Exam Style Question - 5

QuestionX, Y and Z are three different alcohols, each with the molecular formula C₄H₁₀O. Each has an unbranched or branched chain.• All three give steamy fumes when solid phosphorus(V) chloride, PCl₅, is added.• X does not change the colour of warm acidified potassium dichromate(VI).• Y turns acidified potassium dichromate(VI) green, and the only organic product is a ketone.• Z turns acidified potassium dichromate(VI) green, and the product distilled off is butanal.Identify X, Y and Z and explain your reasoning.AnswerThe steamy fumes are hydrogen chloride, HCl, which shows that each compound contains an OH group, so all three are alcohols.ROH + PCl₅ → RCl + POCl₃ + HClX is not oxidised, so it is a tertiary alcohol. The only tertiary alcohol with formula C₄H₁₀O is 2-methylpropan-2-ol, (CH₃)₃COH.Y is oxidised to a ketone, so it is a secondary alcohol: butan-2-ol, CH₃CH(OH)CH₂CH₃. The ketone formed is butanone, CH₃COCH₂CH₃.Z is oxidised to an aldehyde, so it is a primary alcohol. Because the aldehyde is butanal, with an unbranched chain, Z is butan-1-ol, CH₃CH₂CH₂CH₂OH.Examiner tip: 2-methylpropan-1-ol is also a primary C₄H₁₀O alcohol, but it would give 2-methylpropanal, not butanal. Always use the name of the product to decide the carbon chain.