Nitrogen-hydrogen bonds, acidity

Scheme 4.8 Rebek s trifunctional enolization catalyst including a general base (acridine nitrogen), hydrogen bond donor (carboxylic acid), and a binding group (carboxylate).

Even though ammonia, hydrazine, and hydrazoic acid have nitrogen-hydrogen bonds, their photochemistry is very different. In hydrazoic acid the photochemistry is dominated by the weak HN-N bond while in ammonia and hydrazine the fragmentation of the N-H bond appears to be the most important process. 

Enamines that would formally be derived from ammonium cannot be formed by any process that would expose the product to either acid or base, since the conversion to the imine is exothermic as both the carbon-nitrogen ir-bond (74 kcal mol" 1 cal = 4.18 J) and carbon-hydrogen bond (99 kcal mol" ) are stronger than the carbon-carbon rr-bond (63 kcal mol" ) and nitrogen-hydrogen bond (93 kcal mol" ). However, ethenamine and several substituted analogs have been prepared for spectral analysis (at low temperature) by retro-Diels-Alder reactions (equation 10). ... 

Microwave spectra have been observed for a variety of hydrogen-bonded acid-base complexes such as H20 -HF and N2 -HCI. For the latter species, rotation constants (B/MHz) for i4n15n/35ci, 15n2/35c1 and N2/ C1 isotopic species are 1725.67, 1679.26 and 1639.39. What is the distance between the Cl atom and the more distant nitrogen atom Assume that the NN -HCl stracture is linear. 

Nitriles. Nitriles can be prepared by a number of methods, including ( /) the reaction of alkyl haHdes with alkaH metal cyanides, (2) addition of hydrogen cyanide to a carbon—carbon, carbon—oxygen, or carbon—nitrogen multiple bond, (2) reaction of hydrogen cyanide with a carboxyHc acid over a dehydration catalyst, and (4) ammoxidation of hydrocarbons containing an activated methyl group. For reviews on the preparation of nitriles see references 14 and 15. 

Proline is the only amino acid in Table 27.1 that is a secondary amine, and its presence in a peptide chain introduces an amide nitrogen that has no hydrogen available for hydrogen bonding. This disrupts the network of hydrogen bonds and divides the peptide into two separate regions of a helix. The presence of proline is often associated with a bend in the peptide chain. 

Spectroscopic evidence indicates that protonation of 2-fluoro-and 2-chloro-quinoline is not appreciable in O.OlJf aqueous hydrochloric acid. Protonation becomes evident in more strongly acidic solution in the case of the chloro compound without any accompanying decomposition, but the fluoro compound hydrolyzes to carbostyril under the latter conditions. The hydrolysis is acid-catalyzed, but it is doubtful whether protonation on the heterocyclic nitrogen is responsible, owing to its low basicity (presumably below that for the chloro compound). An alternative explanation in this case would be hydrogen bond formation with fluorine, Ar—F. .. H-O+H2. 

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