AQA AS Chemistry Unit 2: Alkenes

Chemistry Unit 2: Alkenes

Alkenes are unsaturated hydrocarbons

Shape of Alkenes

Ethene is a planar molecule so its angles between each bond is 120 degree
There is no rotation around the double bond
There is no rotation due to the pi bond formed from the cloud of electron density above and below the single bond

Isomers

Alkenes with more than 3 carbons can form different isomers
Two types of isomers can be formed involving the double bond:
- Position Isomers
- Geometrical Isomers
Position isomers are where the double bond is positioned differently
Geometrical isomers are a form of stereoisomerism, they stereoisomers have the same structural formula but different bonds are arranged differently, it occurs around the double bond
When two types of the same group e.g. -CH3 are on the same side of a double bond this is called Z and when the groups are on opposite sides this called E
- When atoms of higher atomic number are on the same side this is Z
When atoms of higher atomic number are opposite sides this is E

Physical Properties

Van der Waals are the only intermolecular forces acting between alkene molecules
Physical properties are similar to alkanes
The melting and boiling point increase with the number of carbon atom
Alkenes are not soluble in water as they are non-polar

How Alkenes React

The bond enthaply for a C=C bond is almost double of a C-C bond
Alkenes are more reactive than alkanes
C=C forms an electron rich area in a molecule which can be easily attacked by a positively charged reagent called an electrophile
Electrophiles are electron pair acceptors such as H+
The reaction is known as electrophilic addition

Reactions of Alkenes

Alkenes can burn in air to form CO2 and H2O

Electrophilic Addition Reaction

The four electrons in the C=C bond make an alkene a centre of high electron density
Electrophiles are attracted to it and form a bond by using 2 of its 4 electrons in the C=C bond

The…