Physical Properties of Alkynes

Like alkanes and alkenes, the first three members (ethyne, propyne, 1-butyne) are gases, the next eight are liquids, and the higher alkynes are solids; all are colourless and odourless except acetylene, which has a garlic-like odour due to trace phosphine/arsine impurities from its industrial preparation. They are only slightly soluble in water (the triple bond is only weakly polar) but dissolve readily in organic solvents such as benzene, acetone and ethyl alcohol. Boiling points rise steadily with chain length, and are slightly higher than the corresponding alkanes/alkenes of the same carbon count because the more polarisable triple-bond electron cloud strengthens van der Waals attraction between molecules; melting points, like alkanes, show a characteristic odd-even alternation caused by differences in how odd- and even-c …