The equilibrium acidity of phenylacetylene

Abstract

Equilibrium acidities based on an absolute scale are reported for 2,4-dinitro-, 2,6-dichloro-4-nitro-, 4-nitro-, 2,6-dichloro-, and 2,4-dichloro-aniline indicators in dimethyl sulphoxide (Me₂SO) solution (pK values: 15.85, 17.4, 20.9, 24.8, and 26.2, respectively). Redetermination of the acidity of phenylacetylene in Me₂SO against a new indicator, 9-(p-methoxyphenyl)xanthen (pK= 28.4) gave pK= 28.7, which agrees well with a previous value (28.8) obtained using two other indicators. This value was not affected appreciably by changing the counterion from K⁺ to Na⁺, by the presence of 18-crown-6, or by adding small amounts of water. The calculated pK of PhCCH of 21 in water is discussed in light of the higher value in Me₂SO.