Chemical Reactions and Commercial Importance

• Chemical Reactions of Alcohols: Versatile compounds, reacting both as nucleophiles and electrophiles.
  
  
  1. As Nucleophiles: O-H bond breaks. \(\mathrm{(e.g.,\ R{-}O{-}H^+ + C \rightarrow R{-}O{-}C)}\).
  2. As Electrophiles: C-O bond breaks. Protonated alcohols react this way \(\mathrm{(e.g.,\ R{-}CH_2{-}O^+ + H_2)}\).
• Reactions involving cleavage of O-H bond:
  1. Acidity of Alcohols and Phenols:
     • (i) Reaction with Metals: Alcohols and phenols react with active metals (Na, K, Al) to yield alkoxides/phenoxides and hydrogen. Phenols also react with aqueous NaOH to form sodium phenoxides. These reactions show acidic nature.
     • Alcohols and phenols are Brönsted acids (proton donors).
     • (ii) Acidity of Alcohols: Due to polar O-H bond. Electron-releasing groups (e.g., alkyl groups) increase electron density on oxygen, decreasing O-H polarity, thus decreasing acid strength.
       
       
       • Acid strength order: Primary > Secondary > Tertiary.
       • Alcohols are weaker acids than water.\(\mathrm{(e.g.,\ R{-}O^- + H_2O \rightarrow R{-}OH + OH^-)}\). Alkoxide ions are stronger bases than hydroxide ions.
       • Alcohols also act as Brönsted bases (proton acceptors) due to unshared electron pairs on oxygen.
     • (iii) Acidity of Phenols: Reactions with metals and NaOH show acidic nature.
       
       
       • -OH group directly attached to sp2 hybridised carbon (electron withdrawing) of benzene ring.
       • Charge distribution in resonance structures causes oxygen to be positive.
       • Phenols are stronger acids than alcohols and water.
       • Higher electronegativity of sp2 carbon decreases electron density on oxygen, increasing O-H polarity and ionization.
       • Stability of Phenoxide Ion: In phenoxide ion, negative charge is delocalized over the ring (resonance structures I-V), making it more stable and favoring ionization of phenol. Phenol molecule itself is less stable than phenoxide ion due to charge separation in its resonance structures.
       • Effect of Substituents:
         • Electron withdrawing groups (e.g., nitro group) enhance acidic strength, especially at ortho and para positions (due to effective delocalization). (Table 7.3: p-Nitrophenol pKa 7.1, o-Nitrophenol pKa 7.2).
         • Electron releasing groups (e.g., alkyl groups) decrease acid strength. Cresols are less acidic than phenol. (Table 7.3: Phenol pKa 10.0, Cresols pKa 10.1-10.2).
       • Phenol is a million times more acidic than ethanol (pKa 10.0 vs 15.9).
  2. Esterification: Alcohols and phenols react with carboxylic acids, acid chlorides, and acid anhydrides to form esters.
     
     
     • With carboxylic acid and acid anhydride: Carried out in presence of concentrated H2​SO4​. Reversible, so water removed as formed.
     • With acid chloride: Carried out in presence of a base (pyridine) to neutralize HCl, shifting equilibrium to right.
     • Acetylation: Introduction of acetyl \(\mathrm{(CH_3CO)}\) group. Acetylation of salicylic acid produces aspirin (analgesic, anti-inflammatory, antipyretic).
• Reactions involving cleavage of Carbon-Oxygen (C-O) bond in alcohols: (Phenols only react with zinc).
  
  
  1. Reaction with Hydrogen Halides: Alcohols react to form alkyl halides. \(\mathrm{ROH + HX \rightarrow RX + H_2O}\).
     • Lucas Test: Distinguishes 1°, 2°, 3° alcohols using Lucas reagent \(\mathrm{(conc.\ HCl + ZnCl_2)}\). Halides are immiscible and cause turbidity. Tertiary alcohols produce turbidity immediately. Primary alcohols do not at room temperature.
  2. Reaction with Phosphorus Trihalides: Alcohols converted to alkyl bromides by PBr3​.
  3. Dehydration: Alcohols remove water to form alkenes with protic acids \(\mathrm{(H_2SO_4,\ H_3PO_4)}\) or catalysts \(\mathrm{(anhydrous\ ZnCl_2,\ alumina)}\).
     
     
     • Example: Ethanol with conc. \(\mathrm{H_2SO_4}\)​ at 443 K yields ethene.
     • Secondary and tertiary alcohols dehydrate under milder conditions.
     • Ease of dehydration: Tertiary > Secondary > Primary (Tertiary carbocations are more stable, easier to form).
     • Mechanism (for ethanol): Step 1 (Fast): Protonation of alcohol to form protonated alcohol (ethyl oxonium ion). Step 2 (Slow, rate-determining): Formation of carbocation and water molecule. Step 3 (Fast): Elimination of proton to form ethene.
  4. Oxidation: Formation of C=O double bond with cleavage of O-H and C-H bonds. Also called dehydrogenation.
     
