Trimethylamine oxide, dipole moment

Purine, 6-bromo-9-/3-D-(2,3,5-tri-0-acetyl)ribofuranosyl-synthesis, 5, 598 Purine, 6-carboxy-reactions, 5, 549 Purine, 8-carboxy-reactions, 5, 549 Purine, 2-chloro-reactions, 5, 561 synthesis, 5, 597 Purine, 6-chloro-alkylation, 5, 529 glycosylation, 5, 529 oxidation, 5, 539 3-oxides reactions, 5, 554 synthesis, 5, 595 reactions, 5, 561, 595 with ammonia, 5, 562 with fluorides, 5, 563 with trimethylamine, 5, 562 9- -D-ribofuranoside synthesis, 5, 560 synthesis, 5, 597, 598 Purine, 8-chloro-amination, 5, 542 Purine, 6-chloro-8-ethoxy-synthesis, 5, 591 Purine, 6-chloro-9-ethyl-dipole moment, 5, 522 Purine, 6-chloro-2-fluoro-riboside... 

Since the dipole moments of cyclopropenones are enlarged with respect to simple ketones and compare to other polar systems, e.g. trimethylamine oxide in Table 4, there seems to be evidence for considerable charge separation in the carbonyl group, which was expressed in terms of a cyclopropenium oxide contribution to the ground state. 

A final difference between amine oxides and phosphine oxides lies in the polarity of the molecules. The dipole moment of trimethylamine oxide is 16.7 X 10 C m (5.02 D) compared with 14.6 x 10 C m (4.37 D) for triethylphosphine oxide. A consequence of this polarity is the tendency of the amine oxides to form hydrates, R3NO H2O, and their greater basicity relative to the phosphine oxides. 

A striking difference between pyridines and their A-oxides is the susceptibility of the latter to electrophilic nitration. This can be understood in terms of mesomeric release from the oxide oxygen, and is parallel to electron release by oxygen and hence increased reactivity towards electrophilic substitution in phenols and phenoxides. One can find support for this rationalisation by a comparison of the dipole moments of trimethylamine and its A-oxide, on the one hand, and pyridine and its A-oxide, on the other the difference... 

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