The Organic Chemistry Map: Converting Homologous Series

By Dr. Aisha Rahman, Ph.D. Chemistry·Updated April 18, 2026
A whiteboard schematic showing the chemical conversion pathways between alkanes, alkenes, alcohols, and carboxylic acids.

What is the chemical test for an alkene?

Add aqueous Bromine Water (which is orange/brown). If the substance is an alkene, it undergoes an addition reaction across the carbon double-bond, causing the bromine water to turn colorless immediately. Alkanes will NOT decolorize bromine water (unless under UV light over a long period).

Table of Contents

Organic chemistry is essentially a map. CAIE examiners will give you a starting molecule (like Ethene) and ask you to state the reagents and conditions to reach a final destination (like Ethanoic Acid). This guide from our Ultimate O-Level Chemistry Guide provides the exact 'highway routes' you need to memorize to navigate Paper 2.

1. Alkene Addition Reactions

Alkenes are 'unsaturated' (they have a C=C double bond). Because of this double bond, they are incredibly reactive. The double bond easily breaks open to "add" new atoms.

Alkene → Alkane (Hydrogenation)

Add Hydrogen gas (H2).
Conditions: 200°C and a Nickel Catalyst.
Industrial Use: Margarine production (turning liquid unsaturated vegetable oils into solid saturated fats).

Alkene → Alcohol (Hydration)

Add Steam (H2O gas).
Conditions: 300°C, 60 atmospheres of pressure, and a Phosphoric(V) acid catalyst.
Equation: C2H4 + H2O → C2H5OH

💡 Tutor's Tip
Never write "Water" when describing hydration of an alkene. The reaction takes place at 300°C, so water is physically impossible. You MUST write "Steam".

2. Creating and Oxidizing Alcohols

As seen above, alcohols can be made by hydrating alkenes. The alternative method is Fermentation of glucose (using yeast, in the absence of oxygen, at 30°C).

Alcohol → Carboxylic Acid (Oxidation)

If you expose an alcohol to the air for a long time, microbes oxidize it into a carboxylic acid (which is why old wine tastes like vinegar). To do this instantly in a lab:

Reagents: Acidified Potassium Manganate(VII) (KMnO4) or Acidified Potassium Dichromate(VI). Heat under reflux.
Color Change: If using Manganate(VII), the solution turns from Purple to Colorless to indicate the alcohol has been successfully oxidized into a carboxylic acid.

3. Esterification: The Sweet Smelling Trap

If you react an Alcohol with a Carboxylic Acid, you create an Ester. Esters are volatile, sweet-smelling liquids used in perfumes and artificial food flavorings.

Conditions and Naming

  • Catalyst: Concentrated Sulfuric Acid is required.
  • Reaction Type: It is a condensation reaction because it releases a small molecule of water as a by-product.
  • Naming Rule: The first part of the name comes from the alcohol (ends in -yl). The second part comes from the acid (ends in -oate).
  • Example: Ethanol + Propanoic Acid → Ethyl Propanoate + Water.

Frequently Asked Questions

How do you convert an alkene to an alkane?▼
Hydrogenation: React with H2 gas at 200°C using a Nickel catalyst.
How do you convert an alkene to an alcohol?▼
Hydration: React with steam at 300°C, 60 atm, using a Phosphoric acid catalyst.
How do you convert an alcohol to a carboxylic acid?▼
Oxidation: Heat with an oxidizing agent like acidified Potassium Manganate(VII).
How do you make an ester?▼
Esterification: React an alcohol with a carboxylic acid using a concentrated Sulfuric acid catalyst.

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