Carboxylic Acids

Carboxylic acids are organic compounds containing the –COOH (carboxyl) functional group, which consists of a carbonyl (C=O) and hydroxyl (–OH) group attached to the same carbon atom.

They occur naturally and can also be synthesized. Some carboxylic acids, known as fatty acids, are aliphatic compounds commonly found in natural fats as glycerol esters, typically with chain lengths from C₁₂ to C₁₈.

Carboxylic acids can lose a proton to form carboxylate ions (R–COO⁻), which are used to make salts such as soaps. They also serve as important precursors for compounds like esters, acid chlorides, anhydrides, and amides.

Structure of Carboxylic Acid

A carboxylic acid consists of a -COOH group. The open bond attached to the C atom shows that a long carbon chain can be attached to it. Among the two oxygen atoms, one is attached to the C atom with a double bond, while another O atom is attached to C with an H atom as an alcohol group.

Examples of Carboxylic Acid

• Formic Acid(HCOOH)
• Ethanoic or Acetic Acid (CH₃COOH)
• Citric Acid (C₆H₈O₇)
• Lactic Acid(C₃H₆O₃)
• Fumaric Acid (C₄H₄O₄)
• Oxalic Acid(C₂H₂O₄)
• Stearic Acid(CH₃(CH₂)₁₆COOH)

Nomenclature of Carboxylic Acids

To name any carboxylic acid chain, we can use the following steps:

Step 1: Identify the longest chain: Find the longest continuous carbon chain that contains the carboxyl group (–COOH). This chain serves as the parent chain for the name.

Step 2: Number the carbon atoms: Start numbering from the end closest to the carboxyl group, and assign the lowest possible numbers to the carbon atoms bearing the carboxyl group.

Step 3: Name the alkyl groups: If there are alkyl groups attached to the parent chain, identify and name them as substituents. Use prefixes like methyl, ethyl, propyl, etc., to indicate the number of carbon atoms in the alkyl group.

Step 4: Name the carboxyl group: The carboxyl group is named "oic acid." The carbon atom directly attached to the carboxyl group is numbered carbon 1. For example, a carboxyl group attached to a two-carbon chain is called "ethanoic acid."

Step 5: Use prefixes for multiple carboxyl groups: If there are multiple carboxyl groups present in the compound, use Greek numerical prefixes like "di-" (two), "tri-" (three), etc., to indicate the number of carboxyl groups. The carbon chain containing the carboxyl groups becomes the parent chain.

Step 6: Include substituents: If there are other functional groups or substituents present in the molecule, name and locate them according to the IUPAC rules.

Example: IUPAC name of compound CH₃CH₂COOH.

Solution:

This compound contains a two-carbon chain with a carboxyl group i.e., 3 carbon atoms in parent chain. The parent chain is propane, and the carboxyl group is named "oic acid." Therefore, the name of this compound is "propanoic acid."

Example: IUPAC name of compound CH₃CH₂CH₂COOH.

Solution:

This compound contains a three-carbon chain with a carboxyl group i.e., 4 carbon atoms in longest chain. The parent chain is butane, and the carboxyl group is named "oic acid." Therefore, the name of this compound is "butanoic acid."

Preparation of Carboxylic Acid

Carboxylic can be prepared using various techniques that are discussed below.

Oxidation of Primary Alcohols

Primary alcohols can be oxidized to carboxylic acids using strong oxidizing agents such as potassium permanganate (KMnO₄) or chromic acid (H₂CrO₄). The alcohol is initially converted to an aldehyde, which is further oxidized to the carboxylic acid. The reaction typically requires the presence of an acid catalyst.

Oxidation of Aldehydes

Aldehydes can be oxidized to carboxylic acids using similar oxidizing agents as mentioned above. The aldehyde is directly converted to the carboxylic acid without an intermediate step. The reaction also requires an acid catalyst.

Hydrolysis of Nitriles

Nitriles, which are organic compounds containing a cyano group (C≡N), can be hydrolyzed to carboxylic acids under acidic or basic conditions. In acidic conditions, nitriles are converted to amides, which are then further hydrolyzed to carboxylic acids. In basic conditions, nitriles are directly hydrolyzed to carboxylic acids.

Oxidation of Alkyl Benzenes

Alkyl benzenes, which are aromatic compounds containing an alkyl group attached to a benzene ring, can be oxidized to benzoic acids using strong oxidizing agents like potassium permanganate or chromic acid. The reaction typically requires high temperatures and the presence of an acid catalyst.

Grignard Reaction and Carbon Dioxide

Carboxylic acids can also be prepared by the reaction of a Grignard reagent (organomagnesium compound) with carbon dioxide (CO₂). The Grignard reagent reacts with CO₂ to form a carboxylate salt, which can be subsequently acidified to yield carboxylic acid.

