What are alkenes?

Formulae

 

Alkenes are a family of hydrocarbons (compounds containing carbon and hydrogen only) containing a carbon-carbon double bond. The first two are:

 

                    ethene - C2H4

                    propene - C3H6

 

You can work out the formula of any of them using: CnH2n

 

 

Physical properties of the alkenes

 

Boiling Points

 

The boiling point of each alkene is very similar to that of the alkane with the same number of carbon atoms. Ethene, propene and the various butenes are gases at room temperature. All the rest that you are likely to come across are liquids.

In each case, the alkene has a boiling point which is a small number of degrees lower than the corresponding alkane. The only attractions involved are Van der Waals dispersion forces, and these depend on the shape of the molecule and the number of electrons it contains. Each alkene has 2 fewer electrons than the alkane with the same number of carbons.

 

 

The reactions of alkenes

Like any other hydrocarbons, alkenes burn in air or oxygen, but these reactions are unimportant. Alkenes are too valuable to waste in this way.

The important reactions all centre around the double bond. Typically, the pi bond breaks and the electrons from it are used to join the two carbon atoms to other things. Alkenes undergoaddition reactions.

 

The rather exposed electrons in the pi bond are particularly open to attack by things which carry some degree of positive charge. These are called electrophiles. If you explore the rest of the alkene menu, you will find lots of examples of this kind.

 

 

 

 

http://www.chemguide.co.uk/organicprops/alkenes/background.html