Learning focus
Develop accurate organic nomenclature, connect functional groups to reactions, draw structural and polymer formulae, interpret industrial processes and apply evidence to unfamiliar compounds.
Ethene reaction
Ethene reacts with bromine to produce 1,2-dibromoethane: CH2=CH2 + Br2 -> CH2BrCH2Br. There is one product.
Drawing products
Locate C=C, change it to C-C and attach one Br to each former double-bond carbon. Keep all original hydrogen atoms unless valency requires the bond-order change.
Other alkenes
Propene gives 1,2-dibromopropane. But-2-ene gives 2,3-dibromobutane. The position of bromine atoms follows the original double-bond carbons.
Evidence of unsaturation
The disappearance of bromine colour is evidence that bromine molecules were consumed. It is not a physical dilution effect when equal test conditions are used.
High-value recall and connections
Essential recall: What product forms from ethene and bromine? 1,2-dibromoethane. What happens to C=C? It becomes C-C. Is HBr formed in this addition? No. Practical connection: Draw products for addition of Br2 to ethene, propene, but-1-ene and but-2-ene, checking carbon valency. Examination connection: Add bromine to both double-bond carbons; do not replace hydrogen as in alkane substitution.
Practical or data skill
Draw products for addition of Br2 to ethene, propene, but-1-ene and but-2-ene, checking carbon valency.
Examination tip
Add bromine to both double-bond carbons; do not replace hydrogen as in alkane substitution.
Review questions and suggested answers
Question 1
What product forms from ethene and bromine?
Suggested answer
1,2-dibromoethane.
Question 2
What happens to C=C?
Suggested answer
It becomes C-C.
Question 3
Is HBr formed in this addition?
Suggested answer
No.
