Aldehydes Ketones And Carboxylic Acids Class 12 Chemistry Chapter 8 Notes

Aldehydes Ketones and Carboxylic Acids Class 12 Chemistry Chapter 8 Notes

Nomenclature Of Aldehydes and ketones

Aldehydes and ketones are simple yet highly important carbonyl compounds in organic chemistry. They are distinguished by the placement of the carbonyl group (C=O) within the molecule.

Common Names:

In common nomenclature, aldehydes are often named by deriving their names from the corresponding carboxylic acids, replacing the “-ic” acid ending with “aldehyde.” Additionally, these names may reflect the origin or source of the acid or aldehyde. The position of substituents in the carbon chain is denoted by Greek letters such as α, β, γ, δ, and so on. The α-carbon is directly linked to the aldehyde group, followed by the β-carbon, and so on. For example:

Methanal is commonly known as formaldehyde.
Ethanal is commonly known as acetaldehyde.

Ketones, on the other hand, are named by specifying the two alkyl or aryl groups bonded to the carbonyl group. The position of substituents is indicated using Greek letters like α, α’, β, β’, and so on. The α-carbon atoms are the ones directly attached to the carbonyl group. For example:

The common name acetone corresponds to the IUPAC name propanone.

When an alkylphenyl ketone is named, the name of the acyl group is added as a prefix to the word “phenone.” For example:

Acetophenone is an example of this nomenclature.

IUPAC Names:

In IUPAC nomenclature, open-chain aliphatic aldehydes and ketones are named by replacing the “-e” ending of the corresponding alkane with “-al” for aldehydes and “-one” for ketones. In aldehydes, the longest carbon chain is numbered starting from the carbon atom of the aldehyde group. In ketones, numbering begins from the end nearer to the carbonyl group. Substituents are prefixed in alphabetical order, and their positions in the carbon chain are indicated using numerals.

For example:

Methanal is IUPAC-named as formaldehyde.
Ethanal is IUPAC-named as acetaldehyde.
Propanone is the IUPAC name for acetone.

For cyclic ketones, where the carbonyl carbon is numbered as one, the numbering of the ring carbon atoms begins from the carbon atom attached to the carbonyl group.

When the aldehyde group is attached to a ring, the suffix “carbaldehyde” is added after the full name of the cycloalkane. The numbering of the ring carbon atoms starts from the carbon atom attached to the aldehyde group. For example:

Benzene carbaldehyde is the IUPAC name for benzaldehyde.

Structure of the Carbonyl Group

The carbonyl group (C=O) is a highly significant functional group in organic chemistry. Its structure and properties are as follows:

Hybridization: The carbonyl carbon (C) atom in the C=O group is sp2-hybridized. It forms three sigma (σ) bonds with other atoms. One σ bond is with another carbon or hydrogen atom, one σ bond is with another carbon atom (in the case of ketones), and one σ bond is with an oxygen atom.
Pi (π) Bond: In addition to the three σ bonds, the carbonyl group also contains a pi (π) bond. The pi bond is formed by the overlap of a p-orbital on the carbon atom and a p-orbital on the oxygen atom. This pi bond is responsible for the double bond character of the C=O bond.
Planarity: The carbonyl carbon and the three atoms (two carbons and one oxygen) attached to it all lie in the same plane. This planar arrangement is due to the sp2 hybridization of the carbonyl carbon.
Bond Angles: The bond angles in the carbonyl group are approximately 120 degrees, which is consistent with a trigonal planar geometry. This arrangement minimizes repulsion between electron pairs around the carbon atom.
Polarity: The carbon-oxygen double bond is polarized because oxygen is more electronegative than carbon. As a result, the carbonyl carbon becomes partially positively charged (δ+) and the carbonyl oxygen becomes partially negatively charged (δ-), creating a dipole moment. This polarity makes carbonyl compounds more reactive than nonpolar compounds like ethers.
Resonance: The high polarity of the carbonyl group can be explained by resonance structures. There are two resonance structures, a neutral one (A) and a dipolar one (B), which depict the distribution of electron density in the C=O bond. These resonance structures show that there is partial double bond character in the C=O bond due to resonance, making it stronger and shorter compared to a typical single bond.

Preparation of Aldehydes and Ketones

Aldehydes and ketones can be prepared using various methods, including:

Oxidation of Alcohols: Aldehydes and ketones can be prepared by oxidizing primary and secondary alcohols, respectively. This can be achieved using various oxidizing agents like potassium dichromate (K2Cr2O7) or potassium permanganate (KMnO4).
Dehydrogenation of Alcohols: This method is suitable for volatile alcohols and is used in industrial applications. Alcohol vapors are passed over heavy metal catalysts like silver (Ag) or copper (Cu). Primary alcohols are converted to aldehydes, while secondary alcohols are converted to ketones.
From Hydrocarbons:
- Ozonolysis of Alkenes: Ozonolysis of alkenes followed by treatment with zinc dust and water can yield aldehydes, ketones, or a mixture of both, depending on the substitution pattern of the alkene.
- Hydration of Alkynes: Alkynes can be converted to aldehydes or ketones through the addition of water in the presence of catalysts like sulfuric acid (H2SO4) and mercuric sulfate (HgSO4). Ethyne (acetylene) forms acetaldehyde, while other alkynes yield ketones.

