1. The Reactants:
* Butene: An alkene with a carbon-carbon double bond.
* Bromine solution: Bromine (Br₂) dissolved in a solvent like dichloromethane or water, giving it a reddish-brown color.
2. The Reaction:
* The double bond in butene is electron-rich and acts as a nucleophile.
* The bromine molecule (Br₂) is polarized due to the electronegativity difference between bromine atoms, making one bromine atom slightly electrophilic (electron-loving).
* The electrophilic bromine atom attacks the electron-rich double bond in butene, breaking the double bond and forming a bromonium ion intermediate.
* The other bromine atom, now negatively charged, attacks the bromonium ion from the opposite side, resulting in the formation of a dibromoalkane (1,2-dibromobutane).
3. The Result:
* The bromine solution loses its reddish-brown color as the bromine is consumed in the reaction.
* The colorless dibromoalkane product is formed.
The overall reaction can be represented as follows:
CH₃CH₂CH=CH₂ + Br₂ → CH₃CH₂CHBrCH₂Br
Key points:
* The reaction is stereospecific, meaning the product is formed with a specific stereochemistry. In this case, the bromine atoms are added to opposite sides of the double bond, forming a trans-dibromobutane.
* This decolorization reaction is a common test for the presence of alkenes.
* The reaction is also an example of bromination, a specific type of electrophilic addition.
