IB DP Chemistry Structure 3.2 Functional groups HL Paper 2- Exam Style Questions - New Syllabus
Question
Ethanol is obtained by the hydration of ethene, \(\mathrm{C_2H_4}\).
(a) (i) State the class of compound to which ethene belongs. [1]
(a) (ii) State the molecular formula of the next member of the homologous series to which ethene belongs. [1]
(b) (i) Justify why ethene has only a single signal in its \(^1\mathrm{H}\) NMR spectrum. [1]
(b) (ii) Deduce the chemical shift of this signal. Data booklet (\(^1\)H NMR): vinylic \(\mathrm{HC{=}CH_2}\) protons appear at \(4.5\text{–}6.0\ \mathrm{ppm}\). [1]
(c) Suggest two possible products of the incomplete combustion of ethene that would not be formed by complete combustion. [1]
(d) A white solid was formed when ethene was subjected to high pressure. Deduce the type of reaction that occurred. [1]
(e) Alternative synthetic routes exist to produce alcohols.
(i) Sketch the mechanism for the reaction of propene with hydrogen bromide using curly arrows. [3]
(ii) Explain why the major organic product is 2-bromopropane and not 1-bromopropane. [2]
(iii) 2-bromopropane can be converted directly to propan-2-ol. Identify the reagent required. [1]
(iv) Propan-2-ol can also be formed in one step from a compound containing a carbonyl group. State the name of this compound and the type of reaction that occurs. [2]
(ii) Explain why the major organic product is 2-bromopropane and not 1-bromopropane. [2]
(iii) 2-bromopropane can be converted directly to propan-2-ol. Identify the reagent required. [1]
(iv) Propan-2-ol can also be formed in one step from a compound containing a carbonyl group. State the name of this compound and the type of reaction that occurs. [2]
▶️ Answer/Explanation
Markscheme
(a)
(i) Alkene. A1 [1]
(ii) \(\mathrm{C_3H_6}\) (propene). A1 [1]
(ii) \(\mathrm{C_3H_6}\) (propene). A1 [1]
(b)
(i) All \(^1\mathrm{H}\) nuclei are in the same environment (molecular symmetry) \(\Rightarrow\) one signal. A1 [1]
(ii) \(\delta \approx 4.5\text{–}6.0\ \mathrm{ppm}\) (vinylic \(\mathrm{HC{=}CH_2}\)). A1 [1]
(ii) \(\delta \approx 4.5\text{–}6.0\ \mathrm{ppm}\) (vinylic \(\mathrm{HC{=}CH_2}\)). A1 [1]
(c)
Any two of: carbon monoxide \(\mathrm{CO}\); carbon (soot) \(\mathrm{C}\). A1 [1]
(d)
Addition (chain-growth) polymerization of ethene to poly(ethene). A1 [1]
(e)
(i) Electrophilic addition mechanism (curly arrows):
• \(\pi\)-bond of propene attacks \(\mathrm{H}\) of \(\mathrm{HBr}\); \(\mathrm{H{-}Br}\) bond breaks heterolytically to \(\mathrm{Br^-}\).
• Forms the more stable secondary carbocation \(\mathrm{CH_3{-}CH^+{-}CH_3}\).
• \(\mathrm{Br^-}\) attacks the carbocation \(\rightarrow\) 2-bromopropane.
• Forms the more stable secondary carbocation \(\mathrm{CH_3{-}CH^+{-}CH_3}\).
• \(\mathrm{Br^-}\) attacks the carbocation \(\rightarrow\) 2-bromopropane.
(M1) A1 A1 [3]
(ii) Major product is 2-bromopropane because the reaction proceeds via the more stable secondary carbocation (Markovnikov addition; hyperconjugation/\(+I\) from \(\mathrm{CH_3}\) groups stabilizes \(C^+\)). A2 [2]
(iii) Aqueous hydroxide (e.g. \(\mathrm{NaOH(aq)}\) or \(\mathrm{KOH(aq)}\)). A1 [1]
(iv) Carbonyl compound: propanone; reaction type: reduction to the secondary alcohol (propan-2-ol). A2 [2]
Total Marks: 14