Preparation of Aldehydes

Aldehydes can be prepared through various methods, including:

From Acyl Chlorides (Acid Chlorides): Aldehydes can be obtained by hydrogenating acyl chlorides (acid chlorides) using palladium on barium sulfate as a catalyst. This reaction is known as the Rosenmund reduction.
From Nitriles and Esters:
- Stephen Reaction: Nitriles can be reduced to the corresponding imines using stannous chloride in the presence of hydrochloric acid. These imines can then be hydrolyzed to yield aldehydes.
- DIBAL-H Reduction: Nitriles and esters can also be selectively reduced to aldehydes using diisobutylaluminium hydride (DIBAL-H). The nitriles and esters are first converted to imines, which are subsequently hydrolyzed to form aldehydes.
From Hydrocarbons (Aromatic Aldehydes):
- Oxidation of Methylbenzene: Aromatic aldehydes like benzaldehyde and its derivatives can be prepared from aromatic hydrocarbons (e.g., toluene) through oxidation. Strong oxidizing agents can be used to stop the oxidation at the aldehyde stage. Two methods for achieving this are:
  - Using chromyl chloride (CrO2Cl2), which oxidizes the methyl group to a chromium complex. Hydrolysis of this complex yields the corresponding benzaldehyde (Etard reaction).
  - Using chromic oxide (CrO3) in acetic anhydride to convert toluene to benzylidene diacetate, which can then be hydrolyzed to form the desired benzaldehyde.
- Gatterman-Koch Reaction: Benzaldehydes and substituted benzaldehydes can be synthesized by treating benzene or its derivatives with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminum chloride or cuprous chloride. This reaction is known as the Gatterman-Koch reaction.

Preparation of Ketones

Ketones can be prepared through various methods, including:

From Acyl Chlorides: Ketones can be obtained by reacting acyl chlorides with dialkylcadmium, which is prepared by the reaction of cadmium chloride with a Grignard reagent.
From Nitriles: Ketones can be synthesized by treating a nitrile with a Grignard reagent followed by hydrolysis. This reaction sequence yields a ketone as the final product.
From Benzene or Substituted Benzenes: Ketones can be prepared from benzene or substituted benzenes by treating them with an acid chloride in the presence of anhydrous aluminum chloride. This reaction is known as the Friedel-Crafts acylation reaction.

Physical Properties Of Aldehydes and Ketones

The physical properties of aldehydes and ketones can be summarized as follows:

State at Room Temperature: Methanal (formaldehyde) is a gas at room temperature, ethanal (acetaldehyde) is a volatile liquid, and other aldehydes and ketones are typically liquid or solid at room temperature.
Boiling Points: Aldehydes and ketones generally have higher boiling points than hydrocarbons and ethers with similar molecular masses. This is because of weak molecular associations in aldehydes and ketones due to dipole-dipole interactions.
Comparison to Alcohols: The boiling points of aldehydes and ketones are lower than those of alcohols with similar molecular masses. This is because alcohols can form intermolecular hydrogen bonds, which are stronger than dipole-dipole interactions.
Solubility in Water: Lower molecular weight aldehydes and ketones (e.g., methanal, ethanal, propanone) are miscible with water in all proportions because they can form hydrogen bonds with water molecules. However, solubility decreases as the alkyl chain length increases.
Solubility in Organic Solvents: Aldehydes and ketones are fairly soluble in organic solvents like benzene, ether, methanol, and chloroform.
Odor: Lower aldehydes have sharp, pungent odors, while larger aldehydes and ketones tend to have less pungent and more fragrant odors. Many naturally occurring aldehydes and ketones are used in perfumes and flavorings due to their pleasant fragrances.

Chemical Reactions Of Aldehydes and Ketones

1. Nucleophilic addition reactions

Nucleophilic addition reactions are a key characteristic of aldehydes and ketones. Here, I’ll explain the mechanism and provide examples of these reactions:

Mechanism of Nucleophilic Addition Reactions:

Nucleophile Attack: In nucleophilic addition reactions, a nucleophile (Nu⁻) attacks the electrophilic carbon atom of the polar carbonyl group, specifically the carbon in the carbonyl group (C=O). This attack occurs from a direction approximately perpendicular to the plane of the sp² hybridized orbitals of the carbonyl carbon.
Formation of Tetrahedral Intermediate: The nucleophile’s attack results in the breaking of the π bond between the carbon and oxygen of the carbonyl group. As a result, the carbon changes its hybridization from sp² to sp³, forming a tetrahedral intermediate. This intermediate involves the nucleophile attached to the carbon and the oxygen becoming negatively charged.
Protonation: The next step involves protonation. The tetrahedral intermediate captures a proton (H⁺) from the reaction medium. This protonation results in the neutralization of the negatively charged oxygen and leads to the formation of the electrically neutral product.
Addition of Nu⁻ and H⁺: In the end, the net result is the addition of Nu⁻ and H⁺ across the carbon-oxygen double bond. This addition results in the formation of a new compound.

