Hydrogen bonding, effect pyridines

Aromatic nitrogen heterocycles display considerable medium shifts (Section 3.1.4). Carbon-13 shifts of pyridine decrease in a but increase in [i and y position upon addition of water (Fig. 4.12). The dilution effect is explained in terms of intermolecular hydrogen bonding between pyridine and water [99]. 

Hydrogen bonding between pyridine units in a tetrafunctional compound (12) and benzoic acid units in difunctional compounds (13, Figure 6), resulted in the formation of reversible ladder-like polymers (14) or networks (15). These materials are liquid crystalline, and the large drop in material properties above the isotropisation temperature demonstrates that, also here, the interplay between association and liquid crystallinity is instrumental in the process of polymerization.63 DSC studies on these networks reveal a memory effect, resulting in a consistent decrease of crystallinity as the time the material is in the isotropic state increases.64... 

The.effect of the entropy of activation was noted above for the quaternary pyridine salts (280 and 281). In future work, it may also be found to reflect the electrostatic or hydrogen-bonding interactions in transition states of amination reactions and the effect of reversible cationization of an azine-nitrogen. Brower et observed a substantial rate difference between piperidino-dechlorinations of 2-chloropyrimidine in petroleum ether and in alcohol due partly to the higher entropy of activation in the latter solvent (Table III, lines 3 and 4). 

Nucleophilic substitution of pyridines is discussed in previous sections in relation to the following cyclic transition states (Section II, B, 5), hydrogen bonding and cationization (Section II, C), the leaving group (Section II, D,) and the effect of other substituents (Section II, E) and of the nucleophile (Section II, F). 

Another type of steric effect is the result of an entropy effect. The compound 2,6-di-fert-butylpyridine is a weaker base than either pyridine or 2,6-dimethylpyridine. The reason is that the conjugate acid (8) is less stable than the conjugate acids of nonsterically hindered pyridines. In all cases, the conjugate acids are hydrogen bonded to a water molecule, but in the case of 8 the bulky tert-butyl groups restrict rotations in the water molecule, lowering the entropy. 

Many synthetic water-soluble polymers are easily analyzed by GPC. These include polyacrylamide,130 sodium poly(styrenesulfonate),131 and poly (2-vinyl pyridine).132 An important issue in aqueous GPC of synthetic polymers is the effect of solvent conditions on hydrodynamic volume and therefore retention. Ion inclusion and ion exclusion effects may also be important. In one interesting case, samples of polyacrylamide in which the amide side chain was partially hydrolyzed to generate a random copolymer of acrylic acid and acrylamide exhibited pH-dependent GPC fractionation.130 At a pH so low that the side chain would be expected to be protonated, hydrolyzed samples eluted later than untreated samples, perhaps suggesting intramolecular hydrogen bonding. At neutral pH, the hydrolyzed samples eluted earlier than untreated samples, an effect that was ascribed to enlargement... 

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