Chemistry - 4CH1 PearsonEdexcel

Alkenes

Overview

Alkenes are the reactive cousins of alkanes. A single structural difference - a carbon-carbon double bond - transforms their chemistry completely. Where alkanes are sluggish, alkenes snap up incoming atoms in addition reactions, making them the starting materials for plastics, alcohols, and countless industrial chemicals.

In this lesson you will learn the general formula for alkenes, how to draw and name them, why the double bond makes them unsaturated, and how they react with bromine. You will also discover the bromine water test, a simple way to tell an alkene from an alkane in the laboratory.

Objectives

know that alkenes contain the functional group >C=C<
know the general formula for alkenes
explain why alkenes are classified as unsaturated hydrocarbons
understand how to draw the structural and displayed formulae for alkenes with up to four carbon atoms in the molecule, and name the unbranched-chain isomers knowledge of cis/trans or E/Z notation is not required
describe the reactions of alkenes with bromine to produce dibromoalkanes
describe how bromine water can be used to distinguish between an alkane and an alkene

Lesson Note

Replace just one single bond between two carbon atoms with a double bond and the entire character of the molecule changes. That double bond is a region of high electron density, making alkenes far more reactive than their alkane counterparts. This reactivity is why the petrochemical industry cracks alkanes to produce alkenes, and why alkenes are the building blocks for polymers like polyethene.

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Lesson Evaluation

Congratulations on completing the lesson on Alkenes. Now that youve explored the key concepts and ideas, its time to put your knowledge to the test. This section offers a variety of practice questions designed to reinforce your understanding and help you gauge your grasp of the material.

You will encounter a mix of question types, including multiple-choice questions, short answer questions, and essay questions. Each question is thoughtfully crafted to assess different aspects of your knowledge and critical thinking skills.

Use this evaluation section as an opportunity to reinforce your understanding of the topic and to identify any areas where you may need additional study. Don't be discouraged by any challenges you encounter; instead, view them as opportunities for growth and improvement.

What is the general formula for alkenes? A) CnH2n+2 B) CnH2n C) CnH2n-2 D) CnH2n+1OH Answer: B
Which of the following is the functional group of alkenes? A) -OH B) -COOH C) C=C D) C-C Answer: C
When bromine water is added to an alkene, the solution: A) Turns blue B) Stays orange C) Is decolourised D) Turns green Answer: C
The reaction of ethene with bromine is an example of: A) Substitution B) Decomposition C) Neutralisation D) Addition Answer: D
Why is there no alkene with only one carbon atom? A) One carbon cannot form four bonds B) A C=C double bond needs at least two carbon atoms C) Single-carbon molecules are always gases D) Methene is too unstable to exist Answer: B