Chemistry · Year 12 · Core Organic Chemistry · Spring Term

Alkanes: Structure, Properties, and Reactions

Examining the saturated hydrocarbons, their combustion, and the mechanism of free radical substitution.

• National Curriculum Attainment Targets
• A-Level: Chemistry - Alkanes
• A-Level: Chemistry - Free Radical Substitution

About This Topic

Alcohols and haloalkanes are 'functionalised' organic compounds, where a halogen or an oxygen-containing group replaces a hydrogen atom. This change in structure introduces polarity and new types of reactivity, specifically nucleophilic substitution and elimination. These reactions are the workhorses of synthetic organic chemistry, used to create everything from pharmaceuticals to perfumes.

In the UK curriculum, students must understand the mechanism of nucleophilic substitution (SN1 and SN2) and how the choice of solvent and conditions can favour elimination over substitution. They also explore the oxidation of alcohols to form aldehydes, ketones, and carboxylic acids, which is a fundamental practical skill. This topic requires a high level of precision in drawing curly arrows and identifying the 'leaving group'.

Students grasp these concepts faster through 'mechanism mapping' and collaborative lab work where they can observe the different rates of reaction for primary, secondary, and tertiary haloalkanes.

Key Questions

1. Explain how the mechanism of radical substitution accounts for the formation of multiple products.
2. Justify why alkanes are generally unreactive compared to other functional groups.
3. Analyze the environmental consequences of incomplete combustion of hydrocarbons.

Learning Objectives

• Explain the process of free radical substitution, including initiation, propagation, and termination steps.
• Analyze the factors influencing the formation of multiple products during alkane halogenation.
• Justify the general unreactivity of alkanes based on their bond types and molecular structure.
• Calculate the energy released during the complete combustion of specific alkanes.
• Critique the environmental impact of incomplete hydrocarbon combustion, referencing specific pollutants.

Before You Start

Atomic Structure and Bonding

Why: Students need a solid understanding of electron configuration, covalent bonds, and electronegativity to comprehend bond breaking and radical formation.

Introduction to Organic Chemistry: Hydrocarbons

Why: Familiarity with the basic structure, naming, and properties of alkanes is essential before exploring their reactions.

Key Vocabulary

Free Radical	An atom or molecule with an unpaired electron, making it highly reactive.
Homolytic Fission	The symmetrical breaking of a covalent bond, where each atom retains one of the bonding electrons, forming free radicals.
Combustion	A rapid chemical reaction between a substance and an oxidant, usually oxygen, to produce heat and light; for alkanes, this can be complete or incomplete.
Incomplete Combustion	Combustion that occurs with insufficient oxygen, producing carbon monoxide and/or soot (carbon) in addition to carbon dioxide and water.
Propagation Step	A step in a free radical mechanism where a radical reacts with a molecule to form a new radical and a stable molecule, continuing the chain reaction.

Watch Out for These Misconceptions

Common Misconception

The C-F bond is the most reactive because it is the most polar.

What to Teach Instead

While C-F is the most polar, it is also the strongest bond (highest bond enthalpy). Reactivity in haloalkanes is actually dominated by bond enthalpy, making C-I the most reactive. A 'data-mining' activity comparing bond energies and reaction rates helps correct this.

Common Misconception

All alcohols can be oxidised to carboxylic acids.

What to Teach Instead

Only primary alcohols can be oxidised to carboxylic acids (via an aldehyde). Secondary alcohols only go as far as ketones, and tertiary alcohols don't oxidise at all. Using molecular models to show the 'missing' hydrogen on the carbon atom helps students see why.

Active Learning Ideas

Inquiry Circle: The Haloalkane Hydrolysis Race

Groups test the rate of hydrolysis for 1-chlorobutane, 1-bromobutane, and 1-iodobutane. They use their results to determine whether bond polarity or bond enthalpy is the more important factor in determining reactivity.

45 min·Small Groups

Think-Pair-Share: Substitution vs Elimination

Students are given a set of reaction conditions (e.g., 'hot, ethanolic KOH' vs 'warm, aqueous KOH'). They must work in pairs to predict whether substitution or elimination will occur and draw the resulting products.

25 min·Pairs

Peer Teaching: Alcohol Oxidation Flowcharts

Students create a flowchart showing the oxidation of primary, secondary, and tertiary alcohols. They must explain to a partner why tertiary alcohols cannot be oxidised using standard reagents like acidified potassium dichromate.

30 min·Pairs

Real-World Connections

• Petroleum engineers and chemists in the oil and gas industry analyze the combustion properties of different alkane fractions (like gasoline and diesel) to optimize engine performance and fuel efficiency.
• Environmental scientists monitor air quality in urban centers like London, analyzing the levels of pollutants such as carbon monoxide and particulate matter resulting from incomplete combustion of vehicle fuels.

Assessment Ideas

Quick Check

Present students with a partially completed free radical substitution mechanism for methane and chlorine. Ask them to identify the missing species and draw the curly arrows for the propagation step.

Discussion Prompt

Pose the question: 'Why are alkanes often called 'paraffin' or 'lazy' fuels?' Facilitate a discussion comparing the bond strength and polarity of alkanes to other functional groups students may have encountered, guiding them to explain alkane unreactivity.

Exit Ticket

Ask students to write down two products that can form when propane reacts with excess bromine under UV light, and one reason why incomplete combustion of natural gas is a concern for air quality.

Frequently Asked Questions

What is a nucleophile and how does it react with haloalkanes?

A nucleophile is a 'nucleus-loving' species that has a lone pair of electrons it can donate to form a new covalent bond. In a haloalkane, the carbon-halogen bond is polar, leaving the carbon with a partial positive charge. The nucleophile attacks this delta-plus carbon, displacing the halogen atom as a halide ion.

How do you distinguish between primary, secondary, and tertiary alcohols?

This is based on how many alkyl groups are attached to the carbon atom that is bonded to the -OH group. Primary has one, secondary has two, and tertiary has three. This classification is vital because it determines how the alcohol will react when heated with an oxidising agent.

How can active learning help students master organic synthesis?

Synthesis is like a game of 'chemical chess'. Active learning strategies like 'synthesis relays' or 'retro-synthesis puzzles' encourage students to think backwards from a product to a starting material. By collaborating on these puzzles, students share their knowledge of different reagents and conditions, making the vast array of organic reactions more manageable.

Why are CFCs dangerous to the ozone layer?

Chlorofluorocarbons (CFCs) are haloalkanes that are very stable in the lower atmosphere. However, in the upper atmosphere, UV light causes them to break down, releasing chlorine radicals. These radicals act as catalysts in the destruction of ozone, leading to the 'ozone hole' and increased UV radiation on Earth.