Reactivity:

Aldehydes are generally more reactive than ketones in nucleophilic addition reactions. This is due to both steric and electronic factors.
- Sterically, ketones have two relatively large substituents, which hinder the approach of nucleophiles to the carbonyl carbon compared to aldehydes, which have only one such substituent.
- Electronically, aldehydes are more reactive than ketones because the presence of two alkyl groups in ketones reduces the electrophilicity of the carbonyl carbon more effectively.

Examples of Nucleophilic Addition Reactions:

Addition of Hydrogen Cyanide (HCN): Aldehydes and ketones react with hydrogen cyanide (HCN) to yield cyanohydrins. This reaction is catalyzed by a base, and the cyanide ion (CN⁻) acts as a strong nucleophile.
- Example: RCHO or RCOR’ + HCN → RCH(OH)CN or RC(OH)(CN)R’
Addition of Sodium Hydrogensulfite: Sodium hydrogensulfite adds to aldehydes and ketones to form addition products, which can be used for separation and purification.
- Example: RCHO or RCOR’ + NaHSO₃ → RCH(OH)SO₃Na or RC(OH)(SO₃Na)R’
Addition of Grignard Reagents: Grignard reagents, which are highly reactive organometallic compounds, can add to the carbonyl group, forming alcohols.
- Example: RMgX + RC=O → RCH(OH)MgX
Addition of Alcohols: Aldehydes react with monohydric alcohols in the presence of dry hydrogen chloride to form alkoxyalcohol intermediates, known as hemiacetals, which can further react with more alcohol molecules to form gem-dialkoxy compounds known as acetals.
- Example with an aldehyde: RCHO + ROH + HCl → RCH(OR)OH
- Example with a ketone: RCOR’ + ROH + HCl → RC(OR’)₂OH
Addition of Ammonia and Its Derivatives: Nucleophiles like ammonia (NH₃) and its derivatives (H₂N-Z) can add to the carbonyl group of aldehydes and ketones.
- Example: RCHO or RCOR’ + H₂N-Z → RCH(NHZ)OH or RC(NHZ)(ZR’)OH

These nucleophilic addition reactions are essential in organic chemistry and play a significant role in the synthesis of various organic compounds.

2. Reduction

Reduction reactions are important transformations of aldehydes and ketones, leading to the formation of alcohols or hydrocarbons. Here’s an explanation of these reduction reactions:

1. Reduction to Alcohols:

Aldehydes can be reduced to primary alcohols, while ketones can be reduced to secondary alcohols.
Common reducing agents used for this transformation are sodium borohydride (NaBH4) and lithium aluminum hydride (LiAlH4).
Mechanism: These reducing agents donate hydride ions (H⁻) to the carbonyl carbon, resulting in the reduction of the carbonyl group to an alcohol.
The choice of reducing agent depends on the reaction conditions and the desired product. NaBH4 is milder and typically used for aldehydes and ketones that do not contain sensitive functional groups. LiAlH4 is a strong reducing agent and can reduce a wider range of functional groups.
Example reaction with NaBH4:
- RCHO (Aldehyde) + NaBH4 → RCH2OH (Primary alcohol)
- RCOR’ (Ketone) + NaBH4 → RCHR’OH (Secondary alcohol)

2. Reduction to Hydrocarbons:

Aldehydes and ketones can be reduced to hydrocarbons under specific conditions.
Two common methods for this reduction are the Clemmensen reduction and the Wolff-Kishner reduction.
In the Clemmensen reduction, zinc amalgam and concentrated hydrochloric acid are used to reduce the carbonyl group to a CH2 group. This reaction is particularly useful for converting carbonyl compounds to alkanes.
- Example: RCHO or RCOR’ + Zn/HCl → RH (Alkane)
In the Wolff-Kishner reduction, hydrazine and sodium or potassium hydroxide in a high-boiling solvent like ethylene glycol are used to reduce the carbonyl group to a CH2 group. This method is also employed to convert carbonyl compounds to alkanes.
- Example: RCHO or RCOR’ + N2H4/Hydroxide → RH (Alkane)

These reduction reactions are essential in organic synthesis, allowing chemists to modify the functional groups in aldehydes and ketones, leading to the formation of various organic compounds, including alcohols and hydrocarbons.

