Unit 4: Alcohols and Phenols: A Comprehensive Study Resource

Alcohols and phenols are organic compounds containing hydroxyl (-OH) functional groups. Alcohols have the hydroxyl group attached to an aliphatic carbon atom, while phenols have the hydroxyl group directly attached to an aromatic ring. These compounds are essential in organic chemistry due to their widespread applications and diverse reactivity.

Alcohols

Alcohols are characterized by the presence of a hydroxyl group (-OH) bonded to an sp³ hybridized carbon atom.

Preparation of 1°, 2°, and 3° Alcohols

Alcohols are classified as primary (1°), secondary (2°), or tertiary (3°) based on the number of alkyl groups attached to the carbon atom bearing the hydroxyl group.

1. Using Grignard Reagent

Grignard reagents (RMgX, where R is an alkyl or aryl group and X is a halogen) are powerful nucleophiles that react with carbonyl compounds to form alcohols.

• Formation of Primary (1°) Alcohols: Grignard reagent reacts with formaldehyde (methanal) followed by hydrolysis.

  RMgX + HCHO → R-CH₂-OMgX
  R-CH₂-OMgX + H₃O⁺ → R-CH₂-OH (1°) + Mg(OH)X

• Formation of Secondary (2°) Alcohols: Grignard reagent reacts with any aldehyde (except formaldehyde) followed by hydrolysis.

  RMgX + R'-CHO → RR'-CH-OMgX
  RR'-CH-OMgX + H₃O⁺ → RR'-CH-OH (2°) + Mg(OH)X

• Formation of Tertiary (3°) Alcohols: Grignard reagent reacts with a ketone followed by hydrolysis.

  RMgX + R'-CO-R'' → RR'R''-C-OMgX
  RR'R''-C-OMgX + H₃O⁺ → RR'R''-C-OH (3°) + Mg(OH)X

2. Ester Hydrolysis

Esters can be hydrolyzed to yield a carboxylic acid and an alcohol. This reaction can occur under acidic or basic conditions.

• Acidic Hydrolysis: Reversible, generally conducted by heating the ester with dilute mineral acid.

  R-COOR' + H₂O ⇌ R-COOH + R'-OH

• Basic Hydrolysis (Saponification): Irreversible, involves reaction with a strong base (e.g., NaOH), producing a carboxylate salt and an alcohol.

  R-COOR' + NaOH → R-COONa + R'-OH

3. Reduction of Aldehydes, Ketones, Carboxylic Acids, and Esters

Reducing agents convert carbonyl compounds into alcohols.

• Reduction of Aldehydes: Aldehydes reduce to primary (1°) alcohols. Common reducing agents include lithium aluminium hydride (LiAlH₄) or sodium borohydride (NaBH₄).

  R-CHO + [H] → R-CH₂-OH (1°)

• Reduction of Ketones: Ketones reduce to secondary (2°) alcohols. LiAlH₄ and NaBH₄ are frequently used.

  R-CO-R' + [H] → R-CH(OH)-R' (2°)

• Reduction of Carboxylic Acids: Carboxylic acids reduce to primary (1°) alcohols. LiAlH₄ is required as it is a stronger reducing agent than NaBH₄ for carboxylic acids. NaBH₄ is generally ineffective for carboxylic acid reduction.

  R-COOH + [H] (LiAlH₄) → R-CH₂-OH (1°)

• Reduction of Esters: Esters reduce to two alcohols: a primary (1°) alcohol derived from the acyl part and an alcohol from the alkoxy part. LiAlH₄ is typically used.

  R-COOR' + [H] (LiAlH₄) → R-CH₂-OH (1°) + R'-OH

Reactions of Alcohols

Alcohols participate in a variety of reactions, often involving the hydroxyl group.

1. Reaction with Sodium

Alcohols react with active metals like sodium to liberate hydrogen gas and form alkoxides. This reaction demonstrates the acidic nature of alcohols, albeit weaker than water.

2R-OH + 2Na → 2R-ONa (Sodium Alkoxide) + H₂

2. Reaction with HX (Lucas Test)

Alcohols react with hydrogen halides (HX) to form alkyl halides. The reactivity order is HI > HBr > HCl.

Lucas Test: This test distinguishes between 1°, 2°, and 3° alcohols using Lucas reagent (concentrated HCl and anhydrous ZnCl₂). The test observes the formation of an insoluble alkyl chloride, which appears as turbidity.R-OH + HCl