Physical Properties of Carboxylic Acids

Some physical properties of carboxylic acids are as follows:

• State: Carboxylic acids can exist in different states depending on their molecular weight. Generally, carboxylic acids with a low number of carbon atoms (up to about 5) are liquids at room temperature, while those with a higher number of carbon atoms are typically solids.

• Odor: Carboxylic acids often have strong, pungent odors. Some examples include acetic acid (vinegar-like smell), formic acid (pungent odor), and butyric acid (rancid butter smell).

• Solubility: Carboxylic acids with a low number of carbon atoms (up to about 4) are soluble in water as they form hydrogen bonds with water molecules. But as the number of carbon atoms increases, the solubility of carboxylic acids in water decreases. Other than water, carboxylic acids are more soluble in organic solvents like ethanol and methanol.

• Boiling Points: Carboxylic acids have higher boiling points compared to hydrocarbons of similar molecular weight. This is because the carboxyl group enables the formation of intermolecular hydrogen bonds, which increase the strength of the attractive forces between molecules. Thus, carboxylic acids generally have higher boiling points than aldehydes, ketones, and alcohols of comparable molecular weight.

• Acidity: Carboxylic acids are weak acids that can donate a proton (H⁺) from the carboxyl group. They are typically more acidic than alcohols but less acidic than mineral acids (such as hydrochloric acid). The acidity of carboxylic acids is due to the stability of the resulting carboxylate ion formed after the loss of a proton.

• Reactivity: Carboxylic acids undergo various chemical reactions, including esterification (formation of esters), amidation (formation of amides), decarboxylation (loss of carbon dioxide), and oxidation (to form carbon dioxide and water).

Chemical Reactions of Carboxylic Acid

Carboxylic acids are very reactive compounds; thus, they go through various different chemical reactions. Some of these reactions are as follows:

Reaction with Metals

Carboxylic acids react with metals like potassium, sodium, magnesium, and calcium to create salts. A proton will be liberated from the carboxyl group of the carboxylic acid throughout the reaction step, which is where the metal substitution will take place. H₂ gas is produced in this process.

2CH₃COOH + 2Na ⇢ 2CH₃COONa + H₂

Reaction with Carbonates and Bicarbonates

Carboxylic acids decompose into salts, water, and carbon dioxide gas when they come into contact with carbonates and bicarbonates.

2CH₃COOH + Na₂CO₃ ⇢ 2CH₃COONa + H₂O + CO₂

This reaction can also be used to see if a carboxyl group is present. Effervescence is produced when carboxylic acids combine with a saturated sodium bicarbonate solution, releasing CO₂.. In an aqueous NaHCO₃ solution, however, most phenols do not produce effervescence. As a result, the reaction of bicarbonate with carboxylic acids aids in the differentiation of phenols from carboxylic acids.

Reaction with Alkali

Carboxylic acids react with alkalis, forming salts and water in the process.

CH₃COOH + NaOH ⇢ 2CH₃COONa + H₂O

Reduction of Carboxylic Acid to Alcohol

Reducing Agents: LiAlH₄ in ether solution or with H₂O in the presence of a copper chromite (CuCr₂O₄) catalyst, BH₃/THF (or diborane, B₂H₆), H₂, Ru, and so on.

Nucleophilic Acyl Substitution

Carboxylic acids go through nucleophilic substitution. Some of these nucleophilic substitutions are

• Formation of Acid Chlorides

Acid chlorides are formed when carboxylic acids combine with thionyl chloride (SOCl₂), phosphorus pentachloride (PCl₅), or phosphorus trichloride (PCl₃). To understand the reaction, look at the examples below.

RCOOH + SOCl₂ ⇢ RCOCl + SO₂ + HCl

RCOOH + PCl₅ ⇢ RCOCl + POCl₃ + HCl

• Esterification 

When carboxylic acids and alcohols are heated in the presence of strong sulfuric acid or dry hydrochloric acid, fruity esters result.

Sulfuric acid is used as a dehydrating agent in this reaction. An equilibrium reaction is a reaction, for example. As a result, to keep the reaction moving forward, the ester is distilled.

• Anhydride Formation 

The generation of acid anhydrides occurs when two molecules of carboxylic acid are heated with a dehydrating agent such as phosphorus pentoxide.

2RCOOH ⇢ RCO-O-RCO + H₂O

• Amide Compound Formation 

Ammonium salts are produced by treating carboxylic acids with ammonia. Ammonium salts lose a water molecule when heated, resulting in amide production.

