ALCOHOL, PHENOL & ETHER

Unit Overview

• After studying this Unit, you will be able to:
  • Name alcohols, phenols, and ethers according to the IUPAC system of nomenclature.
  • Discuss the reactions involved in the preparation of alcohols from alkenes, aldehydes, ketones, and carboxylic acids.
  • Discuss the reactions involved in the preparation of phenols from haloarenes, benzene sulfonic acids, diazonium salts, and cumene.
  • Discuss the reactions for the preparation of ethers from alcohols and from alkyl halides and sodium alkoxides/aryloxides.
  • Correlate physical properties of alcohols, phenols, and ethers with their structures.
  • Discuss chemical reactions of the three classes of compounds based on their functional groups.

Importance of Alcohols, Phenols, and Ethers

• Alcohols, phenols, and ethers are crucial in various industries, forming the basis for detergents, antiseptics, and fragrances.
• The replacement of hydrogen atoms in hydrocarbons by -OH groups leads to different compounds with varied properties and applications.
  • Examples:
  • Ethanol (common spirit) for polishing furniture.
  • Hydroxyl-containing compounds in sugar, cotton, paper, etc.

Definitions and Classifications

• Alcohols: Compounds with one or more hydroxyl (-OH) groups attached to carbon atoms of an aliphatic system (e.g., $ext{CH}_3 ext{OH}$).
• Phenols: Compounds with -OH groups attached to carbon atoms of an aromatic system (e.g., $ext{C}<em>6 ext{H}</em>5 ext{OH}$).
• Ethers: Compounds formed by substituting a hydrogen atom in the hydroxyl group of an alcohol or phenol with an alkyl or aryl group (e.g., $ext{CH}<em>3 ext{OCH}</em>3$ - dimethyl ether).

Classification of Alcohols

• Monohydric Alcohols: Containing one hydroxyl group.
• Dihydric Alcohols: Containing two hydroxyl groups.
• Trihydric Alcohols: Containing three hydroxyl groups.
• Polyhydric Alcohols: Containing many hydroxyl groups.

Specific Alcohol Classifications

• Primary, Secondary, Tertiary Alcohols: Based on the hybridization of the carbon atom to which the -OH group is attached.
• Allylic Alcohols: Hydroxyl attached to an sp³ carbon adjacent to a carbon-carbon double bond (e.g., $ext{C}<em>2 ext{H}</em>5 ext{CHOH}$).
• Benzylic Alcohols: Hydroxyl attached to a carbon next to an aromatic ring.

Physical and Chemical Properties

• Alcohols and phenols’ properties are largely due to their hydroxyl group.
• Higher boiling points than hydrocarbons, ethers, and haloalkanes of similar molecular masses due to the existence of hydrogen bonding.
• Increased molecular size and branching results in decreased boiling points for alcohols.
• Alcohols and phenols are generally soluble in water, primarily due to hydrogen bonding capability, which decreases with larger hydrophobic groups.

Nomenclature

Alcohols

• Common name derived from alkyl group + 'alcohol'.
• IUPAC naming involves replacing the 'e' in alkanes with 'ol' while indicating substituent positions with numbers (e.g., $ext{HO-CH}<em>2 ext{-CH}</em>2 ext{-OH}$ -> ethan-1,2-diol).
  • Examples of IUPAC names of specific alcohols:
  • $ext{CH}_3 ext{OH}$ - Methyl alcohol (Methanol)
  • $ext{C}<em>3 ext{H}</em>7 ext{OH}$ - n-Propyl alcohol (Propan-1-ol)

Phenols

• Common and IUPAC names based on hydroxyl derivatives of benzene.
• Terms used: Ortho (1,2-disubstituted), meta (1,3-disubstituted), para (1,4-disubstituted).
  • Examples:
  • Phenol: C6H5OH
  • Ortho-Cresol: 2-Methylphenol

Ethers

• Common names derived from alkyl/aryl groups, listed in alphabetical order, add 'ether'.
• Examples of ethers:
  • $ext{CH}<em>3 ext{OCH}</em>3$ - Dimethyl ether (Methoxymethane)
  • $ext{C}<em>2 ext{H}</em>5 ext{OCH}<em>2 ext{CH}</em>2 ext{CH}_3$ - Methyl n-propyl ether.

Preparation of Alcohols

1. From Alkenes:

   • Acid-Catalyzed Hydration:
     • Alkenes react with water under acidic conditions following Markovnikov’s rule.
     • Mechanism:
     • Protonation of alkene ⇒ Carbocation formation ⇒ Nucleophilic attack by water ⇒ Deprotonation to yield alcohol.
   • Hydroboration-Oxidation:
     • Alkenes react with diborane, oxidized to alcohol by hydrogen peroxide in the presence of base.

2. From Carbonyl Compounds:

   • Reduction of Aldehydes and Ketones:
     • Catalytic reduction in presence of metals.
     • Aldehydes yield primary alcohols; ketones yield secondary alcohols.
   • Reduction of Carboxylic Acids and Esters:
     • Strong reducing agents like lithium aluminum hydride (LiAlH4) yield alcohols from carboxylic acids.

3. From Grignard Reagents:

   • Reaction with aldehydes or ketones produces alcohols after hydrolysis of adducts.

Preparation of Phenols

1. From Haloarenes:
   • Fusion with NaOH under high temperature and pressure, followed by acidification gives phenols.
2. From Benzene Sulfonic Acid:
   • Conversion to sodium phenoxide upon heating with NaOH, acidification yields phenol.
3. From Diazonium Salts:
   • Hydrolysis of diazonium salts under specific temperatures.
4. From Cumene:
   • Oxidation of cumene forms cumene hydroperoxide, which splits into phenol and acetone when treated with acid.

Chemical Reactions of Alcohols and Phenols

Acidity

• Reactions with metals yield alkoxides and hydrogen, illustrating acidic nature.
• Phenols are stronger acids than alcohols due to resonance stabilizing effect of the benzene ring.
  • Comparison of acidity: Phenol > Alcohols > Water.

Nucleophilic and Electrophilic Behavior

• Alcohols act as nucleophiles when the -OH bond breaks.
• Under acidic conditions, protonated alcohols can act as electrophiles.
  • Alcohols undergo nucleophilic substitution and dehydration reactions.
• Examination of specific alcohol reactions emphasizes their functional diversity and reactivity.

Ethers Reaction Characteristics

• Ethers undergo cleavage with hydrogen halides, yielding alkyl halides.
• Ethers demonstrate weaker reactivity overall compared to alcohols and phenols, revealing unique reaction mechanisms when subjected to various conditions.