3. Oxidation

Oxidation reactions are important for distinguishing between aldehydes and ketones and for converting aldehydes into carboxylic acids. Here’s an explanation of these oxidation reactions:

Oxidation of Aldehydes:

Aldehydes can be readily oxidized to carboxylic acids using various oxidizing agents. This is a characteristic reaction that helps differentiate aldehydes from ketones.
Common oxidizing agents for aldehydes include nitric acid (HNO3), potassium permanganate (KMnO4), and potassium dichromate (K2Cr2O7).
Even mild oxidizing agents like Tollens’ reagent and Fehling’s reagent can oxidize aldehydes.
Tollens’ Test: When an aldehyde is warmed with freshly prepared ammoniacal silver nitrate solution (Tollens’ reagent), a bright silver mirror is formed on the inner surface of the test tube. This reaction indicates the presence of an aldehyde. The aldehyde is oxidized to the corresponding carboxylate anion.
Fehling’s Test: Fehling’s reagent consists of two solutions: Fehling solution A (aqueous copper sulfate) and Fehling solution B (alkaline sodium potassium tartarate or Rochelle salt). When an aldehyde is heated with Fehling’s reagent, a reddish-brown precipitate of copper(I) oxide (Cu2O) is formed, indicating the presence of an aldehyde. Aldehydes are oxidized to carboxylate anions in this reaction.
Oxidation of Methyl Ketones by Haloform Reaction: Aldehydes and ketones containing at least one methyl group attached to the carbonyl carbon atom (methyl ketones) can undergo oxidation by sodium hypohalite (NaXO, where X is a halogen, usually chlorine or iodine). This reaction is known as the haloform reaction. In this reaction, the methyl group is converted into a haloform, such as iodoform (CHI3) or chloroform (CHCl3), while the carbonyl group is oxidized to a carboxylate anion. This reaction is used to detect the presence of methyl ketones or compounds containing the CH3CO group.

Oxidation of Ketones:

Ketones are generally more resistant to oxidation compared to aldehydes because they lack a hydrogen atom directly bonded to the carbonyl carbon.
To oxidize ketones, stronger oxidizing agents and elevated temperatures are typically required. Ketones undergo carbon-carbon bond cleavage during oxidation, resulting in a mixture of carboxylic acids with fewer carbon atoms than the original ketone.
Mild oxidizing agents like Tollens’ reagent and Fehling’s reagent do not oxidize ketones.
Oxidation of ketones is not commonly used for synthetic purposes because it involves breaking carbon-carbon bonds, which can lead to a mixture of products.

4. Reactions due to a-hydrogen

Reactions involving α-hydrogen atoms in aldehydes and ketones are significant because these compounds can act as both electrophiles and acids due to the presence of α-hydrogens. Here’s an explanation of some of these reactions:

Acidity of α-Hydrogens:

The α-hydrogen atoms in aldehydes and ketones are relatively acidic due to the electron-withdrawing effect of the carbonyl group and the resonance stabilization of the resulting conjugate base.
This acidity allows these compounds to undergo various reactions, particularly involving the removal of α-hydrogens.

Aldol Condensation:

Aldehydes and ketones containing at least one α-hydrogen undergo a reaction known as the Aldol condensation in the presence of a dilute alkali (base) catalyst.
In the Aldol condensation, the carbonyl compound acts as both an electrophile and an acid. The base removes an α-hydrogen, forming an enolate ion.
The enolate ion can then attack another molecule of the same or a different carbonyl compound (aldol or ketol formation), leading to the formation of a β-hydroxy aldehyde or ketone.
The name “aldol” comes from the combination of “aldehyde” and “alcohol,” reflecting the presence of both functional groups in the product.
The aldol or ketol product can undergo dehydration, losing a water molecule, to form an α,β-unsaturated carbonyl compound (enone or enal), a conjugated system with a double bond between the α and β carbons.
The reaction involving ketones is often referred to as “ketol condensation.”
Aldol condensation is an important reaction in organic synthesis for building complex molecules.

Cross Aldol Condensation:

When aldol condensation is carried out between two different aldehydes, two different ketones, or an aldehyde and a ketone, it is termed “cross aldol condensation.”
Cross aldol condensation can yield a mixture of four possible products because both reactants may contain α-hydrogens.
The products are often a combination of α,β-unsaturated aldehydes and ketones.
Here’s an example: Suppose you have a mixture of ethanal and propanal. In a cross aldol condensation, you can obtain a mixture of four products, including two different aldols, a β-hydroxy aldehyde, and a β-hydroxy ketone.

5. Other reactions

Cannizzaro Reaction:

The Cannizzaro reaction is a unique reaction observed in aldehydes that lack α-hydrogen atoms (i.e., aldehydes where both R groups are bulky). These aldehydes are unable to undergo typical nucleophilic addition reactions or Aldol condensation due to the absence of α-hydrogens.
In the Cannizzaro reaction, when such aldehydes are heated with a concentrated alkali solution (e.g., sodium hydroxide, NaOH), they undergo a disproportionation reaction.
Disproportionation is a redox reaction in which one molecule is simultaneously reduced while another molecule is oxidized.
In the Cannizzaro reaction, one molecule of the aldehyde is reduced to form an alcohol, while another molecule of the same aldehyde is oxidized to form a carboxylic acid salt (usually the sodium or potassium salt).
The reaction often produces a mixture of alcohol and carboxylic acid salt as the final products.

