Alkenes
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ALKENES
What are alkenes?
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 |
The general formula being: CnH2n
Isomerism exhibited by the alkenes
Structural isomerism
All the alkenes with 4 or more carbon atoms in them exhibit structural isomerism. This means that there are more than one different structural formulae that you can draw for each molecular formula.
For example, with C4H8, it isn’t too difficult to come up with these three structural isomers:
Geometric (cis-trans) isomerism
The carbon-carbon double bond doesn’t allow any rotation about it. That means that it is possible to have the CH3 groups on either end of the molecule locked either on one side of the molecule or opposite each other.
These are called cis-but-2-ene (where the groups are on the same side) or trans-but-2-ene (where they are on opposite sides).
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 being 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.
Solubility
Alkenes are virtually insoluble in water, but dissolve in organic solvents. And the explanation holds the same as for alkanes.
Chemical Action
Bonding in the alkenesLet us take the example of ethene. and the same hold true for larger alkenes also.
The structure of ethene can be visualised as
The double bond between the carbon atoms are two pairs of shared electrons. One of the pairs of electrons is held on the line between the two carbon nuclei as you would expect, but the other is held in a molecular orbital above and below the plane of the molecule. A molecular orbital is a region of space within the molecule where there is a high probability of finding a particular pair of electrons.
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 undergo addition reactions.
For example, using a general molecule X-Y . . .
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.