Question 4 of 5: Pentyne-2 Addition Reactions & Butanol Isomer Differentiation

Question 4: Pentyne-2 Addition Reactions & Butanol Isomer Differentiation (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

a) Pentyne-2 is CH₃–C≡C–CH₂CH₃ (pent-2-yne, C₅H₈), an internal, unsymmetrical alkyne.

(i) 1 mol H₂, Pd/BaSO₄ (Lindlar-type poisoned catalyst). This catalyst stops reduction at the alkene stage and delivers both new C–H's from the same face → cis-pent-2-ene, CH₃CH=CHCH₂CH₃ (C₅H₁₀).

(ii) 2 mol H₂, Ni. An unpoisoned metal catalyst reduces all the way to the alkane → pentane, CH₃CH₂CH₂CH₂CH₃ (C₅H₁₂).

(iii) 1 mol Cl₂. Electrophilic addition across the triple bond (one equivalent) gives the vicinal dichloroalkene → 2,3-dichloropent-2-ene, CH₃C(Cl)=C(Cl)CH₂CH₃ (C₅H₈Cl₂).

(iv) 1 mol HCl. Markovnikov addition places Cl on the more stabilised vinylic-cation carbon; here the two alkyne carbons are only mildly unsymmetrical (one flanked by CH₃, the other by CH₂CH₃), so selectivity is modest, but the ethyl-flanked cation is marginally favoured → major product 2-chloropent-2-ene, CH₃C(Cl)=CHCH₂CH₃ (C₅H₉Cl), alongside a minor amount of the 3-chloro regiochemical isomer.

(v) 2 mol HCl. A second Markovnikov addition to the chloroalkene from (iv) places the second Cl on the same carbon (the one that can again form the more stable, now tertiary-like, cation) → the geminal dichloride 2,2-dichloropentane, CH₃C(Cl)₂CH₂CH₂CH₃ (C₅H₁₀Cl₂).

b) i) The four alcohol isomers of C₄H₁₀O. (The same formula also admits three ether isomers — diethyl ether, methyl propyl ether, methyl isopropyl ether — but the question asks specifically for "the alcohol," so only the four alcohols are drawn.)

b) ii) Differentiating the four by simple test-tube reactions. Two classical qualitative tests together distinguish all four: the Lucas test (ZnCl₂/HCl) reacts via an S_N1 mechanism whose rate follows carbocation stability 3° > 2° > 1° — tert-butanol turns cloudy (immiscible alkyl chloride) within a minute at room temperature, 2-butanol takes several minutes to hours, and both primary alcohols (1-butanol, 2-methyl-1-propanol) show no reaction at room temperature. The chromic acid (Jones) oxidation test then splits the two unreactive-to-Lucas primary alcohols apart from any remaining ambiguity and independently confirms class: primary alcohols decolourise the orange Cr(VI) reagent to green Cr(III) (oxidising on to a carboxylic acid), secondary alcohols also decolourise it (oxidising to a ketone), and tertiary alcohols (tert-butanol) do not react at all, since oxidation would require breaking a C–C bond.

b) iii) One of these is a redox reaction. Take the chromic-acid oxidation of a primary or secondary alcohol, e.g. 2-butanol → butan-2-one. The carbinol carbon's oxidation state rises (it loses two bonds to hydrogen and gains a second bond to oxygen: from −1 in the alcohol to +1 in the ketone), while chromium is reduced from +6 (in Cr₂O₇²⁻/CrO₃, orange) to +3 (Cr³⁺, green).