The first step in synthesising pentanal from acetylene is forming the acetylide ion by the addition of NaNH2. The hydrogen atom reacts with the amide ion in NaNH2 to form HCC- and ammonia.The carbon in the acetylide ion is a good nucleophile and reacts with 1-chloropropane in an Sn2 reaction. The chlorine atom in 1-chloropropane takes the electrons from the C-Cl bond forming a positively charged carbon atom on the hydrocarbon chain and a chloride ion. The carbanion attacks the electrophilic carbon forming 1-butyne. 1-butyne then reacts with BH3/THF. The reaction with BH3 is anti-Markovnikov and a syn hydration forming 1-penten-1-ol with mechanism shown below. This molecule then tautomerises to form pentanal.
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