Alcohols, Phenols, and Ethers

Introduction

Alcohols, phenols, and ethers are important classes of organic compounds that find wide applications in industry and daily life. These compounds are formed by the substitution of hydrogen atoms in hydrocarbons with specific functional groups.

These compounds are the basic building blocks for detergents, antiseptics, and fragrances. Common examples include ethanol in spirits, phenol in disinfectants, and diethyl ether as a solvent.

Classification

Classification of Alcohols

Alcohols are classified based on:

1. Number of Hydroxyl Groups

• Monohydric: Contain one -OH group (e.g., CH₃CH₂OH)
• Dihydric: Contain two -OH groups (e.g., HO-CH₂-CH₂-OH)
• Trihydric: Contain three -OH groups (e.g., Glycerol)
• Polyhydric: Contain many -OH groups

2. Hybridization of Carbon Atom

• Compounds with C_sp³-OH bond:
  • Primary (1°): -OH attached to primary carbon (-CH₂-OH)
  • Secondary (2°): -OH attached to secondary carbon (>CH-OH)
  • Tertiary (3°): -OH attached to tertiary carbon (>C-OH)
  • Allylic alcohols: -OH on sp³ carbon adjacent to C=C
  • Benzylic alcohols: -OH on sp³ carbon next to aromatic ring
• Compounds with C_sp²-OH bond:
  • Vinylic alcohols: -OH on sp² carbon (vinylic carbon)

Classification of Phenols

• Monohydric phenols: Contain one -OH group (e.g., Phenol)
• Dihydric phenols: Contain two -OH groups (e.g., Catechol, Resorcinol, Hydroquinone)
• Trihydric phenols: Contain three -OH groups

Classification of Ethers

• Simple/Symmetrical ethers: Both alkyl/aryl groups are the same (e.g., C₂H₅OC₂H₅)
• Mixed/Unsymmetrical ethers: The two groups are different (e.g., C₂H₅OCH₃)

Nomenclature

Naming Alcohols

IUPAC System:

• Replace 'e' of the parent alkane with the suffix '-ol'
• Number the carbon chain to give the -OH group the lowest possible number
• For polyhydric alcohols, retain 'e' of alkane and add ending '-diol', '-triol', etc.
• Indicate positions of -OH groups with appropriate locants

Naming Phenols

The simplest hydroxy derivative of benzene is called phenol (both common and IUPAC name).

For substituted phenols, terms ortho (1,2-), meta (1,3-), and para (1,4-) are often used.

Naming Ethers

Common Names: Derived from names of alkyl/aryl groups in alphabetical order + 'ether'

IUPAC System: Named as alkoxy derivatives of parent hydrocarbon

Preparation Methods

Preparation of Alcohols

1. From Alkenes

• Acid-catalyzed hydration: Alkenes + water in presence of acid catalyst
  CH₃CH=CH₂ + H₂O ⇌ CH₃-CH(OH)-CH₃
  Follows Markovnikov's rule
• Hydroboration-oxidation: Anti-Markovnikov addition
  CH₃-CH=CH₂ + (BH₃)₂ → (CH₃CH₂CH₂)₃B → 3CH₃CH₂CH₂OH

2. From Carbonyl Compounds

• Reduction of aldehydes/ketones: Using catalytic hydrogenation, NaBH₄, or LiAlH₄
  RCHO + H₂ → RCH₂OH (Primary alcohol)
  RCOR' → RCH(OH)R' (Secondary alcohol)
• Reduction of carboxylic acids/esters: Using LiAlH₄ or catalytic hydrogenation

3. From Grignard Reagents

Reaction with carbonyl compounds followed by hydrolysis:

• Formaldehyde → Primary alcohol
• Other aldehydes → Secondary alcohol
• Ketones → Tertiary alcohol

Preparation of Phenols

• From haloarenes: Chlorobenzene + NaOH at high T & P
• From benzenesulphonic acid: Fusion with NaOH
• From diazonium salts: Hydrolysis with warm water
• From cumene (Industrial method): Air oxidation followed by acid cleavage

