Chemistry » Organic Molecules » Organic Molecular Structures

# The Alkenes

## The alkenes

In the alkenes there must be at least one double bond between two carbon atoms. This means that they are unsaturated and are more reactive than the alkanes. The simplest alkene is ethene (also known as ethylene), which is shown in the figure below.

The (a) structural, (b) condensed structural and (c) molecular formula representations of ethene. (d) An atomic model of ethene.

As with the alkanes, the alkenes also form a homologous series. They have the general formula: $$\color{red}{\textbf{C}_{\textbf{n}}\textbf{H}_{\textbf{2n}}}$$. The second alkene in the series would therefore be $$\text{C}_{3}\text{H}_{6}$$. This molecule is known as propene (see figure below).

The (a) structural, (b) condensed structural and (c) molecular formula representations of propene.

There can be more than one double bond in an alkene as shown in the figure below. The naming of these compounds is covered in Chemistry 102, IUPAC naming and formulae.

#### Tip:

Note that if an alkene has two double bonds, it is called a diene.

If you don’t understand the names of compounds, don’t worry. We will go into more detail on this later in the tutorial.

The structural representations of (a) pent-1-ene and (b) pent-1,3-diene.

The alkenes are more reactive than the alkanes because they are unsaturated. As with the alkanes, compounds that have four or less carbon atoms are gases at room temperature. Those with five or more carbon atoms are liquids.

The alkenes have a variety of uses:

• For example, ethene is a chemical compound used in plants to stimulate the ripening of fruits and the opening of flowers.

(a) Unripe (green) and ripe (yellow) bananas and (b) a flowering plant.

• Propene is an important compound in the petrochemicals industry. It is used to make polypropylene (see Chemistry 102 for more information) and is also used as a fuel gas for other industrial processes.