Which of the following laboratory tests can
be performed for terminal alkynes?
ps: A.
Na in ether
B.
All of the mentioned options
+
Ammonium
C.
Silver
nitrate
hydroxide
+
Ammonium
D.
Cuprous chloride
hydroxide
Answers
Answer:
important reaction of ethyne and 1-alkynes is salt (“acetylide”) formation with very strong bases. In such reactions the alkynes behaves as an acids in the sense that they give up protons to suitably strong bases:

Water is too weak a base to accept protons from alkynes; consequently no measurable concentration of H3O⊕H3O⊕ is expected from the ionization of alkynes in dilute aqueous solutions. Therefore we have no quantitative measure of 1-alkyne acidity in aqueous solution other than that it probably is about 10101010 times less acidic than water, as judged from measurements in other solvents to be discussed shortly. In the gas phase, however, the situation is reversed, and ethyne is a stronger acid than water:

This reversal is of little practical value because organic reactions involving ions normally are not carried out in the gas phase. However, it should alert us to the tremendous role that solvents play in determining acidities by their abilities (some much more than others) to stabilize ions by the property known as solvation. (Section 11-8A.)
Liquid ammonia is a more useful solvent than water for the preparation of 1-alkyne salts. A substantial amount of the alkyne can be converted to the conjugate base by amide anions (potassium or sodium amide) because a 1-alkyne is a stronger acid than ammonia.

The acidity of the terminal hydrogen in 1-alkynes provides a simple and useful test for 1-alkynes. With silver-ammonia solution (AgNO3AgNO3 in aqueous ammonia), 1-alkynes give insoluble silver salts, whereas disubstituted alkynes do not:

The silver “acetylides” appear to have substantially covalent carbon-metal bonds and are less ionic than sodium and potassium alkynides. Silver-ammonia solution may be used to precipitate 1-alkynes from mixtures with other hydrocarbons. The 1-alkynes are regenerated easily from the silver precipitates by treatment with strong inorganic acids. It should be noted, however, that silver alkynides may be shock sensitive and can decompose explosively, especially when dry.
11-8A Thermodynamics of Solvation of Ions and Its Importance
Some idea of the importance of solvation can be gained from the calculated ΔHΔH for the following process:
Na+(g)+Cl−(g)→Na+(aq)+Cl−(aq)(11.8.1)(11.8.1)Na+(g)+Cl−(g)→Na+(aq)+Cl−(aq)
with DeltaHo=−187kcalDeltaHo=−187kcal.
The solvation energies of ions are so large that relatively small differences for different ions can have a very large effect on equilibrium constants. Thus, the ratio between the relative acidities of ethyne and water in the gas phase and in water of 10121012 corresponds at 25o25o to an overall ΔG0ΔG0 difference in solvation energies of approximately 16kcal16kcal, which is less than 10%10% of the total solvation energies of the ions. Further difficulties arise because of differences between solvation energies and interactions between the ions in different solvents. Thus the acidities of 1-alkynes relative to other acids have been found to change by a factor of 10111011 in different solvents. For this reason, we must be particularly careful in comparing the rates and equilibrium constants of ionic reactions to take proper account of solvation and ion interaction effects.
An excellent rule of thumb is that, other things being equal, large ions are more stable than small ions in the gas phase, with the opposite being true in polar solvents, where small ions are more strongly solvated (thus more stable) than large ions. For comparison,

From (1) minus (3), the solvation energy of gaseous Li⊕Li⊕ is 47kcal mol−147kcal mol−1 greater than K⊕K⊕; and from (1) minus (2), that of F⊖F⊖ is 35kcal mol−135kcal mol−1 greater than that of I