9.14C - Addition reaction of ethene with bromine

How bromine adds across C=C

In an addition reaction, two reactant molecules join to form one product, with no atoms lost as a smaller molecule. Bromine is $\ce{Br2}$: two Br atoms joined by a single bond.

During bromine addition to an alkene, the C=C double bond becomes a C–C single bond. The Br–Br bond breaks and a new C–Br bond forms at each of the two carbons originally in C=C. The carbon atoms stay connected. All the original carbon and hydrogen atoms remain.

For ethene:

ethene + bromine → 1,2-dibromoethane
CH2=CH2 + Br2 → BrCH2–CH2Br

[DIAGRAM: asset_name: ethene_bromine_addition_structure: Ethene bromine addition structures - diagram 1; asset_slug: 218_9_14c_addition_reaction_of_ethene_with_bromine_diagram1; file: 218_9_14c_addition_reaction_of_ethene_with_bromine_diagram1.png; recommended_method: deterministic_drawn; description: Exact monochrome deterministic structural reaction diagram for Pearson Edexcel GCSE Chemistry 1CH0 9.14C. It shows ethene with all covalent bonds and a C=C double bond, bromine as Br-Br, an arrow to 1,2-dibromoethane with a C-C single bond and one Br atom attached to each carbon. Labels required: ethene, bromine, 1,2-dibromoethane, C=C opens, one Br added to each carbon. The diagram is assessed structural support, so it must be deterministic and monochrome. Colour-change information is taught in nearby prose and not by colouring the asset.]

The figure’s phrase “C=C opens” means its bond order changes from double to single; it does not mean the carbon chain splits. Each product carbon has two C–H bonds, one C–C bond and one C–Br bond: four bonds in total.