Functional derivatives of carboxylic acids

Closely related to carboxylic acids and to each other are a number of chemical families known as functional derivatives of carboxylic acids: Acid halide, anhydride, amide, and esters. These derivatives are compounds in which the OH of the carboxylic acid has been replaced by X(Cl, Br, I), OOCR, NH_2, or OR.

Esters

They have a general formula

Examples

HCO₂CH₃                              Methylmethanoate

CH₃COOCH₂CH₃                 Ethylethanoate

CH₃CH₂COOCH₂CH₃         Ethylpropanoate             

Physical properties

• Insoluble in water but soluble in organic solvents
• They are neutral liquids with a pleasant smell
• They have low boiling points compared to carboxylic acids of comparable molecular mass.

Preparation

(a) By reacting carboxylic acids with alcohol in the presence of mineral acids

Mechanism

(b) The reaction of alcohols with acid halides

 

Chemical properties

1. Hydrolysis

Esters are hydrolyzed by mineral acids or alkalis

Example

(ii) base catalyzed

2. Esters are reduced by LiAlH₄ to alcohols

Examples

3. Esters react with amides to form acid amide

Uses of esters

In formation of perfumes

Acid halides

These have a general formula

Examples

C₆H₅COCl             Benzoyl chloride               

CH₃COBr              Ethanoylbromide

CH₃CH₂COI          Propanoyliodide

Preparation

By reacting carboxylic acid with phosphorus halide or thionyl chloride

Examples

CH₃COOH + PBr₃          →                       CH₃COBr

CH₃CH₂CO₂H + SOCl₂         →                CH₃CH₂COCl

Chemical properties

1. Acid halides are hydrolyzed by water to carboxylic acids

When ethanoyl chloride is exposed to moist air white fumes are seen due to the production of HCl.

i.e. CH₃COCl + H₂O        →    CH₃COOH + HCl

2. Acid halides react with ammonia and amines to produce amides

Example

3. Acid halide reacts with alcohols to form esters

Example

Mechanism

4. Acid halides react with anhydrous sodium salt of carboxylic acid to form acid anhydride

Example

Acid amide

These have a general formula

Examples

CH₃CONH₂                           Ethanamide

CH₃CH₂CONH₂                   Propanamide

Physical properties

• They are white crystalline salts except methanamide
• Lower members are soluble in water but the solubility decreases with increasing molecular mass.
• They have relatively high boiling points due to formation of intermolecular hydrogen bonds

Preparation

1. By the reaction of ammonia with ester, acid anhydride or acid halides

Examples

2. Dehydration of carboxylic acids with ammonium carbonate

Chemical properties

1. They are weaker bases compared to amines and are neutral to litmus. The do not form salts with acids as amines do because their lone pair of electron on the nitrogen atom is delocalized.

2. Amides form nitriles when distilled over phosphorus pentoxide, P₄O₁₀.

3. Amides react with hot alkaline bromine solution to give amines

4. Like amines, amides react with nitrous acid liberating nitrogen

5. Amides are hydrolyzed with mineral acid or alkais

Example

Mechanism

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