Alcohols

1. Preparation of Alcohols

(a) Electrophilic Addition of Steam to an Alkene

Reagents: Steam (H₂O(g)), phosphoric acid (H₃PO₄) catalyst

Conditions: High temperature (300°C), high pressure (~60 atm)

Reaction: CH₂=CH₂+H₂O → CH₃CH₂OH

Ethene → Ethanol

(b) Cold, Dilute, Acidified KMnO₄ (Oxidation of Alkene to Diol)

• Reagents: Cold dilute KMnO₄ + H⁺
• Reaction: CH₂=CH₂ → HOCH₂CH₂OH
• Purple KMnO₄ decolourizes to form a diol.

(c) Substitution of Halogenoalkane

• Reagents: Aqueous NaOH
• Conditions: Heat under reflux
• Mechanism: Nucleophilic substitution R–X+OH− → R–OH+ X⁻
• (d) Reduction of Aldehyde or Ketone
• Reagents: NaBH₄ or LiAlH₄
• Aldehyde → Primary Alcohol RCHO+[H]→RCH2OH
• Ketone → Secondary Alcohol RCOR′+[H]→RCH(OH)R′

(e) Reduction of Carboxylic Acid

• Reagent: LiAlH₄ in dry ether
• Reaction: RCOOH+4[H]→RCH2OH+H2O (Only LiAlH₄ strong enough to reduce carboxylic acids)

(f) Hydrolysis of Esters

• Reagents:
  • Dilute acid (HCl or H₂SO₄) or dilute alkali (NaOH)
  • Conditions: Heat under reflux
• Acidic hydrolysis (reversible): RCOOR′+H2O⇌RCOOH+R′OH
• Alkaline hydrolysis (irreversible): RCOOR′+OH−→RCOO−+R′OH

2. Reactions of Alcohols

(a) Combustion

• Complete combustion forms CO₂ and H₂O C2H5OH+3O2→2CO2+3H2O

(b) Substitution to Form Halogenoalkanes

• With HX (e.g., HCl(g)): ROH+HX→RX+H2O
• With KCl + conc. H₂SO₄ / H₃PO₄ (Lucas reagent):
  • Secondary and tertiary alcohols react faster.
• With PCl₅: ROH+PCl5→RCl+POCl3+HCl l
• With PCl₃: 3ROH+PCl3→3RCl+H3PO3 ​
• With SOCl₂ (thionyl chloride): ROH+SOCl2→RCl+SO2+HCl (Cleanest method – gaseous byproducts)

(c) Reaction with Sodium

• Forms alkoxide and hydrogen gas 2ROH+2Na→2RONa+H2 ​

(d) Oxidation with Acidified K₂Cr₂O₇ or KMnO₄

• Primary alcohols:
  • Gentle heat/distillation → Aldehyde RCH2OH→RCHO+H2O
  • Reflux → Carboxylic acid RCH2OH→RCOOH+H2O
• Secondary alcohols:
  • Reflux → Ketone R2CHOH→R2CO+H2O
• Tertiary alcohols: No reaction

(e) Dehydration to Alkenes

• Reagents: Heated Al₂O₃ or conc. H₂SO₄ or H₃PO₄
• Conditions: Heat
• Reaction: CH3CH2OH→ CH2=CH2+H2O

(f) Esterification

• Reagents: Carboxylic acid + conc. H₂SO₄
• Reaction: RCOOH+R′OH⇌RCOOR′+H2O
• Example: CH3COOH+C2H5OH⇌CH3COOC2H5+H2O

3. Classification of Alcohols

(a) Primary, Secondary, Tertiary

• Primary (1°): –OH on C with one R group
  e.g., ethanol (CH₃CH₂OH)
• Secondary (2°): –OH on C with two R groups
  e.g., propan-2-ol (CH₃CHOHCH₃)
• Tertiary (3°): –OH on C with three R groups
  e.g., 2-methylpropan-2-ol

(b) Distinguishing by Oxidation

• Add acidified K₂Cr₂O₇
  • Primary: orange → green (forms aldehyde/acid)
  • Secondary: orange → green (forms ketone)
  • Tertiary: no colour change

4. Triiodomethane (Iodoform) Reaction

• Alcohols with a CH₃CH(OH)– group (i.e. ethanol and similar) give a yellow ppt of CHI₃
• Reagent: Alkaline iodine (I₂/NaOH)
• Test Reaction: CH3CH(OH)R+3I2+4NaOH→RCOONa+CHI3+3NaI+3H2O
• Positive test: Yellow precipitate of triiodomethane (iodoform) with antiseptic smell.

5. Acidity of Alcohols

• Alcohols are weak acids, weaker than water.
• Order of acidity:
  Water > Alcohols > Alkynes > Alkanes
• Alcohols ionize slightly in water: ROH⇌RO−+H+
• Less acidic than water due to electron-donating alkyl group stabilizing the molecule, making proton loss harder.

Summary Table: Reactions of Alcohols

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