Unsaturated Hydrocarbons: Alkenes are organic compounds consisting entirely of hydrogen and carbon where at least two carbon atoms are linked by a double covalent bond. This 'unsaturation' means the molecule contains fewer hydrogen atoms than the maximum possible for the number of carbon atoms present.
Functional Group: The double bond is the reactive center of the molecule and defines the chemical properties of the alkene homologous series. Because it contains a high density of electrons, it is the site where most chemical reactions, such as additions, occur.
General Formula: All simple alkenes follow the general mathematical relationship , where represents the number of carbon atoms. For example, if an alkene has three carbons (), it must have six hydrogens (), resulting in the molecular formula .
The Addition Mechanism: In an addition reaction, a small molecule (like or ) reacts with the alkene by breaking the component of the double bond. Each carbon atom that was part of the double bond then forms a new single covalent bond with one part of the incoming molecule.
Halogenation (Bromination): When an alkene reacts with a halogen like bromine (), the two bromine atoms add across the double bond. This transforms the unsaturated alkene into a saturated dibromoalkane, such as 1,2-dibromoethane when ethene is used.
Testing for Unsaturation: To perform the bromine water test, an orange bromine solution is added to the sample. If an alkene is present, the orange color disappears (decolourises) as the bromine reacts to form a colorless product; if only alkanes are present, the solution remains orange.
| Feature | Alkenes | Alkanes |
|---|---|---|
| Bonding | Unsaturated () | Saturated ( only) |
| General Formula | ||
| Reactivity | Highly reactive (Addition) | Low reactivity (Substitution) |
| Bromine Water Test | Decolourises (Orange to Colorless) | No reaction (Stays Orange) |
Molecular Formula Verification: Always check that your alkene formula satisfies the rule. A common mistake is using the alkane formula () for alkenes, which will result in an incorrect structure.
Drawing Displayed Formulae: Ensure that every carbon atom in your drawing has exactly four bonds. When drawing the product of an addition reaction, remember to remove the double bond and replace it with a single bond plus two new branches.
Observation Accuracy: In practical descriptions, never use the word 'clear' to describe the result of a positive bromine water test. The correct term is 'colourless', as 'clear' only describes transparency, not the absence of color.
Numbering the Chain: Students often start counting carbon atoms from the wrong end of the molecule. You must always number the chain from the end that gives the lowest possible number to the carbon where the double bond begins.
State of Bromine: Distinguish between pure bromine (a liquid) and bromine water (an aqueous solution). While both work for the test, bromine water is the standard laboratory reagent mentioned in exams due to safety and ease of use.
Industrial Importance: Alkenes are the primary feedstock for the plastics industry. Through addition polymerization, thousands of alkene monomers can link together to form polymers like poly(ethene), which is used in packaging and containers.
Relationship to Cracking: Alkenes are typically produced via the catalytic cracking of long-chain alkanes obtained from crude oil. This process is essential because it breaks down less useful large molecules into smaller, high-demand fuels and reactive alkenes.