Electrophilic Substitution in Aromatic Aldehydes and Ketones:

Aromatic aldehydes and ketones, such as benzaldehyde or acetophenone, can undergo electrophilic aromatic substitution reactions.
In these reactions, the carbonyl group (C=O) attached to the aromatic ring acts as a deactivating and meta-directing group for electrophilic substitution reactions on the aromatic ring.
The presence of the carbonyl group in the aromatic ring system reduces the electron density on the ring, making it less reactive toward electrophiles compared to benzene. This is referred to as deactivation.
Additionally, the carbonyl group directs the incoming electrophile to the meta position with respect to the carbonyl group. This is known as meta-directing behavior.
The electrophilic substitution reaction often leads to the formation of meta-substituted products, where the new substituent is attached at the meta position to the carbonyl group.

Uses of Aldehydes and Ketones

Formaldehyde (Methanal):
- Used in the preparation of formalin (a 40% formaldehyde solution), which is a disinfectant and preservative for biological specimens.
- Utilized in the production of bakelite, a phenol-formaldehyde resin, which is used in the manufacturing of electrical insulators, molded parts, and coatings.
- Used in the synthesis of urea-formaldehyde glues and other polymeric products.
- Employed as a reducing agent in various chemical reactions.
Acetaldehyde (Ethanal):
- Primarily used as a starting material in the production of acetic acid, which finds applications in the food industry, pharmaceuticals, and chemicals.
- Used in the synthesis of ethyl acetate, which is a solvent and flavoring agent.
- Utilized in the production of vinyl acetate, a key monomer in the manufacture of polyvinyl acetate (PVA) and polyvinyl alcohol (PVOH).
- Used in the production of polymers and resins.
Benzaldehyde:
- Commonly used in perfumery and the fragrance industry due to its pleasant almond-like odor.
- Finds applications in dye industries.
Acetone and Ethyl Methyl Ketone:
- Widely employed as industrial solvents for various purposes, including cleaning, degreasing, and as a component in paint and coatings.
- Acetone is used as a nail polish remover and as a solvent in the cosmetics industry.
Other Aldehydes and Ketones:
- Several aldehydes and ketones, such as butyraldehyde, vanillin, acetophenone, and camphor, are known for their characteristic odors and flavors, making them valuable in the fragrance and flavor industry.

Carboxylic Acids:

Fatty Acids: Some higher members of aliphatic carboxylic acids (C12 – C18) known as fatty acids are found in natural fats as esters of glycerol. These fatty acids are essential components of dietary fats and oils and serve as a source of energy in the human diet.
Industrial Use: Carboxylic acids are used in various industrial processes as starting materials for the synthesis of anhydrides, esters, acid chlorides, amides, and other important organic compounds.
Food Industry: Certain carboxylic acids, such as citric acid and acetic acid, are used as food additives for their sour or acidic taste. They are commonly found in soft drinks, jams, and pickles.
Pharmaceuticals: Carboxylic acids can be found in many pharmaceutical compounds. For example, aspirin is acetylsalicylic acid, a derivative of salicylic acid, which is a natural compound found in willow bark.
Chemical Research: Carboxylic acids play a vital role in chemical research and organic synthesis due to their reactivity and ability to form various derivatives, making them versatile starting materials in laboratory experiments.

Nomenclature of Carboxyl Group

The nomenclature of carboxyl groups in carboxylic acids follows the IUPAC naming rules for organic compounds. Here are some key points:

Common Names: Many carboxylic acids have common names that are derived from their natural sources or historical usage. These common names typically end with the suffix “-ic acid.” For example, formic acid (HCOOH) was named after ants (Latin: formica), acetic acid (CH3COOH) was named after vinegar (Latin: acetum), and butyric acid (CH3CH2CH2COOH) was named after butter (Latin: butyrum).
IUPAC Names: In the IUPAC system, aliphatic carboxylic acids are named by replacing the ending “-e” in the name of the corresponding alkane with “-oic acid.” The carbon atom in the carboxyl group (–COOH) is assigned the number 1 in the parent chain. The position of substituents is indicated by numbering the carbon atoms in the alkyl chain.
Multiplicative Prefix: Compounds containing more than one carboxyl group are named by adding a multiplicative prefix to the name of the parent alkyl chain. For example, a compound with two carboxyl groups is called a “dicarboxylic acid,” and a compound with three carboxyl groups is called a “tricarboxylic acid.” The position of each carboxyl group is indicated by an Arabic numeral before the multiplicative prefix.

