4.4 Alkenes

REVISION NOTES

IGCSE Edexcel Chemistry

4.4.1 Know that alkenes contain the functional group >C=C<

Unlike alkanes, alkenes contain the functional group C=C

4.4.2 Know the general formula for alkenes

General formula for alkenes

C_nH_2n

EXAMPLE

Ethene is a type of alkene with the formula C₂H₄

4.4.3 Explain why alkenes are classified as unsaturated hydrocarbons

ALKENES

Alkenes contain one or more C=C bonds
It means that alkenes can still break the double bond and attach to more atoms
So it is classified as unsaturated hydrocarbons

4.4.4 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

The numbers in but-1-ene, but-2-ene refer to the carbon atom the C=C is in

Numbers are not needed in naming ethene and propene, as the C=C is in the first position when counting from the nearest direction

Methene does not exist as a C=C cannot exist with only 1 carbon

4.4.5 Describe the reactions of alkenes with bromine to produce dibromoalkanes

Reaction of Alkenes with Halogens

alkene + bromine → dibromoalkane

As alkenes are unsaturated, they undergo addition reaction

C=C double bond is removed
Each carbon attaches to one bromine atom

Br₂is a diatomic molecule:

Br₂is brown in colour
After reaction, Br2 is reacted
Thus the reaction changes colour from brown to colourless

ethene + bromine → 1,2-dibromoethane
CH₂CH₂ + Br₂ → CH₂BrCH₂Br

The numbers represent the carbon atom that the bromine atoms are attached to.

4.4.6 describe how bromine water can be used to distinguish between an alkane and an alkene

Bromine Water Test

Alkanes react with bromine water ONLY when there is UV (light)
Alkenes react with bromine water even without light
Therefore, to distinguish between an alkane and alkene, add bromine water to the sample in a dark room
If decolourisation happens, it means there is reaction happening, which implies it is an alkene