I am tasked by my textbook to synthesize 2-hexyne from 1-propanol. All carbon atoms must come from the starting material, 1-propanol.

My solutions manual (an older version) does not have this additional problem, so I am forced to ask here.

I understand that in this problem there is a double in carbon count in the product, so we are expected to somehow "fuse" the carbons from the starting material. I also understand that this means making an electrophilic carbon and a nucleophilic carbon from the starting materials. The electrophilic carbons that I know about are alkyl halides, alkyl tosylates, epoxides, and acid chlorides.

I'll choose to make an alkyl halide from the starting material through reaction of the primary alcohol (1-propanol) with thionyl bromide and pyridine as a base. Now I have my electrophilic carbon.

My nucleophilic carbon will be the acetylide anion. I could use a cyanide ion but that only gives me a one carbon increase. Plus that's not an allowed carbon source. So I dehydrate my primary alcohol (is this possible?) and creates 1-propene. I can brominate this in an inert solvent (i.e. methylene chloride). From here with the vicinal dihalide I can form the alkyne and then methyl acetylide. I can then react this with the alkyl halide I made previously.

How does this look?

enter image description here

  • 1
    $\begingroup$ Your approach seems very reasonable. $\endgroup$ – ron Sep 28 '14 at 20:29
  • $\begingroup$ books.google.de/… $\endgroup$ – user7951 Jun 12 '15 at 19:55

Your proposed synthesis looks reasonable. Dehydration of the primary alcohol is indeed possible, it proceeds via the E2 mechanism (although primary alcohols are less reactive than secondary or tertiary ones). Chlorinated solvents, like methylene chloride and chloroform, are commonly used for the bromination of alkenes, for the reason you have provided (inertness).

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