     
     • Primary alcohol → Aldehyde (mild oxidizer) → Carboxylic Acid (strong oxidizer).
     • Strong Oxidizing Agents: Acidified \(\mathrm{KMnO_4}\)​ for direct conversion to carboxylic acids.
     • Mild Oxidizing Agents for Aldehydes: \(\mathrm{CrO_3}\)​ in anhydrous medium; Pyridinium chlorochromate (PCC, complex of \(\mathrm{CrO_3}\)​ with pyridine and HCl) is better.
     • Secondary Alcohols: Oxidized to ketones by \(\mathrm{CrO_3}\).
     • Tertiary Alcohols: Do not undergo oxidation. Strong conditions \(\mathrm{KMnO_4}\), high temp) cause C-C bond cleavage, yielding mixture of carboxylic acids with fewer carbons.
     • Catalytic Dehydrogenation (over heated copper at 573 K): Primary alcohols → aldehyde; Secondary alcohols → ketone; Tertiary alcohols → alkene (dehydration).
     • Biological Oxidation (Methanol Poisoning): Methanol oxidized to methanal, then methanoic acid (causes blindness, death). Treated with intravenous diluted ethanol (ethanol competes for enzyme, allowing methanol excretion).
• Reactions of Phenols (only on aromatic ring):
  1. Electrophilic Aromatic Substitution: -OH is activating and ortho-para directing due to resonance effect.
     
     
     • (i) Nitration:
       • Dilute HNO3​ (298 K): Mixture of ortho- and para-nitrophenols.
       • Isomers separable by steam distillation: o-nitrophenol (steam volatile, intramolecular H-bonding); p-nitrophenol (less volatile, intermolecular H-bonding).
       • Conc. HNO3​: Converts to 2,4,6-trinitrophenol (picric acid), poor yield. Now prepared from phenol-2,4-disulphonic acid (first from conc. H2​SO4​). Picric acid is a strong acid.
     • (ii) Halogenation (Bromination):
       • Low polarity solvents (CHCl3​,CS2​) at low temp (273 K): Monobromophenols (minor ortho, major para).
       • Phenol polarizes bromine even without Lewis acid (FeBr3​) due to highly activating -OH group.
       • Bromine water: Forms 2,4,6-tribromophenol as white precipitate.
  2. Kolbe’s Reaction: Phenoxide ion (from phenol + NaOH) is highly reactive electrophilic substitution with \(CO_2\)​ (weak electrophile). Ortho-hydroxybenzoic acid (salicylic acid) formed.
  3. Reimer-Tiemann Reaction: Phenol with chloroform \(\mathrm{(CHCl_3)}\) in aqueous \(\mathrm{NaOH}\) introduces -CHO group at ortho position, forming salicylaldehyde via an intermediate substituted benzal chloride.
  4. Reaction with Zinc Dust: Phenol converted to benzene on heating with zinc dust.
  5. Oxidation: Phenol with chromic acid \(\mathrm{(Na_2Cr_2O_7\ /\ H_2SO_4)}\) produces conjugated diketone (benzoquinone). Slowly oxidized by air to dark quinones.
• Commercially Important Alcohols:
  
  
  1. Methanol (CH3​OH): ‘Wood spirit’.
     • Historically: destructive distillation of wood.
     • Today: Catalytic hydrogenation of carbon monoxide \(\mathrm{(CO + 2H_2)}\) at high pressure (200-300 atm) and temp (573-673 K) with \(\mathrm{ZnO{-}Cr_2O_3}\)​ catalyst.
     • Properties: Colorless liquid, b.p. 337 K. Highly poisonous. Small quantities cause blindness, large quantities death.
     • Uses: Solvent (paints, varnishes), making formaldehyde.
  2. Ethanol (C2​H5​OH):
     • Commercially: Fermentation of sugars (oldest method).
       
       
       • Sugar (molasses, sugarcane, grapes) invertase​ Glucose + Fructose.
       • Glucose/Fructose zymase (from yeast)​ \(\mathrm{2C_2H_5OH + 2CO_2}\)​.
       • Wine making: Grapes provide sugar and yeast. Fermentation in anaerobic conditions (absence of air), releases \(CO_2\)​.
       • Zymase inhibited if alcohol % exceeds 14%.
       • Air in fermentation mixture oxidizes ethanol to ethanoic acid (destroys taste).
     • Properties: Colorless liquid, b.p. 351 K.
     • Uses: Solvent (paint industry), preparation of carbon compounds.
     • Denaturation: Commercial alcohol made unfit for drinking by mixing copper sulfate (color) and pyridine (foul smell).
     • Nowadays: Large quantities from hydration of ethene.

Physiological Effects: Acts on CNS. Moderate amounts affect judgment/lower inhibitions. Higher concentrations cause nausea, loss of consciousness, can interfere with respiration and be fatal.