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Introduction to Acidity of Terminal Alkynes

Alkanes are without any doubt the weakest Brønsted acids usually encountered in organic chemistry. It is hard to measure such type of weak acids, but estimates put the pKa of ethane at about 48. Hybridizing the carbon hence, as to increase the C-H's s-character increments the acidity, with the maximum change occurring for the sp-C-H groups found in terminal alkynes. So, the pKa of ethene is estimated at 44 and the pKa of ethyne (acetylene) is found to be 25, making it 1023 times stronger an acid than ethane. This increment in acidity allows the isolation of insoluble copper and silver salts of such type of compounds.

RC≡C-H   +   Ag(NH3)2(+)   (in NH4OH)   -->  RC≡C-Ag (insoluble)   +   NH3   +   NH4(+)

In spite of the remarkable increase in acidity of terminal alkynes relative to another hydrocarbons, yet they are very weak acids, mainly when compared with water, which is approximately a billion times more acidic. If we desire to prepare nucleophilic salts of terminal alkynes for use in synthesis, so it will be necessary to use a much stronger base than hydroxide (or ethoxide) anion. Such type of base is sodium amide (NaNH2), described above and with terminal alkynes its reactions may be conducted in liquid ether or ammonia as solvents. The results of this acid-base reaction are ammonia and a sodium acetylide salt. Since the acetylide anion is a great nucleophile it may shift halide ions from 1º-alkyl halides to give a more highly substituted alkyne as a product (SN2 reaction). This synthesis application is elaborated in the equations. The first two equations depict how acetylene can be converted to propyne; the last two equations display a synthesis of 2-pentyne from propyne.

H-C≡C-H   +   NaNH2   (in ammonia or ether)   --> H-C≡C-Na (sodium acetylide)   +   NH3

H-C≡C-Na   +   CH3-I   --> H-C≡C-CH3   +   NaI

CH3-C≡C-H   +   NaNH2   (in ammonia or ether)   -->  CH3-C≡C-Na (sodium propynylide)   +   NH3

CH3-C≡C-Na   +   C2H5-Br   -->  CH3-C≡C-C2H5   +   NaBr

Since RC≡C:(-) Na(+) is a very strong base (approximately a billion times stronger than NaOH), its make use of as a nucleophile in SN2 reactions is limited to 1º-alkyl halides; 2º and 3º-alkyl halides go through elimination by an E2 mechanism.

The terminal's improved acidity alkynes relative to alkanes also leads to metal exchange reactions when these compounds are treated with Grignard reagents or organolithium. This exchange, displayed below in equation 1, can be understood as an acid-base reaction which, as supposed, carries on in the direction of the weaker base and the weaker acid. This issue clearly limits the worth of lithium or Grignard reagents when a terminal triple bond is exist, like in equation 2.

1) RC≡C-H   +   C2H5MgBr   (in ether)   -->  RC≡C-MgBr   +   C2H6

2) HC≡C-CH2CH2Br + Mg (in ether)  --> [ HC≡C-CH2CH2MgBr]   -->  BrMgC≡C-CH2CH2H

The terminal alkynes' acidity also plays a role in product determination when vicinal/geminal dihalides go through base provoked bis-elimination reactions. The following instances demonstrate eliminations of this type starting from 1, 2-dibromopentane, prepared from 1-pentene by the addition of bromine. The primary elimination presumably forms 1-bromo-1-pentene, because base attack at the more acidic and less hindered 1 º-carbon should be favored. The elimination second then produces 1-pentyne. If the very strong base that is sodium amide is employed, the terminal alkyne is trapped as its sodium salt, from which it might be released by mild acid treatment. Though, if the weaker base KOH is employed for the elimination, the terminal alkyne salt is not formed reversibly, or is not formed, and the primarily generated 1-pentyne rearranges to the more stable 2-pentyne from an allene intermediate.

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In the case of non-terminal alkynes, sodium and potassium amide, and associated strong bases from 1 º-amines, are capable to abstract protons from carbon atoms adjacent to the triple bond. The resultant allenic carbanions go through fast proton transfer equilibria, leading to the comparatively stable terminal alkyne conjugate base. This isomerisation might be employed to prepare longer chain 1-alkynes, as displayed in the following conversion of 3-heptyne to 1-heptyne. R and R' substituents on the allenic intermediate range from propyl to hydrogen, like the proton transfers proceed.

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