Alcohols

Introduction

Understanding the chemistry of alcohols is essential in various scientific disciplines due to their numerous applications and the unique characteristics imparted by their hydroxyl (-OH) functional group.

• Significance: Alcohols are indispensable in industries such as cosmetics, medicine, and fuel due to their solvent abilities and antiseptic qualities.
• General Formula: R-OH, where R denotes an alkyl group (e.g., methyl (CH₃) or ethyl (C₂H₅)).

Structural Formulae of Alcohols

Alcohols are classified based on the carbon atom associated with the -OH group:

• Primary: Bound to one other carbon atom.
• Secondary: Bound to two other carbon atoms.
• Tertiary: Bound to three other carbon atoms.

Primary Alcohols

Examples: Methanol (CH₃OH), Ethanol (C₂H₅OH)

Secondary Alcohols

Example: Isopropanol [(CH₃)₂CHOH]

Tertiary Alcohols

Example: Tert-Butanol [(CH₃)₃COH]

Properties of Alcohols

Boiling Points and Solubility

• Hydrogen Bonding: Alcohols exhibit higher boiling points than hydrocarbons, as shown in the table below:

  

• Solubility: Decreases as the chain length increases, due to a reduction in polar interactions with water.

Impact of Chain Length

• Boiling Points: Increase with chain length due to stronger van der Waals forces.

  

Comparison of Alcohol Types

• Primary Alcohols: Exhibit higher boiling points due to less branching.
• Tertiary Alcohols: Display the lowest boiling point due to increased branching.

Reactions of Alcohols

Alcohols are defined by their reactive hydroxyl (OH) group.

Combustion Reactions

• Complete Combustion: Results in carbon dioxide and water, releasing energy.
  • Methanol example: $2\mathrm{CH}_3\mathrm{OH} + 3\mathrm{O}_2 \rightarrow 2\mathrm{CO}_2 + 4\mathrm{H}_2\mathrm{O}$
• Incomplete Combustion: Produces carbon monoxide (CO) and soot, posing risks such as pollution.

Dehydration Reactions

• Convert alcohols to alkenes by removing water, using acid catalysts.

Substitution Reactions

• React with Hydrohalic Acids (HX): Forming alkyl halides, the reactivity order is tertiary > secondary > primary.

Oxidation Reactions

• Primary Alcohols: Convert to aldehydes or carboxylic acids with oxidising agents.
• Secondary Alcohols: Convert to ketones, not further oxidisable.
• Tertiary Alcohols: Resistant to oxidation.

Alcohol Production Processes

Substitution Reactions

• Halogens are replaced by hydroxyl groups with the use of catalysts.

Fermentation

• Biological conversion of sugars into ethanol.
  • Example: Glucose ($\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6$) to ethanol ($\mathrm{C}_2\mathrm{H}_5\mathrm{OH}$)
  • Typical conditions: Temperature 30-35°C, pH 4-5

Enthalpy of Combustion

Definition: The energy change when one mole of alcohol fully combusts.

• Calorimetry: This technique measures energy changes, crucial for assessing fuel efficiency.

Sample Calculations

• Formula: $\Delta H_c = -\frac{q}{n}$ (where q is the heat absorbed, n is moles burnt)

Worked Example:
If burning 0.05 moles of ethanol releases 150 kJ of heat, what is the enthalpy of combustion?

Solution:
$\Delta H_c = -\frac{q}{n} = -\frac{150 \text{ kJ}}{0.05 \text{ mol}} = -3000 \text{ kJ/mol}$

The enthalpy of combustion for ethanol is -3000 kJ/mol.

Safety, Environmental Impact, and Industrial Significance

• Role in Modern Industry: Used extensively as solvents, fuels, and in sanitation.

Overview of Oxidation in Organic Chemistry

Oxidation: Plays a central role in transforming alcohols into more complex compounds, which are crucial for the chemical industry.

• Alcohol Conversion: Primary alcohols convert into aldehydes/carboxylic acids, secondary into ketones, while tertiary show resistance to oxidation.

By understanding alcohols and their diverse reactions, students gain valuable insights into an essential class of compounds that significantly impact both scientific fields and industrial processes.