Alkanes

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Recall the reactions (reagents and conditions) by which alkanes can be produced:

    • (a) addition of hydrogen to an alkene in a hydrogenation reaction, H₂(g) and Pt/Ni catalyst and heat
    • (b) cracking of a longer chain alkane, heat with Al₂O₃

  2. Describe:

    • (a) the complete and incomplete combustion of alkanes
    • (b) the free-radical substitution of alkanes by Cl₂ or Br₂ in the presence of ultraviolet light, as exemplified by the reactions of ethane

  3. Describe the mechanism of free-radical substitution with reference to the initiation, propagation and termination steps

  4. Suggest how cracking can be used to obtain more useful alkanes and alkenes of lower Mr from heavier crude oil fractions

  5. Understand the general unreactivity of alkanes, including towards polar reagents in terms of the strength of the C–H bonds and their relative lack of polarity

  6. Recognise the environmental consequences of carbon monoxide, oxides of nitrogen and unburnt hydrocarbons arising from the combustion of alkanes in the internal combustion engine and of their catalytic removal

Main Content

https://cambridgeassistant.com

What Are Alkanes?

Alkanes are a type of hydrocarbon (compounds containing only carbon and hydrogen atoms). In alkanes, all the carbon atoms are joined together by single covalent bonds. Each carbon atom forms four single bonds arranged in a tetrahedral shape (like a pyramid with a triangular base) with bond angles of about 109.5°.

Alkanes are found naturally in crude oil and natural gas, making them extremely important as fuels and starting materials for the chemical industry.

1. How Alkanes Are Produced

Method 1: Hydrogenation of Alkenes

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond (C=C). We can convert alkenes into alkanes by adding hydrogen gas (H₂) to them. This process is called hydrogenation.

Reagents and Conditions:

  • Reagent: Hydrogen gas, H₂(g)
  • Catalyst: Platinum (Pt) or Nickel (Ni) — these metals speed up the reaction
  • Condition: Heat

The catalyst is finely divided (broken into very small pieces) to increase its surface area, which makes the reaction happen faster.

Example: Butene + Hydrogen → Butane

In this reaction, the double bond in butene breaks, and hydrogen atoms are added to form butane, which only has single bonds.

Real-world application: Hydrogenation is used to make margarine from vegetable oil. Vegetable oil contains many C=C double bonds. When partially hydrogenated, the oil becomes more solid at room temperature, turning into margarine.

Sign in to view full notes