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Formation constants in CH hydrogen bonding. 2. Alkynes and polyhalobenzenes with hexamethylphosphoric triamide in cyclohexane solution
Author(s) -
Lorand John P.,
Nelson Jonathan P.,
Gilman R. Dean,
Staley Kathy L.,
Chambers Jeffery R.,
Kirk Harold D.,
Moeggenborg Kevin J.,
Farlow David L.
Publication year - 1990
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.610031007
Subject(s) - chemistry , cyclohexane , benzene , hydrogen bond , medicinal chemistry , halogen , hydrogen , benzene derivatives , computational chemistry , stereochemistry , molecule , organic chemistry , chemical synthesis , biochemistry , alkyl , in vitro
Formation constants, K , for hydrogen bonding of weak CH 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 CCl 4 . For RCCH, K decreases in the order R = BrCH 2 > C 6 H 5 > C 2 H 5 O > 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 C 6 HX 5 , 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 C 6 HX 5 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.