Characterization tests alkynes

Alkynes and dienes respond to characterization tests in the same way as alkenes they decolorize bromine in carbon tetrachloride without evolution of hydrogen bromide, and they decolorize cold, neutral, dilute permanganate they are not oxidized by chromic anhydride. They are, however, more unsaturated than alkenes. This property can be detected by determination of their molecular formulas (CnH2n-2) and by a quantitative hydrogenation (two moles of hydrogen are taken up per mole of hydrocarbon). 

Two common types of unsaturated compounds are alkenes and alkynes, characterized by the carbon-carbon double and triple bonds, respectively, as the functional group. The two common qualitative tests for unsaturation are the reaction of the compounds with bromine in dichloromethane and with potassium permanganate. In both cases, a positive test is denoted by decoloration of the reagent. There are no simple, direct ways to prepare solid derivatives of unsaturated aliphatic compounds having no other functional groups. 

The competitive experiments were carried out with a series of terminal al-kynes characterized by similar electronic properties but differing in portions of the molecule remote from the reaction site. For aliphatic alkyne, the three substrates ethynyl cyclohexane, 1-octyne (which represents an acyclic isomer of the former substrate), and 1-dodecyne were tested, with the free catalyst compared to the encapsulated catalyst. In the former case, after a short induction time, the initial reaction rate for the three substrates showed similar behavior for 1-dodecyne and 1-octyne, while the cyclic isomer, being slightly more electron rich, reacted 1.5 times faster (Fig. 7.8a). Encapsulation of the catalyst led to a magnification of the favorable hydration of the cyclic substrate that reacted more than twice as rapidly as the longer substrate. It is likely that extended linear substrates, that in their extended conformation are approximately 1.4 and 2.1 times longer than ethynyl cyclohexane, have to fold to better complement the residual space left available within Ihe cavity occupied by the catalyst. 

In order to exclude folding effects, three rigid aromatic terminal alkynes like phenylacetylene, 4-methyl-phenylacetylene, and 4-t-butyl-phenylacetylene, characterized by the same general shape but different lengths and electronic properties, were tested with the free and the encapsulated catalyst. In the bulk... 

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