2025_J2_H2Chemistry_HydroxyCompounds_Notes&Tutorial_Student

Introduction to Hydroxy Compounds

1. Alcohols

1.1 Introduction (Nomenclature / Structure)

  • General formula: C_nH_{2n+1}OH or C_nH_{2n+2}O.

  • Naming convention: Replace the ending ‘−e’ of the corresponding alkane with the suffix ‘−ol’.

  • Specify hydroxyl (–OH) location by inserting number before ‘−ol’.

  • Identify the longest chain containing the carbon with the –OH group when determining the parent alcohol.

  • Example: Hexan-2-ol has the -OH on the second carbon.

1.2 Physical Properties
1.2.1 Boiling Point

  • Alcohols have higher boiling points than alkanes due to intermolecular hydrogen bonding.

  • Increased molecular size correlates with increased boiling points (e.g., Methanol 64.5 °C, Ethanol 78.4 °C, Propan-1-ol 97.0 °C).

1.2.2 Solubility

  • Short-chain alcohols (C1 to C3) are miscible with water due to hydrogen bond formation.

  • As hydrocarbon chain length increases, miscibility decreases because the impact of hydrophobic alkyl chains overcomes hydrogen bonding.

1.2.3 Acidity

  • Alcohols can donate protons (acting as acids): ROH ⇌ RO− + H+.

  • Weaker acids than water due to destabilization of alkoxide ions (RO−) from charge intensification.

1.3 Laboratory Preparation

  • Alcohols can be synthesized from:

    1. Alkenes: Electrophilic addition of water (steam + acid catalysis).

    2. Halogenoalkanes: Nucleophilic substitution with aqueous NaOH.

    3. Carboxylic Acids: Reduction with LiAlH_4 or NaBH_4.

    4. Aldehydes: Reduction to alcohols.

    5. Ketones: Reduction to form secondary alcohols.

1.4 Reactions
1.4.1 Complete Combustion

  • General combustion equation: CxHyOH + (x+ y+1)/4 - 1/2 O2 → xCO2 + (y+1)/2 H2O.

1.4.2 Nucleophilic Substitution to Halogenoalkanes

  • Hydroxyl group in alcohols can be replaced by halides. Example: Reaction with HCl.

1.4.3 Redox Reaction with Sodium

  • Alcohol reacts with sodium to form alkoxide and hydrogen gas. Na + ROH → RO−Na+ + ½ H2.

1.4.4 Oxidation to Carbonyl Compounds and Carboxylic Acids

  • Primary alcohols oxidized to aldehydes or carboxylic acids. Secondary alcohols to ketones.

  • E.g., RCH2OH + [O] → RCHO (aldehyde); RCHO + [O] → RCO2H (carboxylic acid).

  • KMnO4 or K2Cr2O7 reagent used for oxidation.

1.4.5 Dehydration to Alkenes

  • Alcohol dehydration forms alkene: Alcohol → Alkene + H2O in presence of acid.

  • Apply Saytzeff's rule for product distribution.

1.4.6 Condensation to Esters

  • Alcohol + Carboxylic Acid → Ester + Water (with H2SO4 as catalyst).

1.4.7 Tri-Iodomethane (Iodoform) Formation

  • Alcohols with CH3CH(OH)– groups react with iodine to form structure CHI3.

2. Phenols

2.1 Introduction

  • Phenols are compounds with an -OH group directly attached to an aromatic ring.

2.2 Physical Properties
2.2.1 Appearance

  • Colorless, hygroscopic crystals with m.p. 42 °C.

2.2.2 Boiling Point

  • Higher boiling point compared to hydrocarbons due to strong intermolecular hydrogen bonding.

2.2.3 Solubility

  • Partially soluble in water; solubility increases with temperature.

2.2.4 Acidity

  • Relative acidities: Phenol > Water > Alcohol.

  • Stability of phenoxide ion (charge delocalization in the benzene ring) increases acidity.

2.3 Reactions
2.3.1 Hydroxy Group Reactions

  • Redox with sodium: Reacts with Na to yield hydrogen gas.

  • Acid-base reactions: Neutralization with sodium hydroxide; phenol reacts with bases, not with Na2CO3.

2.3.2 Electrophilic Substitution at Benzene Ring

  • Hydroxy group activates the benzene, directing substitution to 2 and 4 positions.

  • E.g., Nitration and Bromination occur more readily than with alkyl benzene.

2.3.3 Reaction with Neutral FeCl3(aq)

  • Produces a violet coloration, confirming phenol presence.

Summary

  • Candidates should understand the chemistry of alcohols and phenols, including nomenclature, physical properties, laboratory preparations, and characteristic reactions.