Hello Future Chemist! Understanding Alcohols

Welcome to the study of Alcohols! If you’ve successfully grasped Alkanes and Alkenes, you’re already halfway there. Alcohols are organic molecules where a hydrogen atom has been replaced by a special functional group containing oxygen. They are extremely important in everyday life—from the fuel in certain cars to the hand sanitiser you use every day.

Note: Alcohols are examined in Paper 2 (Chemistry only). We will break down the structure, reactions, and manufacture of alcohols step-by-step, making sure you understand the essential concepts needed for your exams!


Section 1: The Basics of Alcohols

1.1 What Defines an Alcohol? The Functional Group

In organic chemistry, a functional group is the atom or group of atoms that determines how the molecule will react.

  • The functional group for all alcohols is the hydroxyl group: -OH.
  • We can think of an alcohol as an alkane chain where one hydrogen atom has been replaced by this -OH group.

Key Term: The hydroxyl group (-OH) makes the molecule an alcohol.

General Formula

Since unbranched alcohols contain a saturated alkyl chain, the general formula is:

\(\text{C}_n\text{H}_{2n+1}\text{OH}\)

Quick Review: Alcohols form a homologous series containing the -OH functional group.

Section 2: Naming and Formulae of Alcohols

Naming alcohols follows standard organic nomenclature rules:

  1. Count the number of carbon atoms in the chain (meth-, eth-, prop-, but-).
  2. Replace the standard alkane ending -ane with the alcohol ending -ol.

For Edexcel IGCSE, you must know the names and formulae of the first four unbranched alcohols:

# Carbons (\(n\))Name (IUPAC)Molecular FormulaStructural Formula
1Methanol\(\text{CH}_4\text{O}\)\(\text{CH}_3\text{OH}\)
2Ethanol\(\text{C}_2\text{H}_6\text{O}\)\(\text{CH}_3\text{CH}_2\text{OH}\)
3Propan-1-ol (or propanol)\(\text{C}_3\text{H}_8\text{O}\)\(\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}\)
4Butan-1-ol (or butanol)\(\text{C}_4\text{H}_{10}\text{O}\)\(\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH}\)

Section 3: Oxidation Reactions of Ethanol

You must know three different ways that ethanol can be oxidised:

3.1 Method 1: Complete Combustion (Burning in Air or Oxygen)

Ethanol burns completely in a plentiful supply of oxygen, releasing heat energy in an exothermic reaction. The products are carbon dioxide and water:

\(\text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}\)

3.2 Method 2: Microbial Oxidation

When an open bottle of wine or beer is left exposed to the air, it slowly turns sour. This happens because microorganisms (bacteria) use oxygen from the air to oxidise ethanol into ethanoic acid (the acid in vinegar) and water:

\(\text{C}_2\text{H}_5\text{OH} + \text{O}_2 \rightarrow \text{CH}_3\text{COOH} + \text{H}_2\text{O}\)

3.3 Method 3: Heating with Potassium Dichromate(VI) in Dilute Sulfuric Acid

In the laboratory, ethanol can be oxidised chemically by heating it with an oxidising agent: potassium dichromate(VI) (\(\text{K}_2\text{Cr}_2\text{O}_7\)) in dilute sulfuric acid (\(\text{H}_2\text{SO}_4\)).

  • Product: Ethanol is oxidised to ethanoic acid (\(\text{CH}_3\text{COOH}\)).
  • Colour change: The mixture changes colour from orange to green as the dichromate(VI) ions are reduced.
  • Simplified equation using [O] as the oxidising agent:
    \(\text{CH}_3\text{CH}_2\text{OH} + 2[\text{O}] \rightarrow \text{CH}_3\text{COOH} + \text{H}_2\text{O}\)

Section 4: Manufacture of Ethanol

There are two industrial methods for manufacturing ethanol, each with specific conditions and reasons for those conditions:

4.1 Method 1: Fermentation of Glucose

  • Raw Material: Glucose (\(\text{C}_6\text{H}_{12}\text{O}_6\)) from renewable plant matter (e.g., sugar cane or corn).
  • Catalyst: Enzymes in yeast.
  • Temperature: About 30°C (an optimum temperature; if too cold, the reaction is too slow; if too hot, the yeast enzymes denature).
  • Conditions: Absence of air (anaerobic). If air is present, the yeast respires aerobically producing carbon dioxide and water instead of ethanol, or bacteria will oxidise the ethanol to ethanoic acid.

Equation:

\(\text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\text{yeast, } \sim 30^\circ\text{C}} 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2\)

4.2 Method 2: Hydration of Ethene with Steam

  • Raw Materials: Ethene (\(\text{C}_2\text{H}_4\), from cracking crude oil fractions) and steam (\(\text{H}_2\text{O}\)).
  • Catalyst: Phosphoric acid (\(\text{H}_3\text{PO}_4\)).
  • Temperature: About 300°C.
  • Pressure: 60–70 atm.

Equation:

\(\text{C}_2\text{H}_4 + \text{H}_2\text{O} \xrightarrow{\text{H}_3\text{PO}_4, \ 300^\circ\text{C}, \ 60\text{--}70\text{ atm}} \text{C}_2\text{H}_5\text{OH}\)

Comparison of the Two Methods
FeatureFermentationHydration of Ethene
Raw materialRenewable (sugar cane / crops)Non-renewable (crude oil)
Type of processBatch process (slow)Continuous process (fast)
PurityDilute aqueous solution (requires distillation)Pure ethanol
Reaction conditionsGentle (about 30°C, atmospheric pressure)High temperature (~300°C) and pressure (60–70 atm)

Summary of Key Facts

  • Functional Group: Hydroxyl group, -OH.
  • First 4 Alcohols: Methanol, ethanol, propan-1-ol, butan-1-ol.
  • Three oxidations of ethanol: Complete combustion (produces \(\text{CO}_2\) + \(\text{H}_2\text{O}\)), microbial oxidation (produces \(\text{CH}_3\text{COOH}\) + \(\text{H}_2\text{O}\)), and heating with acidified potassium dichromate(VI) (orange to green, produces \(\text{CH}_3\text{COOH}\)).
  • Fermentation: Glucose with yeast enzymes at ~30°C in absence of air.
  • Hydration of ethene: Ethene + steam over phosphoric acid at ~300°C and 60–70 atm.