Bromine water goes colorless because bromine molecules react with the alkene's carbon-carbon double bond and are consumed. This electrophilic addition reaction produces a colorless dibromo compound in the standard IB model.
Bromine water is orange-brown because it contains dissolved bromine, . An alkene contains a region of high electron density in its carbon-carbon double bond. As a bromine molecule approaches, the electrons in the double bond repel its electrons and induce a temporary dipole.
The electron-rich double bond attracts the partially positive end of the polarized bromine molecule. During the reaction, the weaker component of the double bond breaks, and bromine atoms are added to the two carbon atoms. A carbon-carbon single bond remains, while two new carbon-bromine bonds form.
Because colored is converted into a colorless organic product, the orange-brown color disappears. For ethene, the simplified equation is:
The product is 1,2-dibromoethane. This reaction therefore provides a qualitative test for carbon-carbon unsaturation: it indicates that a multiple bond is present but does not determine its location.
| Substance tested | Observation with bromine water | Explanation |
|---|---|---|
| Alkene | Orange-brown to colorless | is consumed by addition across the double bond |
| Alkane under ordinary conditions | No color change | No carbon-carbon double bond is available for electrophilic addition |
A common misconception is that bromine merely becomes diluted. It does not: the color disappears because bromine undergoes a chemical reaction. Another mistake is to describe this as substitution; addition occurs because atoms join across the double bond without replacing another atom. This is SL-accessible content under Reactivity 3.4; detailed electrophilic-addition mechanisms using curly arrows are AHL.
Exam technique: If asked to explain the observation, state both that the alkene's double bond is electron-rich and that colored bromine is consumed during electrophilic addition. Include the displayed formula or equation when requested. For an AHL mechanism, start every curly arrow at an electron pair or bond, never at an atom.
