Alcohols & Carboxylic Acids

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Quiz 2:
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Alcohols & Carboxylic Acids

IGCSE Chemistry

18.1 Alcohols

Alcohols are a homologous series of organic compounds containing the elements Carbon, Hydrogen, and Oxygen.

  • Functional Group: Hydroxyl group (-OH).
  • General Formula: CnH2n+1OH.
  • Molecular Shape: The bonding around the oxygen atom is bent (V-shaped), similar to water.

Example: Ethanol (C2H5OH)

H H | | H - C - C - O - H | | H H

Physical Properties

As the carbon chain length increases (Methanol → Butanol):

  1. Boiling Point: Increases.
  2. Solubility in Water: Decreases (Methanol is very soluble; Butanol is only slightly soluble).
  3. Volatility: Decreases (they evaporate less easily).

Manufacture of Ethanol

There are two primary methods to produce ethanol.

Method A: Fermentation

Biological process using yeast to break down glucose (sugar).

C6H12O6(aq) → 2C2H5OH(aq) + 2CO2(g)

Essential Conditions:

  • Temperature: 25°C – 35°C (Optimum for enzymes).
  • Anaerobic: Absence of oxygen. (If oxygen is present, bacteria oxidize ethanol into vinegar/acid).
  • Limit: Yeast dies at ~15% alcohol concentration.

Method B: Hydration of Ethene

Industrial chemical process using crude oil fractions.

C2H4(g) + H2O(g) ↔ C2H5OH(g)

Essential Conditions:

  • Catalyst: Phosphoric(IV) Acid (H3PO4).
  • Temperature: 300°C.
  • Pressure: 6000 kPa (60 atmospheres).

Comparison Table

Feature Fermentation Hydration
Resources Renewable (Plants) Non-renewable (Petroleum)
Speed Slow (Batch process) Fast (Continuous process)
Purity Impure (requires distillation) Pure Ethanol
Energy Low energy input High energy input

18.2 Uses & Reactions

Combustion

Alcohols burn in excess oxygen to produce carbon dioxide and water. The flame is clean and blue.

C2H5OH + 3O2 → 2CO2 + 3H2O

Uses

  • Solvent: Dissolves substances insoluble in water (e.g., perfumes, cosmetics, glues).
  • Fuel: Used in cars (e.g., in Brazil using sugarcane ethanol) and spirit burners. It is a cleaner fuel than petrol.

18.3 Carboxylic Acids

  • Functional Group: Carboxyl (-COOH).
  • General Formula: CnH2n+1COOH.
  • Acidity: They are weak acids because they only partially dissociate in water to release H+ ions.

Structure of Ethanoic Acid

H O | // H - C - C | \ H O - H

Chemical Reactions

They react typically like acids, but slower.

1. With Metals (MASH):

2CH3COOH + Mg → (CH3COO)2Mg + H2

Observation: Effervescence (hydrogen gas). Salt formed: Magnesium Ethanoate.

2. With Bases (Neutralisation):

CH3COOH + NaOH → CH3COONa + H2O

Salt formed: Sodium Ethanoate.

3. With Carbonates:

2CH3COOH + Na2CO3 → 2CH3COONa + CO2 + H2O

Observation: Vigorous fizzing (Carbon dioxide).

18.4 Making Ethanoic Acid

Ethanoic acid is produced by the oxidation of ethanol.

1. Chemical Oxidation (Lab Method)

Heating ethanol with acidified Potassium Manganate(VII).

  • Observation: Color change from Purple to Colourless.
  • Role: Ethanol is the reducing agent; KMnO4 is the oxidising agent.
CH3CH2OH + 2[O] → CH3COOH + H2O

Note: [O] represents oxygen from the oxidising agent.

2. Bacterial Oxidation (Natural Method)

Bacteria (Acetobacter) in the air react with ethanol to form ethanoic acid (vinegar).

18.5 Esters

Esters are organic compounds responsible for sweet, fruity smells and flavours.

  • Functional Group: Ester linkage (-COO-).
  • Formation: Reaction between a Carboxylic Acid and an Alcohol.

Esterification

Acid + Alcohol ↔ Ester + Water
  • Catalyst: Concentrated Sulfuric Acid (H2SO4).
  • Conditions: Heat (often reflux or water bath).

Naming Esters

The name has two parts:

  1. From Alcohol: Ends in -yl (e.g., Ethanol → Ethyl).
  2. From Acid: Ends in -oate (e.g., Ethanoic Acid → Ethanoate).

Example: Ethyl Ethanoate

H O H H | || | | H - C - C -- O -- C - C - H | | | H H H (Acid Part) (Alcohol Part)

Note: Water is eliminated (removed) when the bond forms.

© Study Guide Handout - Chapter 18

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