Reactions of Carboxyl Group

Carboxyl group compounds, i.e., derivatives of carboxylic acids, are also very reactive compounds in nature. Thus, they also go

• Decarboxylation: When sodium salts of carboxylic acids are distilled with soda lime (NaOH + CaO), a decarboxylation process occurs, resulting in alkanes.

• Dry Distillation of Calcium Salts:

(RCOO)₂Ca{Calcium Salt} ⇢ RCOR{Ketone} + CaCO₃

Formaldehyde is produced by heating calcium formate.

(HCOO)₂Ca⇢ HCHO{Formaldehyde} + CaCO₃

An aldehyde is generated when a calcium salt of an acid (other than formic acid) and calcium formate are heated together.

(CH₃COO)₂Ca{calcimu Acetate} + (HCOO)₂Ca{Calcium Formate} ⇢ 2CH₃CHO{Acetaldehyde} + CaCO₃

• Conversion to Nitriles:

Alkane nitrile is formed by passing a combination of acid and NH₃ over overheated alumina.

R-(C=O)-OH  + NH₃⇢ R-CN + 2H₂O

Name Reactions with Carboxylic Acid

There are various name reactions that involve carboxylic acids; some of these name reactions are

• Kolbe's Synthesis
• Hunsdiecker Reaction
• Hell-Volhard Zelinsky Reaction
• Simonini Reaction 

Let's understand these reactions in detail.

Kolbe's Synthesis 

Alkanes are produced by the electrolysis of fatty acid salts (concentrated aqueous solution).

At the anode: 2RCOO⁻⇢ R-R + 2CO₂ + 2e⁻

At Cathode: 2Na⁺ + 2e⁻ ⇢ 2Na ⇢ 2NaoH + H₂

Radial intermediates are present during the reaction.

Hunsdiecker Reaction 

When monocarboxylic acid's silver salt is treated with halogen, an alkyl halide is produced.

Hell-Volhard Zelinsky Reaction

In the presence of a catalyst, a carboxylic acid with an α-hydrogen is treated with Cl₂ or Br₂.. Chlorine or bromine atoms substitute for the α-hydrogen.

RCH₂COOH{Carboxylic Acid} + Cl₂ ⇢ RCHClCOOH{α-Chlorinated Acid} ⇢ RCCl₂COOH{α, α-dichloro Acid}

Simonini Reaction 

Simonini Relations refer to the reaction of silver carboxylic acid salts with iodine.

Uses of Carboxylic Acid 

Carboxylic acids are one of the most used compounds in organic chemistry. Some of these use cases of carboxylic acid are as follows:

Food and Beverage Industry

Carboxylic acids such as acetic acid, citric acid, lactic acid, etc. are used as food additives for flavoring and preservation. For example, synthetic white vinegar is a diluted solution of acetic acid, citric acid is used as a flavoring agent in the majority of beverages, and lactic acid is a well-known flavor enhancer.

Pharmaceuticals and Cosmetics

Carboxylic acids are used in the production of pharmaceutical drugs and cosmetic products either as active ingredients, pH adjusters, or preservatives. For instance, salicylic acid is used in acne treatments, and citric acid is used in skincare products for its exfoliating properties.

Polymer Production

Many carboxylic acids are utilized as monomers in the production of various different polymers and plastics. For example, acrylic and methacrylic acids are used in the production of acrylic polymers, which find applications in paints, adhesives, and textiles.

Detergents and Cleaning Products

Long-chain fatty acids (which are examples of carboxylic acids) are used in the production of soaps, detergents, and cleaning products.

Agriculture

Carboxylic acids are used in agricultural applications as herbicides, fungicides, and plant growth regulators. For example, acetic acid-based herbicides are used for weed control, and auxin-like carboxylic acids are used to promote plant growth.

Chemical Intermediates

Carboxylic acids serve as important intermediates in the synthesis of various chemicals. They can be used to produce esters, amides, anhydrides, and other derivatives. These derivatives have diverse applications in industries such as pharmaceuticals, perfumes, and plastics.

Textile Industry

Carboxylic acids are used in textile dyeing and printing processes. They act as mordants, which help fix the dyes onto the fabric, improving color fastness.

Industrial Processes

Carboxylic acids find applications in various industrial processes. For example, formic acid is used in leather tanning, acetic acid is used in the production of vinyl acetate for adhesives, and adipic acid is used in the synthesis of nylon.

Carboxylic Acid Derivatives

The compounds that are derived from the carboxylic acid are called the derivatives of the carboxylic acid. Various derivatives of the carboxylic acid are,

• Ester
• Acidic Anhydride
• Amide
• Alkyl halide
• Thioester, etc.