ALKENES AT A GLANCE

Introduction

Alkenes are a very important family of hydrocarbons. Unlike alkanes, which contain only single carbon–carbon bonds, alkenes contain at least one carbon–carbon double bond (C=C).

Because of this double bond, alkenes are described as unsaturated hydrocarbons and they are generally more reactive than alkanes.

The simplest and most important member of the series is ethene.

What are Alkenes?

Alkenes are unsaturated hydrocarbons containing at least one carbon–carbon double bond (C=C).

For open-chain alkenes containing one double bond, their general formula is:

CₙH₂ₙ

Some examples are:

i. Ethene — C₂H₄

ii. Propene — C₃H₆

iii. Butene — C₄H₈

iv Pentene — C₅H₁₀

Remember

Alkanes → single bond → saturated

Alkenes → double bond → unsaturated

Physical Properties

Most lower members of the alkene series are:

i. Colourless gases.

ii. Slightly soluble or insoluble in water.

iii. Soluble in organic solvents.

iv. Flammable.

v. Less dense than water.

Vi. Their boiling points increases generally with increase in the number of carbon atom.

Chemical Properties

The presence of the C=C double bond makes alkenes quite reactive.

Their important reactions include:

1. Addition of Hydrogen-: Alkenes react with hydrogen in the presence of a suitable catalyst such as nickel.

CH₂=CH₂ + H₂ → CH₃–CH₃

Ethene produces ethane.

This reaction is called hydrogenation.

2. Addition of Bromine-: Alkenes react readily with bromine.

CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br

One important use of this reaction is to test for unsaturation.

Test for Unsaturation

When an alkene is shaken with bromine water, the reddish/brown colour of bromine water disappears

This is a common test for a carbon–carbon double bond.

3. Addition of Steam

An alkene can react with steam to form an alcohol.

For ethene:

CH₂=CH₂ + H₂O → CH₃CH₂OH

Ethene produces ethanol.

4. Combustion

Alkenes burn in oxygen.

For complete combustion of ethene:

C₂H₄ + 3O₂ → 2CO₂ + 2H₂O

Carbon dioxide and water are produced.

5. Polymerization-: This is the linking of smaller molecules called monomers to form a large molecules called polymers.

Many alkene molecules can join together to form a very large molecule called a polymer.

For example:

nCH₂=CH₂ → (–CH₂–CH₂–)ₙ

Ethene produces poly(ethene), a widely used plastic.

ETHENE

Ethene is the simplest member of the alkene homologous series. It has molecular formula C₂H₄ and a Structural formula: CH₂=CH₂

Ethene has two carbon atoms joined by a double bond, with each carbon atom bonded to two hydrogen atoms.

The carbon atoms in ethene are sp² hybridized.

Preparation of Ethene

LABORATORY PREPARATION OF ETHENE

Ethene can be prepared in the laboratory by removing components of water from ethanol (C2H5OH) using concentrated H2SO4. This process is known as dehydration

DIAG.

Laboratory preparation of Ethene

equation for the reaction-: the reaction is a 2- stage reaction

stage i: C2H5OH + H2SO4 → C2H5HSO4 + H2O.

stage ii. C2H5HSO4 →C2H4 + H2SO4

When ethylhydrogentetraoxosulphate (VI) is heated, it releases ethene which is collected over water.

Ethene can also be produced by passing ethanol vapour over heated aluminium oxide under suitable conditions.

Physical Properties

Ethene is:

i.A colourless gas.

ii. Slightly soluble in water.

iii. Highly flammable.

iv Less dense than air

Chemical Properties

Ethene readily undergoes addition reactions because of its C=C double bond.

