific SDS for SKU F729030.

Frequently asked questions

What is 4-fluoro-3,5-dimethylbenzoic acid used for in research?

It is primarily used as a fluorinated aromatic building block in medicinal chemistry, agrochemical synthesis, and materials science. Its carboxylic acid group enables amide and ester coupling reactions, while the para-fluoro substituent can modulate electronic properties or participate in nucleophilic aromatic substitution, making it valuable for synthesizing drug candidates and functional materials.

Can the para-fluoro group in this compound undergo nucleophilic aromatic substitution?

Yes. The para-fluoro group can undergo SNAr reactions when the aromatic ring bears sufficient electron-withdrawing activation. In 4-fluoro-3,5-dimethylbenzoic acid, the carboxylic acid group provides some activation, though the electron-donating methyl groups partially counteract this effect. Reaction feasibility will depend on the nucleophile and reaction conditions employed, and researchers should optimize conditions empirically.

How should I store 4-fluoro-3,5-dimethylbenzoic acid to maintain its purity?

Store the compound in a tightly sealed container in a cool, dry, and well-ventilated area, protected from moisture, light, and incompatible substances such as strong bases and oxidizing agents. Always refer to the Aladdin Scientific SDS for SKU F729030 for full storage requirements.

What reactive handles does this compound offer for further derivatization?

The compound offers three primary handles: (1) the carboxylic acid, which can be converted to esters, amides, acyl chlorides, or reduced to alcohols/aldehydes; (2) the para-fluoro group, which can be displaced via SNAr with appropriate nucleophiles; and (3) the aromatic methyl groups, which can be oxidized to aldehydes or carboxylic acids under suitable oxidative conditions.

References

1. Clayden, J.; Greeves, N.; Warren, S. Organic Chemistry, 2nd ed.; Oxford University Press: Oxford, 2012. (General reference for aromatic substitution and carboxylic acid reactivity.)
2. Purser, S.; Moore, P. R.; Swallow, S.; Gouverneur, V. Fluorine in medicinal chemistry. Chem. Soc. Rev. 2008, 37, 320–330. doi:10.1039/b610213c
3. Müller, K.; Faeh, C.; Diederich, F. Fluorine in pharmaceuticals: looking beyond intuition. Science 2007, 317, 1881–1886. doi:10.1126/science.1131943

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