In an addition reaction, atoms add across a carbon to carbon double bond and the molecule becomes saturated. This skill appears in organic reactions whenever a question gives an alkene and asks for a product or a test result.
How does the double bond change in addition?
The double bond in an alkene is made of two shared pairs of electrons. In addition, one of those pairs is used to form bonds to new atoms. The carbon to carbon link drops from double to single, and each carbon gains one new atom.
The general pattern is: C=C + X-Y gives a saturated molecule with X on one carbon and Y on the other. Nothing leaves, so the product is a single molecule containing every atom of both reactants.
Three additions are worth knowing for ethene, C₂H₄:
- With hydrogen, H₂, it gives ethane, C₂H₆, using a catalyst and heat. This is hydrogenation.
- With bromine, Br₂, it gives 1,2-dibromoethane, C₂H₄Br₂, at room temperature. This is the basis of the bromine water test.
- With steam, H₂O, it gives ethanol, C₂H₅OH, under conditions your syllabus states. This is hydration.
Worked example
Propene, C₃H₆, reacts with bromine. Draw and name the product.
Step 1, write the reactants: CH₃-CH=CH₂ and Br-Br.
Step 2, open the double bond: the two carbons keep a single link to each other and give up the second bond.
Step 3, attach one bromine to each of those carbons: CH₃-CHBr-CH₂Br.
Step 4, name it: 1,2-dibromopropane, C₃H₆Br₂.
Step 5, check the atoms: left C₃H₆ + Br₂ gives 3 C, 6 H and 2 Br. Right C₃H₆Br₂ gives 3 C, 6 H and 2 Br. Balanced.
Amounts check with ethene: 2.8 g of ethene is 0.10 mol (Mr 28).
It reacts with 0.10 mol of bromine and forms 0.10 mol of C₂H₄Br₂. The Mr is 24 + 4 + 160 = 188, so the mass is 18.8 g. Check by adding: 2.8 g + 16.0 g of bromine is 18.8 g.
The mistake to watch for
A common slip is to write only one bromine atom in the product.
Mistaken equation: C₂H₄ + Br₂ → C₂H₄Br
This loses a bromine atom. The right side has 1 Br and the left has 2.
The correction: in addition, both atoms of the molecule attach, one per carbon. Write the product with both, C₂H₄Br₂, and count before you move on. A second slip is to keep the double bond in the product, which would mean nothing has added.
Check yourself
1. Write the equation for the addition of hydrogen to propene.
Show answer
C₃H₆ + H₂ → C₃H₈. Check: 3 C and 8 H on both sides. The product is propane, which is saturated.
2. Which of these decolourises bromine water: C₃H₈, C₃H₆, C₄H₁₀? Explain.
Show answer
C₃H₆. It is an alkene, so it has a double bond and bromine adds across it. The orange-brown colour disappears. The other two are alkanes with only single bonds.
3. But-1-ene, C₄H₈, reacts with bromine. Give the product formula and a check on the atoms.
Show answer
C₄H₈Br₂ (1,2-dibromobutane). Left: 4 C, 8 H, 2 Br. Right: 4 C, 8 H, 2 Br.
Where this leads next
Hydration links straight to alcohol and acid transformations, and the same addition idea repeats in polymer repeat units. To revisit what makes a molecule unsaturated, see recognising saturated and unsaturated structures. The equation balance reasoning trainer is a quick way to practise the atom count.
If the structure drawing still costs you marks, our teachers can work through it with you in online one-to-one Chemistry tuition.