INTRODUCTION TO ALKENES AND THEIR PHYSICAL PROPERTIES

What are alkenes?

Formulae

Alkenes are an unsaturated form of hydrocarbons that are formed by double bonding between the carbon atoms. There is at least one such double bond in their structure. The simplest alkene with one double bond is ethene (C₂H₄). Alkenes are an important part of our everyday lives, and also have many industrial uses. Let us take a look at some properties of alkenes.

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

You can work out the formula of any of them using: C_nH_2n

The table is limited to the first two, because after that there are isomers which affect the names.

Isomerism in the alkenes

Structural isomerism

All the alkenes with 4 or more carbon atoms in them show structural isomerism. This means that there are two or more 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:

There is, however, another isomer. But-2-ene also exhibits geometric isomerism.

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).

Cis-but-2-ene is also known as (Z)-but-2-ene; trans-but-2-ene is also known as (E)-but-2-ene. For an explanation of the two ways of naming these two compounds, follow the link in the box below.

Physical Properties of Alkenes

Physical properties of alkenes are quite similar to those of alkanes. Let us take a look at few physical properties

• Alkenes exist naturally in all three states. The first three alkenes are gases, and the next fourteen are liquids. Alkenes higher than these are all solids.
• All alkenes are insoluble in water, due to the weak van der Waal forces.
• But alkenes are soluble in organic solvents like benzene or acetone because here the van der Waal forces will be replaced by new ones, making alkenes fully soluble.
• The boiling points of alkenes depend on their molecular structure. The bigger their molecular chain the higher the boiling points. So the higher alkenes have very high boiling points
• The polarity of alkenes will depend on their functional groups

Chemical Reactivity

Bonding in the alkenes

We just need to look at ethene, because what is true of C=C in ethene will be equally true of C=C in more complicated alkenes.

Ethene is often modelled like this:

The double bond between the carbon atoms is, of course, two pairs of shared electrons. What the diagram doesn’t show is that the two pairs aren’t the same as each other.

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.

In this diagram, the line between the two carbon atoms represents a normal bond – the pair of shared electrons lies in a molecular orbital on the line between the two nuclei where you would expect them to be. This sort of bond is called a sigma bond.

The other pair of electrons is found somewhere in the shaded part above and below the plane of the molecule. This bond is called a pi bond. The electrons in the pi bond are free to move around anywhere in this shaded region and can move freely from one half to the other.

The reactions of alkenes

The pi electrons are not as fully under the control of the carbon nuclei as the electrons in the sigma bond and, because they lie exposed above and below the rest of the molecule, they are relatively open to attack by other things.

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.