Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
Formation constants, K, for hydrogen bonding of weak C—H donors with hexamethylphosphoric triamide were measured in cyclohexane at 35°C by PMR spectroscopy; data were analyzed by the Higuchi method. Donors include terminal alkynes and series of polyfluoro-, polychloro- and polybromobenzenes. The K values for many of these are too small to measure in CCl4. For RC≡CH, K decreases in the order R = BrCH2 〉 C6H5 〉 C2H5O 〉 tert-butyl. For polyhalobenzenes, K is generally larger for F than Cl or Br, and meta halogens facilitate hydrogen bonding most effectively. Among tetrahalobenzenes, 1,2,3,4-isomers show the least evidence of 2:1 complex formation. Pentachlorobenzene has a smaller K than its 1,2,3,4-tetrachlor analog even after statistical correction; for C6HX5, K decreases in the order F 〉 Cl 〉 Br; the K values for 1,3,5-trihalobenzenes are closely similar; and 1,3,5-tribromo-2,4-dimethylbenzene shows no evidence of hydrogen bonding by PMR. The last three observations suggest that buttressing is important in the C6HX5 structure, at least with Cl and Br. A value of K for benzene was measured, suggesting for the first time that the H atoms of benzene engage in hydrogen bonding. The result compares well with values extrapolated from data for polyfluoro- but not polychlorobenzenes, again probably because of buttressing in the latter.
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