Study Notes on Carboxylic Acids

Classification and Nomenclature

• Carboxylic Acids: Organic compounds containing the $-\text{COOH}$ functional group.
• Aliphatic Carboxylic Acids: Fatty acids where $-\text{COOH}$ is attached to an alkyl group.
  • Formic acid ($HCOOH$): Methanoic acid.
  • Acetic acid ($CH_3COOH$): Ethanoic acid.
  • Oxalic acid ($(COOH)_2$): Ethanedioic acid.
• Aromatic Carboxylic Acids: Where $-\text{COOH}$ is attached to an aryl group.
  • Benzoic acid (C_6_H_5COOH): Benzene carboxylic acid.

Preparation of Carboxylic Acids

• Oxidation of Primary Alcohols:
  • $CH_3OH + [O] \xrightarrow{Aci. K_2Cr_2O_7} HCHO \xrightarrow{[O]} HCOOH$
  • $C_2H_5OH + [O] \xrightarrow{Aci. K_2Cr_2O_7} CH_3CHO \xrightarrow{[O]} CH_3COOH$
• Oxidation of Cane Sugar:
  • $C_{12}H_{22}O_{11} + 18[O] \xrightarrow{Conc. HNO_3 + V_2O_5} 6(COOH)_2 + 5H_2O$
  • Process: $80\,gm$ sugar and $150\,ml$ $conc.\,HNO_3$; crystals obtained as $(COOH)_2 \cdot 2H_2O$.
• Oxidation of Benzyl Alcohol:
  • $C_6H_5CH_2OH + [O] \xrightarrow{Alk. KMnO_4} C_6H_5CHO \xrightarrow{[O]} C_6H_5COOH$

Physical Properties

• State and Odour:
  • $C_1$–$C_3$: Colourless liquids with sharp irritating smells.
  • $C_4$–$C_9$: Oily liquids; Butyric acid ($C_4$) smells like rancid butter.
  • $C_{10}+$: Waxy solids.
• Solubility:
  • Soluble up to $C_4$. Solubility decreases as the hydrophobic alkyl chain length increases.
  • Ability to form intermolecular hydrogen bonds with water through both carbonyl oxygen ($C=O$) and hydroxylic hydrogen ($O-H$).
• Melting Points:
  • Even number carbon chains have higher melting points than adjacent odd-numbered chains because they fit better into the crystal lattice.
• Boiling Points:
  • Higher than alcohols and hydrocarbons of comparable mass (e.g., Acetic acid at $391\,K$ vs. n-propyl alcohol at $370\,K$).
  • Attraction occurs via strongly polarized $O-H$ bonds and the formation of cyclic dimers.

Chemical Properties and Neutralization

• Acidity: Carboxylic acids react with bases to form salts and water.
  • $HCOOH + NaOH \rightarrow HCOONa + H_2O$
  • $(COOH)_2 + 2NaOH \rightarrow (COONa)_2 + 2H_2O$
• Carbonate/Bicarbonate Reaction: Standard test for the carboxyl group; evolves $CO_2$ gas.
  • $2CH_3COOH + Na_2CO_3 \rightarrow 2CH_3COONa + H_2O + CO_2\uparrow$
• Esterification: Reaction with alcohols and $conc.\,H_2SO_4$ to produce esters with fruity smells.
  • $CH_3COOH + C_2H_5OH \rightarrow CH_3COOC_2H_5 + H_2O$

Specific Acid Tests and Uses

• Formic Acid ($HCOOH$):
  • Uses: Reducing agent, coagulating agent for rubber, antiseptic.
  • Tests: Tollen’s reagent (silver mirror) and Fehling’s solution (red $Cu_2O$ precipitate).
• Acetic Acid ($CH_3COOH$):
  • Uses: Vinegar, solvent for resins, rubber coagulation.
  • Tests: Neutral $FeCl_3$ produces a wine red colour.
• Oxalic Acid ($(COOH)_2$):
  • Uses: Ink stain removal, metal polishes, volumetric analysis.
  • Tests: Decolourises acidified $KMnO_4$. Reacts with $CaCl_2$ to form white precipitate ($CaC_2O_4$). Decomposes with $conc.\,H_2SO_4$ to give $CO$ gas (blue flame).
• Benzoic Acid ($C_6H_5COOH$):
  • Uses: Dyes (aniline blue), medicine, food preservative (sodium benzoate).
  • Tests: Gives a buff coloured precipitate with neutral $FeCl_3$. Heating with soda lime produces benzene vapours (sooty flame).