Chemistry – Organic chemistry - Alkenes | e-Consult

Description of Reaction: Ethene reacts with bromine (Br₂) in solution to undergo an addition reaction. The bromine molecule adds across the carbon-carbon double bond, resulting in the formation of 1,2-dibromoethane. The reaction is typically carried out in an inert solvent like carbon tetrachloride (CCl₄) or dichloromethane (CH₂Cl₂) to prevent unwanted side reactions. The bromine solution will turn from a reddish-brown colour to a colourless solution as the bromine is consumed.

Chemical Equation:

C₂H₄(g) + Br₂(l) → C₂H₄Br₂(l)

Mechanism: The reaction proceeds via a 1,2-addition mechanism. The bromine molecule approaches the alkene. The π electrons of the double bond attack one of the bromine atoms, forming a cyclic bromonium ion intermediate. The bromide ion then attacks the bromonium ion from the opposite side (anti-addition), opening the ring and forming the 1,2-dibromoethane. This is a concerted reaction, meaning all bond breaking and bond forming occur in a single step.

Structural Formulae:

• Ethene (Reactant): CH₂=CH₂
• Bromine (Reactant): Br₂
• 1,2-Dibromoethane (Product): BrCH₂-CH₂Br