When cyclohexane interacts with bromine water, the reaction is very slow, and crucially, no decolourisation of the bromine water is typically observed. This outcome is a key indicator of its chemical nature.
Understanding Cyclohexane's Reactivity
Cyclohexane is a saturated hydrocarbon, belonging to the class of cycloalkanes. This means that all the carbon atoms in its ring structure are bonded to the maximum possible number of hydrogen atoms, with only single bonds between carbon atoms. The absence of carbon-carbon double or triple bonds significantly influences its reactivity with reagents like bromine water.
Bromine water, which has a distinct brownish colour due to the dissolved bromine (Br₂), is commonly used as a chemical test to detect the presence of carbon-carbon double or triple bonds (unsaturation) in organic compounds.
Why No Decolourisation?
The decolourisation of bromine water primarily occurs through an addition reaction. In this type of reaction, bromine atoms add across a carbon-carbon double or triple bond, breaking the pi bond and forming new single bonds with the carbons. Since cyclohexane contains only single carbon-carbon bonds, it cannot undergo this characteristic addition reaction under normal conditions. As a result, the bromine in the solution is not consumed, and its brown colour persists.
While cyclohexane can react with bromine, this typically requires specific conditions such as the presence of ultraviolet (UV) light and proceeds via a free radical substitution reaction. In this mechanism, hydrogen atoms are replaced by bromine atoms. However, this process is much slower and does not lead to the rapid decolourisation characteristic of the test for unsaturation.
Comparison with Cyclohexene
To further illustrate cyclohexane's behaviour, it is helpful to contrast it with cyclohexene, an unsaturated hydrocarbon containing a carbon-carbon double bond. The differing reactivity highlights the significance of unsaturation in organic chemistry.
| Property | Cyclohexane (Saturated) | Cyclohexene (Unsaturated) |
|---|---|---|
| Structure | Cycloalkane (all C-C single bonds) | Cycloalkene (contains a C=C double bond) |
| Reaction with Bromine Water | Reacts very slowly; no decolourisation occurs. | Reacts quickly and spontaneously; bromine water is decolourised from brown to colourless. |
| Reaction Type | Typically requires UV light for substitution. | Undergoes rapid addition reaction at the double bond. |
| Indicator of Unsaturation | Negative test (no decolourisation). | Positive test (rapid decolourisation). |
In summary, the key outcome when cyclohexane reacts with bromine water is the lack of observable decolourisation, indicating its saturated nature and differentiating it from unsaturated compounds like cyclohexene.
