What is a Carboxylic Acid?
A carboxylic acid is an organic compound containing a carboxyl group (-COOH) attached to an alkyl or aryl group. The carboxyl group consists of a carbonyl (C=O) and a hydroxyl (-OH) group. Carboxylic acids are characterized by the presence of at least one carboxyl group and can be classified as monocarboxylic acids (containing one carboxyl group) or polycarboxylic acids (containing two or more carboxyl groups).
Properties of Carboxylic Acid
Structure and Reactivity
- Carboxylic acids contain the carboxyl group (-COOH), which consists of a carbonyl (C=O) and a hydroxyl (-OH) group.
- The carbonyl group is polar and moderately reactive, allowing carboxylic acids to participate in nucleophilic addition reactions, condensations, and other transformations.
- Carboxylic acids can form salts and esters by reacting with bases and alcohols, respectively.
Physical Properties
- Lower carboxylic acids (up to C4) are soluble in water due to hydrogen bonding.
- Higher carboxylic acids have higher melting and boiling points due to increased van der Waals forces.
- Carboxylic acids exhibit hydrogen bonding, which influences their physical properties like boiling point and solubility.
Acidity
- Carboxylic acids are weak acids due to the moderate polarity of the carbonyl group and the stabilization of the conjugate base by resonance.
- The acidity of carboxylic acids increases with the electronegativity of substituents and the ability to stabilize the conjugate base through resonance or inductive effects.
Synthesis of Carboxylic Acid
Synthesis from Carbonyl Compounds
Carboxylic acids can be synthesized from aldehydes, ketones, and their derivatives via oxidation reactions. Common oxidizing agents include chromic acid, potassium permanganate, and Jones reagent.
- Aldehydes: Oxidation of primary alcohols or aldehydes yields carboxylic acids, e.g., oxidation of acetaldehyde gives acetic acid.
- Ketones: Oxidation of methyl ketones yields carboxylic acids, e.g., oxidation of acetone gives acetic acid.
- Enol ethers: Hydrolysis of enol ethers followed by oxidation yields carboxylic acids.
Synthesis from Alkenes
Carboxylic acids can be prepared from alkenes via oxo processes, ozonolysis, or hydroboration-oxidation.
- Oxo process: Alkenes react with carbon monoxide and hydrogen in the presence of a metal catalyst to form aldehydes, which can be oxidized to acids.
- Ozonolysis: Alkenes react with ozone to form ozonides, which upon reductive workup yield aldehydes or carboxylic acids.
- Hydroboration-oxidation: Alkenes undergo hydroboration followed by oxidation to yield alcohols, which can be further oxidized to carboxylic acids.
Synthesis from Halides and Alcohols
- From alkyl halides: Conversion of alkyl halides to carboxylates via carbonylation, carboxylation, or oxidation reactions.
- From alcohols: Oxidation of primary alcohols using strong oxidizing agents like chromic acid, Jones reagent, or potassium permanganate.
Synthesis from Carbonic Acid Derivatives
Carboxylic acids can be obtained by hydrolysis of carbonic acid derivatives like esters, amides, nitriles, and acid chlorides.
Rearrangement Reactions
Certain rearrangement reactions like the Arndt-Eistert synthesis, Baeyer-Villiger oxidation, and Favorskii rearrangement can be used to synthesize carboxylic acids.
Miscellaneous Methods
Other methods include oxidative cleavage of alkenes, carbonylation of Grignard reagents, and hydrolysis of nitriles.
The choice of synthetic method depends on the starting materials, desired product, and functional group compatibility. Protecting groups and selective oxidation/reduction may be required for complex substrates.
Uses & Benefits of Carboxylic Acid
Carboxylic acids are versatile organic compounds with a wide range of applications and benefits. They are widely used in organic synthesis, nanotechnology, and polymer chemistry due to their reactive nature and polar structure.
Organic Synthesis
Carboxylic acids serve as valuable intermediates and building blocks in organic synthesis. They can undergo various reactions, such as substitution, elimination, oxidation, and coupling, to produce small molecules, macromolecules, and natural or synthetic polymers.
Nanotechnology
In nanotechnology, carboxylic acids are employed as surface modifiers to promote the dispersion and incorporation of metallic nanoparticles or carbon nanostructures (e.g., carbon nanotubes and graphene) into various materials. This enhances their compatibility and facilitates their integration into nanocomposites.
Polymer Chemistry
Carboxylic acids find applications in polymer chemistry as monomers, additives, and catalysts. They can be used to synthesize polyesters, polyamides, and other polymers. Additionally, they can act as compatibilizers, improving the properties and performance of polymer blends and composites.
