Nucleophilic reaction resonance effects

The least squares value for the p constant obtained by this procedure is +6.2 it wiU be obviously subject to change as more meta and epi substituents become available. Only the cata-NO group was excluded from the above plot because it causes a strongly enhanced resonance effect in nucleophilic substitution (Section IV,C, l,a) and an anomalous effect of uncertain origin in the dissociation of carboxylic acids. It can be assumed that the reaction constant for 4-chloro-... 

One further comparison aromatic aldehydes, such as benzaldehyde, are less reactive in nucleophilic addition reactions than aliphatic aldehydes because the electron-donating resonance effect of the aromatic ring makes the carbonyl group less electrophilic. Comparing electrostatic potential maps of formaldehyde and benzaldehyde, for example, shows that the carbonyl carbon atom is less positive (less blue) in the aromatic aldehyde. 

The mechanism of imine formation is standard, as seen in the other examples. The cyclization reaction is then like the Mannich reaction, attack of an enol on to the iminium cation. This time though, the nucleophile is provided by the resonance effect from the phenol system. 

Similarly, those reactions that are strongly assisted by withdrawal of electrons from the reaction site, such as nucleophilic aromatic substitution, give a poor fit to a Hammett plot for the substituents that are capable of withdrawing electrons by delocalization (—N02, —N2 , —C=N, and so on). An example is Reaction 16 in Table 26-7. To correlate reactivity data with structures where strong resonance effects operate, different sets of substituent constants are required.1... 

Numerous authors189-191 have compared the reactions of Fischer carbene complexes with nucleophiles to the corresponding reactions of carboxylic esters.183,185-187 Our view is that there is much more resemblance between the reactions of Fischer carbene complexes and SNV reaction than between the reactions of Fischer carbene complexes and reactions with esters because in the latter reactions there are no strong resonance effects. 

Acid-catalysed solvolysis of N-aryl phosphoramidates is characterised by the negative value of the reaction constant (p = -1.2).These inversed substituent effects illustrate two points discussed before. First, if the N-protonated form repre -sents the reactive intermediate in solvolysis of (2), much stronger dependence of the protonation preequilibrium on the effect of N-substitution is expected. Secondly, if the resonance effects are poorly transmitted to the P atom through the -NH- bridge, structural variation in the N-aryl substituent should have weak effect upon the ability of phosphorus to accept a nucleophile. 

In particular, called resonance polar effect [Taft, 1956] is defined for any benzene derivative where there is no direct conjugation between substituent and reactive it can be considered constant for a particular solvent, therefore expressing resonance interactions between substituent and skeletal group. 6r is usually referred to as the effective resonance constant and of hold for electrophilic and nucleophilic reaction series, respectively. 

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