Here are some examples of IUPAC names for carboxylic acids:

CH3CH2COOH: Propanoic acid (Derived from propane)
CH3CH2CH2COOH: Butanoic acid (Derived from butane)
CH3CH(COOH)CH2COOH: 2-Methylbutanoic acid (Derived from 2-methylbutane)
HOOCCH2COOH: Oxalic acid (Dicarboxylic acid with two carboxyl groups)
HOOCCH2CH2COOH: Malonic acid (Dicarboxylic acid with two carboxyl groups)
HOOCCH2CH2CH2COOH: Glutaric acid (Dicarboxylic acid with two carboxyl groups)
HOOCCH2CH(COOH)CH2COOH: 3-Methylglutaric acid (Dicarboxylic acid with three carboxyl groups, including a methyl group)

These rules help provide a systematic way to name carboxylic acids based on their molecular structure and substituent positions.

Structure of Carboxyl Group

The carboxyl group (-COOH) in carboxylic acids consists of a carbonyl group (C=O) and a hydroxyl group (OH) attached to the same carbon atom. The structural formula of the carboxyl group can be represented as follows:

   O
   |
C--C
   |
   H

In this structure:

The carbon atom at the center is known as the carboxyl carbon.
The oxygen atom bonded to the carboxyl carbon is part of the carbonyl group (C=O).
The oxygen atom bonded to the carboxyl carbon via a single bond represents the hydroxyl group (OH).
The other bonds of the carboxyl carbon may be connected to other atoms or groups depending on the specific carboxylic acid.

The bonds in the carboxyl group are typically arranged in one plane, and the angle between them is approximately 120 degrees. This planar arrangement allows for resonance stabilization of the carboxyl group, which can be represented as follows:

    O
   // \
C--C
   \\ /
    O-

In this resonance structure:

The carbon-oxygen double bond (C=O) is retained.
The oxygen atom on the right carries a negative charge (O-) due to the movement of electrons.

The resonance stabilization makes the carboxyl group less electrophilic than a typical carbonyl group (C=O) and influences the reactivity of carboxylic acids in various chemical reactions, including nucleophilic addition and acid-base reactions.

Methods of Preparation of Carboxylic Acids

The methods of preparation of carboxylic acids are as follows:

From Primary Alcohols and Aldehydes:
- Primary alcohols can be oxidized to carboxylic acids using common oxidizing agents like potassium permanganate (KMnO4) or potassium dichromate (K2Cr2O7) in various media (neutral, acidic, or alkaline). Jones reagent, which consists of chromium trioxide (CrO3) in acidic media, can also be used for this purpose.
- Carboxylic acids can also be prepared from aldehydes using mild oxidizing agents.
From Alkylbenzenes:
- Aromatic carboxylic acids can be prepared by oxidizing alkylbenzenes using strong oxidizing agents such as chromic acid or acidic or alkaline potassium permanganate. This method oxidizes the entire side chain of the alkylbenzene to the carboxyl group.
From Nitriles and Amides:
- Nitriles can be hydrolyzed to amides and then to carboxylic acids in the presence of acid (H+) or base (OH-) as catalysts. Mild reaction conditions can be used to stop the reaction at the amide stage.
From Grignard Reagents:
- Grignard reagents can react with carbon dioxide (dry ice) to form salts of carboxylic acids. These salts can then be acidified with a mineral acid to yield the corresponding carboxylic acids. This method is useful for converting alkyl halides into carboxylic acids with one more carbon atom.
From Acyl Halides and Anhydrides:
- Acid chlorides (acyl halides) can be hydrolyzed with water to directly yield carboxylic acids. They can also be hydrolyzed with aqueous base to form carboxylate ions, which can be subsequently acidified to obtain carboxylic acids. Anhydrides can be hydrolyzed with water to form carboxylic acids.
From Esters:
- Carboxylic acids can be prepared from esters by acidic hydrolysis, which directly yields carboxylic acids. Alternatively, basic hydrolysis of esters forms carboxylates, which can be acidified to produce carboxylic acids.

Physical Properties of Carboxylic Acids

Carboxylic acids exhibit the following physical properties:

State at Room Temperature: Aliphatic carboxylic acids with up to nine carbon atoms are typically colorless liquids at room temperature. However, higher carboxylic acids are waxy solids and are essentially odorless due to their low volatility.
Odor: Aliphatic carboxylic acids often have unpleasant odors. For example, acetic acid (a two-carbon carboxylic acid) is responsible for the distinctive smell of vinegar. The odor becomes less pronounced as the size of the carboxylic acid molecule increases.
Boiling Points: Carboxylic acids have higher boiling points compared to aldehydes, ketones, and even alcohols of similar molecular masses. This elevated boiling point is attributed to the extensive intermolecular hydrogen bonding that occurs among carboxylic acid molecules. These hydrogen bonds are not fully broken even in the vapor phase. In fact, many carboxylic acids exist as dimers (pairs of molecules) in the vapor phase or in aprotic solvents.
Solubility in Water: Simple aliphatic carboxylic acids containing up to four carbon atoms are miscible in water due to the formation of hydrogen bonds with water molecules. However, the solubility of carboxylic acids decreases as the number of carbon atoms in the molecule increases. Higher carboxylic acids are practically insoluble in water because of the increased hydrophobic interactions between the hydrocarbon part of the molecule and water molecules. For instance, benzoic acid, the simplest aromatic carboxylic acid, is nearly insoluble in cold water.
Solubility in Organic Solvents: Carboxylic acids are generally soluble in less polar organic solvents such as benzene, ether, alcohol, chloroform, and others. This solubility makes them useful in various organic reactions and extractions.

