Substituent effects electron-withdrawing substituents

Hydrolysis reactions involving tetrahedral intermediates are subject to steric and electronic effects. Electron-withdrawing substituents faciUtate, but electron-donating and bulky substituents retard basic hydrolysis. Steric effects in acid-cataly2ed hydrolysis are similar to those in base-cataly2ed hydrolysis, but electronic effects are much less important in acid-cataly2ed reactions. Higher temperatures also accelerate the reaction. 

Substitution on the carbon atoms of the azole rings has the expected effect electron-withdrawing substituents such as nitro or halogen increase the reactivity of the azolides, whereas alkyl substituents lead to a decrease in transacylation rates. 101... 

The reaction rates of diisocyanates are strongly influenced by their molecular structure. The reactivity of isocyanate groups is enhanced by adjacent electron-withdrawing substituents. Aromatic rings are very effective electron withdrawing groups, and it is for this reason that the majority of commercial diisocyanates are aromatic. Many of the diisocyanates used commercially consist of mixtures of isomers. Some of the more important commercial diisocyanates are illustrated in Fig. 25.6. Diisocyanates must be handled carefully to avoid exposing workers to their hazardous vapors. 

The Dimroth-type rearrangements are sensitive to both solvent and electronic effects. Electron-withdrawing substituents on the pyridine nucleus (—HNC=S or—NC=SH —NCNH2) facilitate the rearrangement, while the solvent effect is based on empirical observations. For example, 5-amino-l,4-dihydro-7//-[l,2,3]triazolo[4,5-6]pyridine-7-thione 3-riboside (258) is converted in DMF... 

Azides and tetrazoles usually equilibrate slowly at room temperature and the position of the equilibrium, which varies with temperature and solvent, is also susceptible to steric and electronic effects, electron-withdrawing substituents favouring the azide tautomer. The azinyl-azide/tetrazole tautomeric equilibrium, which has been extensively investigated and reviewed [9, 10], also arises when the C = N double bond comprises part of a heterocyclic ring, as exemplified by 2-azidoadenosine (5a, R = J3-d-ribofuranosyl) synthesized for use, as the diphosphate, as a photoaffinity reagent for the ADP receptor [11]. The azide was found to equilibrate with one or both of the two possible tetrazoles (5b and 5c) with an azide. tetrazole ratio of 1 1 at pH 7.0, and in contrast to the azide, which was readily photolyzed at 270nm, the tetrazole tautomer was resistant to photolysis. Azides that exist predominantly as tetrazoles may thus exhibit different... 

The downfield shift caused by chlorine is what we might have expected from its inductive effect electron withdrawal lowers the electron density in the vicinity of the proton and thus causes deshielding. The effect of a substituent on the chemical shift is unquestionably the net result of many factors yet we shall often observe chemical shifts which strongly suggest that an inductive effect is at least one of the factors at work. 

Cr(CO)3 is an effective electron-withdrawing group. The coordination of this organometallic unit to the arene substituents of the isophthalamide in 89 increases the acidity of the NH protons and hence their affinity for anions. 

Table 19 3 lists the ionization constants of some substituted benzoic acids The largest effects are observed when strongly electron withdrawing substituents are ortho to the carboxyl group An o nitro substituent for example increases the acidity of benzoic acid 100 fold Substituent effects are small at positions meta and para to the carboxyl group In those cases the values are clustered m the range 3 5-4 5... 

Carboxylic acids are weak acids and m the absence of electron attracting substituents have s of approximately 5 Carboxylic acids are much stronger acids than alcohols because of the electron withdrawing power of the carbonyl group (inductive effect) and its ability to delocalize negative charge m the carboxylate anion (resonance effect)... 

Electron releasing substituents attached to the ring have a negligible effect on the acidity of phenols Strongly electron withdrawing groups increase the acidity The compound 4 nitro 3 (tnfluoromethyl)phenol for example is 10 000 times more acidic than phenol... 

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