Acid-Base Properties and Charge Delocalization

Electron delocalization in an acid, or its conjugate base can have a large impact on both stability and reactivit Consider the following acids and their conjugate bases. 

Obtain energies for each ion and for their correspondin precursors benzoic acid,phenol and cyclohexanol). Us this information to calculate the energy for each of the abov deprotonation reactions. (The energy of proton is given left.) Is the trend consistent with the experimental pKa dat (see table at left) Does deprotonation energy parade charge delocalization in these systems Explain ho electron delocalization affects the reactivity of these acidf 

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Phenol has different chemical properties from those of typical alcohols. Display the electrostatic potential map for phenol. Does this suggest that phenol is likely to be a stronger or weaker acid than any of the compounds discussed above Compare the electrostatic potential map for 4-nitrophenol to that for phenol. What effect does substitution by nitro have on acid strength Explain your result by considering charge delocalization in the conjugate base. Draw all reasonable Lewis structures for phenoxide anion and for 4-nitrophenoxide anion. Which is more delocalized Is this consistent with experimental pKa s ... 

All studies show the zwitterionic nature of the spiropyran merocyanine beyond any doubt and we can further see how the acid-base characteristics of the C9 oxygen and indoline nitrogen are delocalized as alternating charges on the methine bridge, leading to the labile properties of the bridge itself. 

Because of the rather localized negative charge at the phenoHc oxygen atom , the standard dye (44) is capable of specific HBD/HBA and Lewis acid/base interactions. Therefore, in addition to the nonspecific dye/solvent interactions, the betaine dye (44) predominately measures the specific HBD and Lewis acidity of organic solvents. On the other hand, the positive charge of the pyridinium moiety of (44) is delocalized. Therefore, the solvent Lewis basicity will not be registered by the probe molecule (44). If this solvent property is relevant for the system under study, other empirical measures of Lewis basicity should be used cf. Section 7.7. 

Finally, the solvent also interacts with Lewis acid and Lewis base sites that are not directly involved in mutual coordination, thereby altering the electronic properties of the complex. For example, delocalization of charges onto the surrounding solvent molecules causes ions in solution to be softer than in gas phase241. Again, water is particularly effective in this respect because it can act as an efficient electron-pair acceptor and donor. 

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