Chemistry

4. Organic Chemistry
4.1 Introduction>4.2 Crude Oil>4.3 Alkanes>4.4 Alkenes>4.5 Alcohols>4.6 Carboxylic Acids>4.7 Esters>4.8 Synthetic Polymers>

1. Principles of Chemistry

2. Inorganic Chemistry

3. Physical Chemistry

Unlike alkanes, alkenes contain the functional group C=C

General formula for alkenes

C_nH_2n

EXAMPLE

Ethene is a type of alkene with the formula C₂H₄

ALKENES

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

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.

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