Exam Style Question - 3

QuestionButan-2-ol, CH₃CH(OH)CH₂CH₃, can be dehydrated to form alkenes.(a) Give the reagent and conditions for the dehydration.(b) State the type of reaction and the small molecule removed.(c) Name all the alkenes formed from butan-2-ol and explain why there is more than one.(d) Name the only alkene formed from butan-1-ol.Answer(a) Heat with concentrated phosphoric acid (H₃PO₄). The alkene can be distilled off as it forms.(b) An elimination reaction (dehydration). A molecule of water is removed: the OH group and a hydrogen atom on a neighbouring carbon atom are lost.C₄H₉OH → C₄H₈ + H₂O(c) In butan-2-ol, the carbon bearing the OH group has two neighbouring carbon atoms that carry hydrogen atoms (C1 and C3). Removing H from C1 gives but-1-ene, CH₂=CHCH₂CH₃. Removing H from C3 gives but-2-ene, CH₃CH=CHCH₃. But-2-ene has two different groups on each carbon of the C=C, so it exists as E and Z isomers (E-but-2-ene and Z-but-2-ene). So butan-2-ol gives a total of three alkenes.(d) In butan-1-ol the OH group is on the end carbon, which has only one neighbouring carbon atom, so only but-1-ene forms.Examiner tip: when asked how many alkenes form, always check whether any of the products can show E/Z isomerism, and count each stereoisomer separately.