An alkene belongs to a homologous series of molecules. It is classified as a hydrocarbon because it consists of hydrogen and carbon atoms ONLY. This is a crucial negative feature: if a molecule contains oxygen or any other element, it is not a hydrocarbon.
Alkenes are described as unsaturated because they contain at least one carbon-carbon double covalent bond (). Because of this double bond, they do not contain the maximum possible number of hydrogen atoms that a carbon chain could hold.
The characteristic feature of an alkene is its functional group, the double bond. This group determines the chemical properties of the molecule and dictates exactly how it will react.
When drawing fully displayed structural formulas for alkenes, every carbon atom must have exactly four bonds. First, draw the double bond between two adjacent carbon atoms. Then, fill in single bonds to other carbon or hydrogen atoms until every carbon has exactly four lines connecting to it.
The composition of any alkene can be found using the general formula:
Where represents the number of carbon atoms. The name of every alkene ends in the suffix "-ene", and standard organic prefixes tell you the number of carbon atoms: eth- (2), prop- (3), but- (4), and pent- (5).
The first four members of the alkene series are:
There is no such thing as "methene" (). This molecule does not exist because a carbon-carbon double bond strictly requires a minimum of two carbon atoms.
Determine the molecular formula for pentene (5 carbon atoms).
Step 1: Identify the number of carbon atoms ().
Step 2: Use the general formula to calculate the number of hydrogen atoms.
Step 3: Write the final molecular formula.
For a given number of carbon atoms, an alkene contains exactly two fewer hydrogen atoms than the corresponding alkane. For example, butane is , but butene is ().
| Feature | Alkanes | Alkenes |
|---|---|---|
| Type of bonds | Only single bonds | At least one double bond |
| Saturation | Saturated | Unsaturated |
| General Formula | ||
| Hydrogen count | Maximum possible | Two fewer hydrogen atoms than alkanes |
Students often define a hydrocarbon as 'containing carbon and hydrogen' without adding the word 'only' — AQA mark schemes strictly require 'only' for the mark.
In 'explain' questions about unsaturation, examiners expect two specific points: state that it contains a double bond AND that it has exactly two fewer hydrogen atoms than the corresponding alkane.
Examiners may use 'methene' as a distractor in multiple-choice questions; remember that it does not exist because a double bond requires a minimum of two carbon atoms.
Homologous series
A family of compounds with the same general formula, same functional group, and similar chemical properties.
Alkene
A homologous series of unsaturated hydrocarbons with the functional group and the general formula .
Hydrocarbon
A compound consisting of hydrogen and carbon atoms ONLY.
Unsaturated
A molecule that contains at least one carbon-carbon double covalent bond ().
Functional group
The atom or group of atoms in a molecule that determines its chemical properties and how it reacts.
General formula
A mathematical formula that represents the composition of any member of an entire chemical family.
Put your knowledge into practice — try past paper questions for Chemistry
Homologous series
A family of compounds with the same general formula, same functional group, and similar chemical properties.
Alkene
A homologous series of unsaturated hydrocarbons with the functional group and the general formula .
Hydrocarbon
A compound consisting of hydrogen and carbon atoms ONLY.
Unsaturated
A molecule that contains at least one carbon-carbon double covalent bond ().
Functional group
The atom or group of atoms in a molecule that determines its chemical properties and how it reacts.
General formula
A mathematical formula that represents the composition of any member of an entire chemical family.