Preparation of Ethers

• Dehydration of alcohols: Using protic acids at 413 K
  2C₂H₅OH → C₂H₅OC₂H₅ + H₂O
• Williamson synthesis: Alkyl halide + sodium alkoxide/phenoxide
  R-X + R'-O⁻Na⁺ → R-O-R' + NaX

Physical Properties

Boiling Points

• Alcohols > Phenols > Ethers ≈ Hydrocarbons of comparable molecular masses
• This order is due to intermolecular hydrogen bonding in alcohols and phenols
• Boiling points increase with molecular mass (increased van der Waals forces)
• Branching decreases boiling points (decreased surface area)

Solubility

• Lower members of alcohols and phenols are soluble in water due to H-bonding with water molecules
• Solubility decreases with increase in size of hydrophobic alkyl/aryl groups
• Ethers have limited solubility in water due to their ability to form H-bonds with water

Acidity

• Phenols > Alcohols > Water in terms of acidity
• Alcohols are weaker acids than water
• Phenols are stronger acids due to resonance stabilization of phenoxide ion
• Acidity order in alcohols: Primary > Secondary > Tertiary

Chemical Reactions

Reactions of Alcohols and Phenols

A. Reactions Involving Cleavage of O-H Bond

1. Acidity and Reaction with Metals

• Alcohols and phenols react with active metals (Na, K, Al) to form alkoxides/phenoxides and H₂
• Phenols also react with NaOH to form phenoxides

2. Esterification

Reaction with carboxylic acids, acid chlorides, or acid anhydrides to form esters

B. Reactions Involving Cleavage of C-O Bond (Primarily Alcohols)

1. Reaction with Hydrogen Halides

Reactivity order: Tertiary > Secondary > Primary (basis of Lucas Test)

2. Dehydration

Forms alkenes in presence of acidic catalysts (H₂SO₄, H₃PO₄)

Ease of dehydration: Tertiary > Secondary > Primary

3. Oxidation

• Primary alcohols:
  • Mild oxidation (PCC) → Aldehydes
  • Strong oxidation (KMnO₄/K₂Cr₂O₇) → Carboxylic acids
• Secondary alcohols: Oxidation → Ketones
• Tertiary alcohols: Resistant to oxidation

Reactions of Phenols

1. Electrophilic Aromatic Substitution

The -OH group activates the ring and directs incoming groups to ortho and para positions

• Nitration:
  • With dilute HNO₃ → o- and p-nitrophenol
  • With concentrated HNO₃ → 2,4,6-trinitrophenol (picric acid)
• Halogenation:
  • With Br₂ in CS₂ at low T → o- and p-bromophenol
  • With Br₂ water → 2,4,6-tribromophenol

2. Kolbe's Reaction

Phenoxide ion + CO₂ → Salicylic acid (o-hydroxybenzoic acid)

3. Reimer-Tiemann Reaction

Phenol + CHCl₃ + NaOH → Salicylaldehyde (o-hydroxybenzaldehyde)

4. Reaction with Zinc Dust

Phenol → Benzene + ZnO

Reactions of Ethers

1. Cleavage of C-O Bond

Ethers are cleaved by concentrated HI or HBr at high temperature

Reactivity order: HI > HBr > HCl

2. Electrophilic Substitution

The alkoxy group (-OR) activates the aromatic ring and directs incoming groups to ortho and para positions

• Halogenation
• Friedel-Crafts reaction
• Nitration

Commercially Important Compounds

Methanol (CH₃OH)

• Also known as 'wood spirit'
• Produced by catalytic hydrogenation of CO
• Colorless liquid, boiling point: 337 K
• Highly poisonous - causes blindness or death if ingested
• Uses: Solvent in paints, varnishes; making formaldehyde

Ethanol (C₂H₅OH)

• Obtained by fermentation of sugars
• Colorless liquid, boiling point: 351 K
• Acts on central nervous system
• Uses: Solvent in paint industry; preparation of carbon compounds
• Denatured alcohol: Made unfit for drinking by adding CuSO₄ (color) and pyridine (foul smell)