Organic Chemistry: Carboxylic Acids and Derivatives

This document provides detailed notes on carboxylic acids and their derivatives, covering their properties and key reactions, as outlined in the Cambridge A-Level Chemistry 9701 syllabus.

Carboxylic Acids

Properties

Carboxylic acids are organic compounds containing the carboxyl group (-COOH). They are generally weak acids, less acidic than mineral acids like HCl and H₂SO₄. This is due to the resonance stabilization of the carboxylate anion after deprotonation.

Physical Properties:

• Typically liquids or solids at room temperature, depending on the length of the alkyl chain.
• Saturated with water, forming acidic solutions.
• Have a characteristic pungent odor (especially short-chain acids).
• Boiling points are higher than alcohols of similar molecular weight due to hydrogen bonding.

Chemical Properties:

• Undergo typical acid reactions: react with bases to form salts, react with metals to produce hydrogen gas.
• Their acidity is influenced by inductive and resonance effects of the alkyl group attached to the carboxyl group. Electron-withdrawing groups increase acidity, while electron-donating groups decrease it.

Acidity

The acidity of carboxylic acids is quantified by their pK_a values. A lower pK_a indicates a stronger acid.

$$pK_a \approx 4 - 5$$

The acidity arises from the ability of the carboxylate anion to delocalize the negative charge over the two oxygen atoms, making it more stable than the protonated acid.

Reactions of Carboxylic Acids

1. Neutralization Reactions: React with bases (e.g., NaOH, KOH) to form carboxylate salts and water.

$$RCOOH(aq) + NaOH(aq) \rightarrow RCOONa(aq) + H_2O(l)$$

2. Esterification: React with alcohols in the presence of an acid catalyst (e.g., concentrated sulfuric acid) to form esters and water. This is a reversible reaction.

$$RCOOH(aq) + R'OH(aq) \xrightarrow{H_2SO_4} RCOOR'(aq) + H_2O(l)$$

3. Amide Formation: React with ammonia (NH₃) or amines to form amides and water. This reaction typically requires heating.

$$RCOOH(aq) + NH_3(g) \xrightarrow{\Delta} RCONH_2(aq) + H_2O(l)$$

4. Reduction: Can be reduced to primary alcohols using strong reducing agents like lithium aluminium hydride (LiAlH₄).

$$RCOOH \xrightarrow{LiAlH_4} RCH_2OH$$

Carboxylic Acid Derivatives

Esters

Esters are formed by the reaction of a carboxylic acid with an alcohol. They have a characteristic pleasant odor and are used as solvents and flavoring agents.

Properties:

• Generally colorless liquids with fruity odors.
• Relatively insoluble in water but soluble in organic solvents.
• Undergo hydrolysis (saponification) in the presence of a base.

Reactions:

• Hydrolysis (Saponification): Reaction with a base (e.g., NaOH) to form a carboxylate salt and an alcohol. This is used in the production of soaps.

$$RCOOR'(aq) + NaOH(aq) \xrightarrow{\Delta} RCOONa(aq) + R'OH(aq)$$

• Reduction: Can be reduced to primary alcohols using strong reducing agents like lithium aluminium hydride (LiAlH₄).

$$RCOOR' \xrightarrow{LiAlH_4} RCH_2R'$$

Amides

Amides are formed by the reaction of a carboxylic acid with ammonia or an amine. They are generally more stable than esters and have different properties.

Properties:

• Solid at room temperature.
• Less reactive than esters.
• Form hydrogen bonds, leading to higher melting points.

Reactions:

• Hydrolysis: Can be hydrolyzed back to a carboxylic acid and an amine under acidic or basic conditions.
• Reduction: Difficult to reduce to alcohols under mild conditions.

Acid Chlorides

Acid chlorides are formed by the reaction of a carboxylic acid with a chlorinating agent like thionyl chloride (SOCl₂) or phosphorus pentachloride (PCl₅).

Properties:

• Highly reactive compounds.
• React readily with water, alcohols, and amines.

Reactions:

• Hydrolysis: Reacts rapidly with water to form a carboxylic acid and hydrochloric acid.

$$RCOCl(aq) + H_2O(l) \rightarrow RCOOH(aq) + HCl(aq)$$

• Alcoholysis: Reacts with alcohols to form esters.

$$RCOCl(aq) + R'OH(aq) \rightarrow RCOOR'(aq) + HCl(aq)$$

• Amination: Reacts with amines to form amides.

$$RCOCl(aq) + R'NH_2(aq) \rightarrow RCONHR'(aq) + HCl(aq)$$

Derivative	Reaction with Water (Hydrolysis)	Reaction with Alcohol	Reaction with Amine
Ester	Slow (requires acid or base catalyst)	Forms ester (requires acid catalyst)	Forms amide (requires heat)
Amide	Requires acid or base catalyst	No reaction	No reaction
Acid Chloride	Reacts readily to form carboxylic acid and HCl	Forms ester	Forms amide