# internal alkyne stability to strong alkali

Internal alkynes are relatively electron rich and are not usually prone to attack by strong (nucleophilic) bases such as hydroxide or alkoxide.

However, this isn't always the case. A very nice overview of the most common reactions of alkynes says:

The sp-hybrid carbon atoms of the triple-bond render alkynes more electrophilic than similarly substituted alkenes. As a result, alkynes sometimes undergo addition reactions initiated by bonding to a nucleophile. This mode of reaction, illustrated below, is generally not displayed by alkenes, unless the double-bond is activated by electronegative substituents, e.g. $\ce{F2C=CF2}$, or by conjugation with an electron withdrawing group.

$\ce{HC≡CH + KOC2H5 —>[in C2H5OH at 150 ºC] H2C=CH-OC2H5}$

Unfortunately, the linked page doesn't provide a reference for this reaction, and searching in Google Scholar for "internal alkyne" "potassium hydroxide" doesn't provide useful results.

Thus my questions are:

1. Does hydroxide ever react similarly with internal alkynes, at any temperature? If so, under what conditions?
2. How long does the ethoxide reaction indicated above (at 150 °C) take? Days? Hours? Minutes? Would it be significantly different for an internal alkyne rather than acetylene?
3. In the case of a hydroxide reaction, would the ultimate product be a ketone (a.k.a. base catalyzed hydration)?
• I can't read German, but this paper is probably relevant. From what I can tell, it is describing the reaction between acetylene, an alcohol/phenol (including ethanol), and KOH at elevated pressures to give a vinyl aryl or vinyl alkyl ether. – orthocresol Dec 4 '16 at 20:22
• There are also multiple patents describing how the reaction can be carried out in industry, for example this one. Page 9 states that the reaction with MeOH (to form vinyl methyl ether) was run with KOMe catalyst at 45 psi, 145 deg C, for 40 hours giving a 96% yield. – orthocresol Dec 4 '16 at 20:39
• And I did another search on Reaxys for vinyl ethers of the form $\ce{R^1HC=C(OR^2)R^3}$ but sadly no syntheses from internal alkynes turned up. I'll leave somebody else more qualified to try to rationalise it. – orthocresol Dec 6 '16 at 2:51

According to A Textbook of Organic Chemistry by V. K. Ahluwalia and Madhuri Goyal

1-Butyne may be isomerised to a more stable 2-butyne in presence of alcoholic potassium hydroxide and the reverse of this reaction may occur with sodamide

(see 17.2.1 of Best Synthetic Methods: Acetylenes, Allenes and Cumulenes for reaction conditions in the 2-alkyne to 1-alkyne direction, actually many relevant reactions are in this book)

So in the present of the strong base $\ce{NaNH2}$ the internal alkyne $\ce{H3CC#CCH3}$ rearranges to $\ce{HC#CCH2CH3}$

My thinking is that with just hydroxide the more stable 2-butyne is favored, while if an even-stronger base is used, that fact that the acetylenic-H can deprotonate pulls the equilibrium toward 1-butyne.

Additionally, see 17.2.11 Isomerisation of 1,4 bis(alkoxy)-2-butynes to the corresponding allenes, which is catalyzed by t-BuOK.

Add see table 17.1 "Base catalyzed isomerisations of acetylenic compounds" about 10 different internal acetylene reactions are listed in the table.

• Thanks for this! I'll probably accept & award in the next day or two (unless a better answer materializes). I'm especially impressed w/ table 17.1. It seems like alkynes can readily isomerize to allenes or other slightly more stable isomers under basic conditions. – Curt F. Dec 8 '16 at 17:55