2019 O Level Chem P1 Q36 — Organic Chemistry / Alkenes and Isomers Solution

Answer: B (2 statements correct). Compare but-1-ene, CH₂=CHCH₂CH₃, with but-2-ene, CH₃CH=CHCH₃.

'Both decolourise bromine water': correct, since both contain a C=C double bond and undergo addition with bromine, decolourising the orange bromine water.

'Both produce the same molecule with bromine': wrong. But-1-ene gives 1,2-dibromobutane while but-2-ene gives 2,3-dibromobutane — different positions, different products.

'Both produce the same molecule with hydrogen': correct. Adding hydrogen across either double bond gives the same saturated product, butane.

'When polymerised, the same polymer is produced': wrong. The position of the double bond gives different repeating units, so polybut-1-ene and polybut-2-ene are different addition polymers.

Two of the four statements are correct, namely both decolourising bromine water and both giving butane on hydrogenation. The other two are wrong because the products of bromine addition and of polymerisation depend on where the double bond sits in the chain. With exactly two correct statements, the answer is B.