Chemical Reactions of Carboxylic Acids

1. Reactions Involving Cleavage of O–H Bond

Carboxylic acids exhibit various reactions involving the cleavage of O-H bonds and display distinct acidity properties. Here’s an explanation of some key points:

1. Acidity:

Carboxylic acids are acidic compounds due to the presence of the carboxyl group (-COOH). The O-H bond in the -COOH group can be easily cleaved, releasing a proton (H+) and forming a carboxylate anion (RCOO⁻). This acidity is attributed to the resonance stabilization of the conjugate base (carboxylate anion) and the strong electron-withdrawing effect of the carbonyl group.
The strength of an acid is often indicated by its pKa value, which is the negative logarithm (base 10) of the acid dissociation constant (Ka). Smaller pKa values correspond to stronger acids. For instance, hydrochloric acid has a pKa of -7.0, while the pKa values for some carboxylic acids are as follows: trifluoroacetic acid (0.23), benzoic acid (4.19), and acetic acid (4.76).
Generally, strong acids have pKa values below 1, moderately strong acids have pKa values between 1 and 5, weak acids have pKa values between 5 and 15, and extremely weak acids have pKa values greater than 15.
Carboxylic acids are stronger acids compared to alcohols and many simple phenols but weaker than mineral acids. This increased acidity can be attributed to the resonance stabilization of the carboxylate ion compared to the phenoxide ion.

2. Effect of Substituents on Acidity:

Substituents attached to the carboxylic acid molecule can influence its acidity. Electron-withdrawing groups (EWG) increase the acidity of carboxylic acids by stabilizing the conjugate base through inductive and resonance effects. Conversely, electron-donating groups (EDG) decrease acidity by destabilizing the conjugate base.
A common trend is that halogens (F < Cl < Br < I), cyano (CN), and nitro (NO2) groups are electron-withdrawing and increase the acidity of carboxylic acids. Conversely, groups like methyl (CH3) and ethyl (CH3CH2) are electron-donating and decrease acidity.
Direct attachment of groups such as phenyl or vinyl to the carboxylic acid can increase acidity due to the greater electronegativity of the sp2 hybridized carbon to which the carboxyl carbon is attached.

2. Reactions Involving Cleavage of C–OH Bond

Reactions involving the cleavage of the C-OH bond in carboxylic acids result in the formation of various functional groups and products. Here are some key reactions:

1. Formation of Anhydride:

Carboxylic acids can be converted into anhydrides when heated with mineral acids such as H2SO4 or with dehydrating agents like P2O5.
The general reaction involves the removal of a water molecule between two carboxylic acid molecules, resulting in the formation of an anhydride. Anhydrides have the general structure (RCO)2O.

2. Esterification:

Esterification is a common reaction of carboxylic acids where they react with alcohols or phenols in the presence of a mineral acid catalyst, such as concentrated H2SO4 or HCl gas.
In this reaction, the -OH group of the carboxylic acid reacts with the -OH group of the alcohol or phenol, leading to the formation of an ester. The general reaction can be represented as: RCOOH + R’OH → RCOOR’ + H2O, where R and R’ represent alkyl or aryl groups.

3. Reactions with PCl5, PCl3, and SOCl2:

Carboxylic acids can undergo reactions with various phosphorus halides, such as PCl5 and PCl3, or thionyl chloride (SOCl2).
The hydroxyl (-OH) group of the carboxylic acid is replaced by a chlorine atom in these reactions. Thionyl chloride (SOCl2) is often preferred because the other two products (HCl or PCl3) are gaseous and can be easily removed, making the purification of the product more straightforward.

4. Reaction with Ammonia:

Carboxylic acids can react with ammonia (NH3) to form ammonium salts. When these salts are heated at high temperatures, they undergo further reactions to yield amides.
The general reaction with ammonia can be represented as: RCOOH + NH3 → RCOONH4.
Subsequent heating of the ammonium salt leads to the formation of amides: RCOONH4 → RCONH2 + H2O, where R represents an alkyl or aryl group.

