Alkyne 6:

Problem

Solution

Internal (disubstituted) alkynes are reversibly isomerized with catalytic NaNH₂ at elevated temperatures. Thus, 3-hexyne is isomerized to 2-hexyne and visa versa through the intermediacy of the allene, 2,3-hexadiene. 2-Hexyne can also be isomerized to 1-hexyne (pKa = 25), which is deprotonated by NaNH₂ (NH₃; pKa = 35). Because of this irreversible step, stoichiometric quantities of NaNH₂ must be used. The same reaction may be accomplished with fused KOH at 200^oC. Under these conditions all three hexynes are in equilibrium because KOH is not basic enough to effect deprotonation. For a mechanism of the isomerization, click here.