How many atoms of chlorine are required to saturate the double bonds of a compound containing a triene?

Analyze the question: A triene is a compound with three C=C double bonds. We need the number of chlorine atoms needed to saturate all of them.

Concept: Chlorine adds across a C=C double bond (addition reaction). Each double bond takes one Cl₂ molecule, which is 2 Cl atoms. The double bond breaks and one Cl attaches to each carbon.

R–CH=CH–R' + Cl₂ → R–CHCl–CHCl–R'

Solution:

• Number of double bonds in a triene = 3
• Each double bond needs 1 Cl₂ molecule = 2 Cl atoms
• Total Cl atoms = 3 × 2 = 6 atoms

(That is 3 molecules of Cl₂.)

Option B (3) would be correct only if the question asked for the number of Cl₂ molecules, so read the wording carefully: "atoms" means 6.

Exam tip: In addition questions, always check whether the question asks for atoms or molecules. For diatomic halogens (Cl₂, Br₂, H₂), multiply the number of molecules by 2 to get atoms. Quick rule: number of C=C bonds × 2 = atoms of halogen needed.