Carboxylic Acids & Derivatives

Introduction to Carboxylic Acids

• Definition: Carboxylic acids are compounds that contain the carboxyl functional group, -COOH.

• General Formula: The general formula of a carboxylic acid is $C<em>nH</em>{2n}COOH$, which can also be expressed as $RCOOH$ where R is an alkyl group.

• Alternative Name: In some regions, they may also be referred to as alkanoic acids.

• Nomenclature: The naming follows the structure "alkane + -oic acid" (e.g., propanoic acid).

  • Key Rule: The numbering of carbon atoms does not need to be specified for the carboxyl group since it is always located on the first carbon atom.

Properties of Carboxylic Acids

• Water Solubility: Carboxylic acids with fewer than six carbon atoms are typically water-soluble due to hydrogen bonding capabilities with water.

• Weak Acids: Carboxylic acids are classified as weak acids because they are only partially ionized in aqueous solution, resulting in low concentrations of hydronium ions (H₃O⁺) and carboxylate ions (RCOO⁻).

• Equilibrium State: The equilibrium of their dissociation lies towards the left:
  $RCOOH <br>ightleftharpoons RCOO^- + H^+$

• Reaction with Carbonates: Carboxylic acids react with sodium carbonate and sodium hydrogen carbonate, indicating their acidity:

  • $2RCOOH + Na<em>2CO</em>3 <br>ightarrow 2RCOO^- Na^+ + CO<em>2 + H</em>2O$

  • $RCOOH + NaHCO<em>3 ightarrow RCOO^- Na^+ + CO</em>2 + H_2O$

Esters

• Definition: Esters are derivatives of carboxylic acids containing the ester functional group, -COO-.

• Formation: They are produced through a condensation reaction between a carboxylic acid and an alcohol, typically in the presence of a strong acid catalyst (e.g., sulfuric or hydrochloric acid). This process is known as esterification, which involves the elimination of a small molecule, commonly water.

Nomenclature of Esters

• Naming Convention: An ester is named based on the parent carboxylic acid from which it is derived:

  • Remove the -oic acid suffix from the carboxylic acid and replace it with -oate.

  • Add the alkyl chain (from the alcohol) as the first word in the name (e.g., propanol becomes propyl).

  • Note: The naming can be counterintuitive as it is written backward compared to the structure.

Uses of Esters

• Fragrances and Flavourings: Esters are aromatic compounds often utilized in the fragrance and flavouring industries because they possess fruity and sweet scents, unlike the less pleasant odors of aldehydes and ketones.

  • Example: Ethyl methanoate is used as a raspberry flavoring.

  • Testing for Alcohols: The sweet smell from esters can indicate the presence of alcohols or carboxylic acids when warmed with a carboxylic acid in the presence of concentrated sulfuric acid, which can be confirmed by subsequent reactions with sodium carbonate to produce carbon dioxide.

• Plasticisers: Esters function as plasticisers that increase the flexibility of polymers. An example is Poly(chloroethene) (PVC), which is rigid unless modified with suitable esters, allowing it to become pliable and flexible.

  • Mechanism: The plasticiser intersperses between polymer chains, weakening intermolecular attractive forces, thus reducing rigidity.

• Solvents: Esters like ethyl ethanoate are favorable solvents due to their low toxicity, low volatility, and cost-effectiveness, ideal for applications in glues, perfumes, and nail products.

• Naturally Occurring Esters: Triglycerides, found in fats and oils, are tri-esters formed from glycerol and fatty acids. Their reaction with acid leads to the formation of the triglyceride, yielding three moles of water as a by-product.

Biodiesel

• Definition: Biodiesel is made from renewable resources such as vegetable oils instead of non-renewable petrochemicals.

• Production Process: The transesterification process converts triglycerides into fatty acid methyl esters (FAME) by reacting with methanol. This reaction is catalyzed by either an acid or an alkali, although alkaline catalysts (e.g., NaOH) are more common due to efficiency.

  • Yield: Optimal conditions can provide yields as high as 98%.

Ester Hydrolysis

• Definition: Hydrolysis is the reverse reaction of esterification. It breaks down esters into their original carboxylic acids and alcohols using either dilute acid or alkali under heat.

  • Acidic Hydrolysis: This process is reversible, establishing an equilibrium. For example:
    $RCOOR' + H_2O <br>ightleftharpoons RCOOH + R'OH$

  • Alkaline Hydrolysis: This is an irreversible reaction that proceeds to completion:
    $RCOOR' + NaOH <br>ightarrow RCOO^- Na^+ + R'OH$

• Worked Example: The hydrolysis of ethyl ethanoate with hot dilute sulfuric acid produces:
  $CH<em>3COOCH</em>2CH<em>3 + H</em>2O <br>ightleftharpoons CH<em>3COOH + CH</em>3CH_2OH$

• Saponification: This is the hydrolysis of triglycerides in the presence of alkaline substances, yielding soaps and glycerol during the process.

Acylation

• Definition: Acyl groups can be introduced into compounds utilizing acyl chlorides or acid anhydrides. Acyl chlorides, derived from carboxylic acids, have the -OH group replaced by a chlorine atom and are named with the suffix -oyl chloride.

• Reactivity: Acyl chlorides undergo nucleophilic addition-elimination reactions, which include:

  • Hydrolysis: Producing carboxylic acids and HCl.

  • Reactions with Alcohols: Yielding esters.

  • Reactions with Ammonia/Primary Amines: Producing amides.

Manufacture of Aspirin

• Acylation Applications: Used in pharmaceutical and textile manufacturing.

• Aspirin Production: Involves the acylation of salicylic acid with ethanoic anhydride:
  $C<em>7H</em>6O<em>3 + (C</em>2H<em>4O</em>3) <br>ightarrow C<em>9H</em>{10}O<em>4 + C</em>2H<em>4O</em>3$

• Preference for Acid Anhydrides: They are generally favored over acyl chlorides due to being cheaper, less reactive, and producing a less corrosive by-product (carboxylic acid instead of HCl).