AS-Level Alkenes

  • Alkanes are hydrocarbons in which two or more of the carbon atoms are bonded together with a double bond.

  • A carbon double bond is made by the merging of two p-orbitals from two carbon atoms, creating an area of high electron density between the two atoms, called a ‘pi-bond’.

  • Electron deficient species (electrophiles) are attracted to the electrons in the double bond and this makes alkenes more reactive than simple alkanes.

QUICK NOTES

Alkenes

 

Carbon can form double bonds between atoms of itself. A carbon chain can still exist, as carbon atoms are still bonded together, but the number of hydrogen atoms bonded to each carbon will change. Remember a carbon atom can only form four covalent bonds. A double bond counts as two of those four, so only two bonds are left for each carbon to make with other atoms.

 

 

 

 

 

 

 

 

Hydrocarbons that have one or more double bonds within their carbon chain are called alkenes. Alkenes are more reactive than alkanes, and they can be very useful when making new molecules in organic chemistry.

 

The double bond

 

A covalent bond is the sharing of a pair of electrons between two atoms. In order for a bond to form, electron orbitals have to overlap so both atoms can have ‘access’ to both electrons. As there are different shapes of electron orbitals, there are different ways orbitals can overlap between atoms.

 

How ‘well’ two orbitals overlap can indicate how strong a particular bond is likely to be. Poor overlap results in a weak bond, good overlapping results in a stronger bond. Perfect overlapping creates a single bond called a sigma bond.

 

In a carbon-carbon double bond p-orbitals cannot fully overlap, so they have to ‘merge’. This causes two regions of electron density to form – above and below the sigma single bond between the carbon atoms.   

 

 

Electrons are negatively charged particles. Wherever electrons exist, there is a region of negative charge. In a carbon-carbon double bond the merged p-orbitals contain electrons, so are highly vulnerable to ‘attack’ from positive ions and electron deficient species (electrophiles). This is why carbon-carbon double bonds are, relatively, easily broken.

Addition polymerisation

 

The carbon-carbon double bond in an alkene enables polymers to be formed.

 

A polymer is just a very long chain molecule, made up of the same basic unit that is bonded over and over again.

 

 

A repeating is the basic unit that makes up a polymer.

 

Due to the large size of polymer molecules, they have a high relative molecular mass. Also, due to their large size, they have large amounts of intermolecular forces between molecules, which make them solids at room temperature. However, their intermolecular forces are usually very weak, so overall melting points can be quite low, making polymers very useful in manufacturing.