Carboxylic Acids and Its Derivatives

Carboxylic acids are organic compounds characterized by the presence of the carboxyl functional group (-COOH). This functional group consists of a carbonyl group (C=O) and a hydroxyl group (-OH) attached to the same carbon atom. The carbonyl group is double-bonded to an oxygen atom, and the hydroxyl group is single-bonded to the same carbon atom.

1.0Structure of Carboxylic Acid

Organic compounds that contain a –COOH group are referred to as having a carboxylic acid group. This functional group consists of a carbonyl (C=O) group and a hydroxyl (–OH) group:

2.0Classification of Carboxylic Acids

Carboxylic acids can be classified based on the number of carboxyl (-COOH) groups they contain. Here's a brief overview of the classification of mono-, di-, and tricarboxylic acids:

1. Monocarboxylic Acids:

• Monocarboxylic acids contain one carboxyl group (-COOH) per molecule.
• They are the most common type of carboxylic acids.

Examples include:

Formic acid (HCOOH)

Acetic acid (CH₃COOH)

2. Dicarboxylic Acids:

• Dicarboxylic acids contain two carboxyl groups (-COOH) per molecule.
• They are characterized by having two functional groups separated by a carbon  chain.

Examples include:

Oxalic acid (HOOC-COOH)

Malonic acid (HOOCCH₂COOH)

3. Tricarboxylic Acids:

• Tricarboxylic acids contain three carboxyl groups (-COOH) per molecule.
• They have three functional groups separated by carbon chains.

Examples include:

Citric acid (HOOCCH₂C(OH)(COOH)CH₂COOH)

Aconitic acid (HOOCC(CH=CHCOOH)COOH)

3.0Carboxylic Acid Derivatives

Acid derivatives are organic compounds derived from carboxylic acids by replacing the hydroxyl group (-OH) of the carboxyl group (-COOH) with another functional group. These derivatives retain some characteristics of carboxylic acids and can undergo similar chemical reactions. Common acid derivatives include:

1. Esters:

• Esters are derived from carboxylic acids by replacing the hydroxyl group (-OH)     with an alkoxy group (-OR).
• They have the general formula RCOOR'.
• Esters are commonly formed through the reaction of a carboxylic acid with an alcohol in the presence of an acid catalyst. 
• Examples include methyl acetate (CH₃COOCH₃), ethyl acetate (CH3COOCH2CH3), and benzyl acetate (C₆H₅COOCH₂CH₃).

2. Amides:

• Amides are derived from carboxylic acids by replacing the hydroxyl group    (-OH) with an amino group (-NH₂).
• They have the general formula RCONH₂.Amides are typically formed through the reaction of a carboxylic acid with ammonia or an amine.
• Examples include acetamide (CH₃CONH₂), benzamide (C₆H₅CONH₂), and N,N-dimethylacetamide (CH₃CON(CH₃)₂).

3. Acid Anhydrides:

• Acid anhydrides are formed by the removal of water from two carboxylic acid molecules.
• They have the general formula (RCO)₂O.
• Acid anhydrides are commonly formed by the reaction of a carboxylic acid with another carboxylic acid in the presence of a dehydration agent.
• Examples include acetic anhydride ((CH₃CO)₂O), phthalic anhydride (C₆H₄(CO)2O), and succinic anhydride ((CH₂CO)₂O).

4. Acyl Chlorides (Acid Chlorides):

• Acyl chlorides are derived from carboxylic acids by replacing the hydroxyl group (-OH) with a chlorine atom (-Cl).
• They have the general formula RCOCl.
• Acyl chlorides are typically formed by the reaction of a carboxylic acid with thionyl chloride (SOCl₂) or phosphorus pentachloride (PCl₅).
• Examples include acetyl chloride (CH₃COCl), benzoyl chloride (C₆H₅COCl), and oxalyl chloride (C₂Cl₂O₂).

Reactivity order of acid derivatives-

The reactivity order of acid derivatives generally follows this trend-

RCOCl > (RCO)₂O > RCOOR > RCONH₂

We will learn the physical and chemical properties of carboxylic acids and its derivatives.

4.0Physical Properties of Carboxylic Acids

5.0Chemical Reactions of Carboxylic Acids

1. Formation of Anhydrides- 

Carboxylic acids can react with another molecule of carboxylic acid to form an anhydride and a molecule of water. This reaction typically occurs under acidic or dehydrating conditions.

2. Esterification Reaction- 

Carboxylic acids react with alcohols or phenols in the presence of an acid catalyst (such as concentrated sulfuric acid or dry hydrogen chloride) to form esters and water. This method, named after Emil Fischer, is widely used in organic synthesis for the preparation of esters, which find applications in fragrance, flavor, and pharmaceutical industries.

3. Reaction with PCl₃, PCl₅, SOCl₂-

Carboxylic acids can undergo reactions with various reagents like PCl₃, PCl₅ and SOCl₂ to form acid chlorides, which are important intermediates in organic synthesis. Here's how these reactions proceed:

6.0Chemical Reactions of Acid Derivatives (RCOZ)-

Z may be OR, X, OCOR, NH₂

1. Nucleophilic substitution reactions:

        Nucleophilic substitution reactions (NSR) are characteristic reactions of acid derivatives. These reactions involve the substitution of a nucleophile for a leaving group on the carbonyl carbon of the acid derivative. The leaving group could be a halide ion, an alkoxide ion, or an amide ion, depending on the specific type of acid derivative.

Mechanism-

In this reaction Z is leaving the group and weak bases are good leaving groups.

Reactivity order: CH₃COCl > CH₃COOCOCH₃ > CH₃COOC₂H₅ > CH₃CONH₂

2. Reaction with water (Hydrolysis)-

Hydrolysis of acid derivatives typically yields carboxylic acids as one of the products. The specific conditions and reagents used for hydrolysis may vary depending on the type of acid derivative. 

Reaction with alcohol- 

When acid derivatives react with alcohols, they can undergo various types of reactions depending on the conditions and the specific type of acid derivative. The most common reaction is esterification, where an ester is formed.        

7.0General Preparation Method of Carboxylic Acids

1. By oxidation of primary alcohol with acidic KMnO₄ or acidic K₂Cr₂

The preparation of carboxylic acids through the oxidation of primary alcohols is a fundamental transformation in organic chemistry.

2. By oxidation of aldehydes  

The oxidation of aldehydes to form carboxylic acids is a straightforward and commonly employed method in organic synthesis. This reaction ensures that the carboxylic acid produced retains the same number of carbon atoms as the original aldehyde.