Other Applications
Carboxylic acids have numerous other applications, including in the medical field, pharmaceuticals, and as intermediates for the production of various chemicals and materials. Their versatility stems from their ability to undergo various reactions and their polar nature, which facilitates interactions with other molecules.
Application Case
| Product/Project | Technical Outcomes | Application Scenarios |
|---|---|---|
| Carboxylic Acid-Based Polymers | Utilising carboxylic acids as monomers or catalysts enables the synthesis of high-performance polymers with tailored properties, such as improved thermal stability, mechanical strength, and chemical resistance. | Advanced materials for aerospace, automotive, and construction industries. |
| Carboxylic Acid-Functionalised Nanocomposites | Carboxylic acid-modified nanoparticles or nanostructures exhibit enhanced compatibility and dispersion within polymer matrices, resulting in nanocomposites with superior mechanical, thermal, and electrical properties. | Lightweight and high-strength materials for transportation, energy storage, and electronics. |
| Carboxylic Acid-Based Drug Delivery Systems | Carboxylic acid-containing polymers can be engineered to control drug release rates, improve bioavailability, and target specific tissues or cells, enhancing therapeutic efficacy and reducing side effects. | Targeted drug delivery for cancer treatment, regenerative medicine, and chronic disease management. |
| Carboxylic Acid-Based Catalysts | Carboxylic acids can act as efficient and environmentally friendly catalysts for various organic transformations, enabling greener and more sustainable chemical processes with reduced waste and energy consumption. | Sustainable chemical synthesis, biofuel production, and environmental remediation. |
| Carboxylic Acid-Based Corrosion Inhibitors | Carboxylic acids can form protective films on metal surfaces, inhibiting corrosion and extending the lifespan of metallic structures and components, leading to significant cost savings and improved safety. | Protection of infrastructure, transportation systems, and industrial equipment from corrosion. |
Latest Innovations of Carboxylic Acid
Renewable Carboxylic Acids and Derivatives
Novel substituted and unsubstituted terephthalic acid and carboxylate derivatives with significant renewable content have been developed. These are prepared from renewable starting materials like starch or cellulosic materials from agricultural crops and byproducts. The processes involve converting these renewable feedstocks to cyclohexene-based intermediates, which are then converted to the desired terephthalic and carboxylate derivatives with renewable content.
Carboxylic Acid Polymer Compositions
Carboxyl group-containing polymer compositions with improved properties have been reported. One example is a composition containing a carboxyl polymer with specific ratios of acrylic acid and sulfonic acid units, along with an adduct of hydrogen sulfite, exhibiting excellent anti-soil redeposition ability for fabric washing. Another is a copolymer with unsaturated monocarboxylic, dicarboxylic, and cationic units, showing better calcium ion trapping and carbon black dispersing abilities.
Carboxylic Acid Synthesis Innovations
New synthetic methods have emerged for preparing carboxylic acids and derivatives:
- Oxidation of alkynyl boronates, generated from terminal alkynes, to access carboxylic acids, esters, and amides, tolerating a broad range of functional groups. [Supplementary Information]
- A process for preparing a carboxylic acid compound from a precursor via hydrolysis and decarboxylation using an acidic resin.
- Synthesis of silicon-containing carboxylic acid derivatives as potential pharmaceutical compounds.
Liquid Crystal and Polymer Applications
Carboxylic acid compounds have found applications in liquid crystals and polymers. A carboxylic acid silyl ester alkoxysilane prevents hydrolysis and deprotection, which is useful for polymer synthesis. Carboxyl polymers act as thickeners, moisturizing agents, emulsifiers, and suspension stabilizers across various fields.
These innovations highlight the continued research efforts to develop renewable sources, improved polymer properties, new synthetic methods, and novel applications for carboxylic acids and their derivatives in diverse areas like materials, pharmaceuticals, and liquid crystals.
Technical challenges
| Renewable Carboxylic Acid Derivatives | Developing novel substituted and unsubstituted terephthalic acid and carboxylate derivatives with significant renewable content from renewable feedstocks like starch or cellulosic materials. |
| Carboxylic Acid Polymer Compositions | Formulating carboxyl group-containing polymer compositions with improved properties, such as anti-soil redeposition ability for fabric washing, calcium ion trapping ability, and carbon black dispersing ability. |
| Carboxylic Acid Synthesis Innovations | Exploring new synthetic methods for preparing carboxylic acids and their derivatives with improved efficiency, selectivity, and sustainability. |
| Carboxylic Acid Silyl Ester Stabilisation | Developing carboxylic acid silyl ester group-containing alkoxysilane compounds that prevent hydrolysis and deprotection from progressing, enhancing stability. |
| Renewable Carboxylic Acid Intermediates | Producing renewable intermediates like cyclohexene-based compounds from renewable feedstocks for subsequent conversion to terephthalic and carboxylate derivatives with renewable content. |
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