Addition Reaction

i. With Hydrogen ( hydrogenation reaction)

CH₂=CH₂ + H₂ → CH₃CH₃

Ethane

ii. With Bromine (bromination)

CH₂=CH₂ + Br₂ → CH₂BrCH₂Br

1,2-dibromoethane

With Steam

CH₂=CH₂ + H₂O → CH₃CH₂OH

Ethanol

Polymerization

nCH₂=CH₂ → (–CH₂–CH₂–)ₙ

Product: Poly(ethene)

Uses of Ethene

Ethene is an important raw material in the chemical industry. It is used:

To manufacture poly(ethene) and other plastics.
To produce ethanol.
In the manufacture of other useful organic chemicals.
As a starting material for producing several industrial chemicals.
To promote the ripening of fruits under controlled conditions.

Alkenes and Alkanes: Quick Comparison

Feature	Alkanes	Alkenes
Type	Saturated	Unsaturated
Carbon-carbon bond	C–C	C=C
General formula	CₙH₂ₙ₊₂	CₙH₂ₙ
Reactivity	Generally lower	Generally higher
Main reaction	Substitution	Addition
Bromine water	No rapid reaction	Decolourises
Example	Ethane	Ethene

Summary

Definition: Unsaturated hydrocarbons containing at least one C=C double bond.

General formula: CₙH₂ₙ

Functional group: C=C

First member: Ethene

Formula of ethene: C₂H₄

Structure: CH₂=CH₂

Laboratory preparation: Dehydration of ethanol.

Important test: Decolourises bromine water.

Major reactions: Addition, combustion, oxidation and polymerization.

Important product from ethene: Poly(ethene).

Important use of ethene: Manufacture of plastics and production of other useful chemicals.

Objective Questions

Choose the correct answer from options A–D.

Alkenes are hydrocarbons that contain at least one
A. C–C single bond
B. C=C double bond
C. C≡C triple bond
D. C–H double bond
The general molecular formula of alkenes is
A. CₙH₂ₙ₊₂
B. CₙH₂ₙ
C. CₙH₂ₙ₋₂
D. CₙHₙ
The simplest member of the alkene homologous series is
A. Methene
B. Ethene
C. Propene
D. Butene
The molecular formula of ethene is
A. CH₄
B. C₂H₄
C. C₂H₆
D. C₃H₆
The functional group present in alkenes is
A. –OH
B. –COOH
C. C=C
D. –CHO
Which of the following is an alkene?
A. C₂H₆
B. C₃H₈
C. C₃H₆
D. C₄H₁₀
Ethene can be prepared in the laboratory by dehydrating
A. methane
B. ethanol
C. ethanoic acid
D. ethane
The reagent commonly used to test for the presence of an alkene is
A. limewater
B. acidified potassium manganate(VII)
C. bromine water
D. sodium hydroxide
When ethene is passed through bromine water, the bromine water
A. turns blue
B. becomes colourless
C. produces a white precipitate
D. turns green
The reaction between ethene and hydrogen produces
A. ethanol
B. ethyne
C. ethane
D. ethanoic acid
The addition of hydrogen to ethene is known as
A. hydration
B. hydrogenation
C. hydrolysis
D. oxidation
Ethene reacts with steam in the presence of a suitable catalyst to form
A. ethanol
B. ethane
C. ethyne
D. ethanoic acid
The polymer formed from ethene is
A. polyethene
B. PVC
C. nylon
D. starch
Which of the following is a major use of ethene?
A. Manufacture of polyethene
B. Manufacture of sodium chloride
C. Production of glass only
D. Manufacture of ammonia only
The type of reaction commonly associated with alkenes is
A. substitution
B. addition
C. neutralization
D. precipitation

Theory Questions

(a) Define an alkene.
(b) State the general molecular formula of alkenes.
(c) Give the names and molecular formulae of the first four members of the alkene homologous series.
(a) Describe the laboratory preparation of ethene from ethanol.
(b) Write the chemical equation for the reaction.
(c) State one condition necessary for the reaction.
(a) State four physical properties of ethene.
(b) State three chemical properties of ethene and give equations for any two.
(a) What is an addition reaction?
(b) Explain what happens when ethene reacts with:
i. hydrogen
ii. bromine
iii. steam
Write the appropriate equations.
(a) What is polymerization?
(b) Explain how ethene forms polyethene.
(c) State three uses of polyethene.

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Friday, 10 October 2025