3. Reactions Involving –COOH Group

Reactions involving the -COOH group in carboxylic acids can lead to various transformations. Here are two important reactions:

1. Reduction:

Carboxylic acids can be reduced to primary alcohols by specific reducing agents such as lithium aluminum hydride (LiAlH4) or diborane (B2H6).
Lithium aluminum hydride (LiAlH4) is a commonly used reagent for this purpose. It reduces the carboxyl group (-COOH) to a primary alcohol group (-CH2OH). The general reaction is: RCOOH + 4[H] → RCH2OH + H2O, where R represents an alkyl or aryl group.
Diborane (B2H6) is another reducing agent that can be used for the reduction of carboxylic acids to primary alcohols. Diborane is more selective and does not easily reduce other functional groups like esters, nitro groups, or halogens. Sodium borohydride (NaBH4), a milder reducing agent, is not effective in reducing the carboxyl group.

2. Decarboxylation:

Decarboxylation is a chemical reaction in which carboxylic acids lose carbon dioxide (CO2) to form hydrocarbons. This reaction is commonly carried out by heating the sodium salt of a carboxylic acid with sodalime (a mixture of sodium hydroxide, NaOH, and calcium oxide, CaO) in a specific ratio (usually 3:1 NaOH to CaO).
The general reaction is: RCOONa + NaOH + CaO → R-H (hydrocarbon) + Na2CO3 + CaCO3, where R represents an alkyl or aryl group.
Alkali metal salts of carboxylic acids can also undergo decarboxylation during the process of Kolbe electrolysis. In this reaction, the alkali metal salt of the carboxylic acid is electrolyzed in its aqueous solution, resulting in the formation of hydrocarbons with twice the number of carbon atoms as in the alkyl group of the acid. Kolbe electrolysis is a useful method for the synthesis of hydrocarbons from carboxylic acids.

4. Substitution Reactions in the Hydrocarbon Part

Substitution reactions in the hydrocarbon part of carboxylic acids can lead to various transformations. Here are two important types of substitution reactions:

1. Halogenation (Hell-Volhard-Zelinsky Reaction):

Carboxylic acids containing an α-hydrogen atom can be halogenated at the α-position by treating them with chlorine (Cl2) or bromine (Br2) in the presence of a small amount of red phosphorus (P4). This reaction is known as the Hell-Volhard-Zelinsky reaction.
The α-hydrogen atom in the carboxylic acid undergoes substitution with a halogen atom (Cl or Br), resulting in the formation of α-halocarboxylic acids.
The general reaction for the halogenation of carboxylic acids is: RCH2COOH + X2 (Cl2 or Br2) + P4 → RCHXCOOH + HX + P4, where R represents an alkyl or aryl group, and X represents the halogen atom.
This reaction is useful for the synthesis of α-halocarboxylic acids.

2. Ring Substitution (Aromatic Carboxylic Acids):

Aromatic carboxylic acids, which contain both an aromatic ring and a carboxyl group (-COOH), can undergo electrophilic aromatic substitution reactions.
In these reactions, the carboxyl group serves as a deactivating and meta-directing group. This means that the presence of the carboxyl group decreases the reactivity of the aromatic ring and directs the incoming substituents to the meta position relative to the carboxyl group.
Aromatic carboxylic acids do not undergo Friedel-Crafts reactions (such as Friedel-Crafts alkylation or acylation) because the carboxyl group deactivates the aromatic ring, and the Lewis acid catalyst, such as aluminum chloride (AlCl3), gets bonded to the carboxyl group, rendering it inactive.
Typical electrophilic substitution reactions on aromatic carboxylic acids include nitration, sulfonation, and halogenation, where the substituents are introduced at the meta position with respect to the carboxyl group.

These substitution reactions can lead to the modification of the hydrocarbon part of carboxylic acids, allowing the introduction of halogen atoms or other substituents into the molecule.

Uses of Carboxylic Acids

Carboxylic acids find various applications in different industries due to their diverse properties. Here are some common uses of carboxylic acids:

Formic Acid (Methanoic Acid):
- Used in the rubber industry for coagulating latex.
- Used in the textile industry for dyeing and finishing textiles.
- Employed in leather processing as a tanning agent.
- Used in electroplating processes as a reducing agent.
Acetic Acid (Ethanoic Acid):
- Widely used as a solvent in various industries, including chemical, pharmaceutical, and food.
- Commonly used as vinegar in the food industry.
- An essential component in the production of synthetic fibers like acetate and triacetate.
- Used in the manufacture of various chemicals and pharmaceuticals.
Adipic Acid (Hexanedioic Acid):
- Primarily used in the production of nylon-6,6, a type of synthetic polymer used in textiles and plastics.
- Used as a food additive and flavor enhancer.
Benzoic Acid and Its Esters:
- Esters of benzoic acid are frequently used in perfumery to create pleasant fragrances.
- Sodium benzoate, the sodium salt of benzoic acid, is a common food preservative used to extend the shelf life of various food and beverage products.
Higher Fatty Acids:
- Utilized in the soap and detergent industry for the production of soaps and cleaning products.
- Serve as raw materials for the synthesis of surfactants, which are essential components of detergents and cleaning agents.

Carboxylic acids and their derivatives play crucial roles in various chemical processes and industries, contributing to the development of a wide range of products and applications.