Resonance stabilization acylating agent

The acylating agent R—C(=0)—X is generally higher in energy, the lower the resonance stabilization of its C=0 double bond by the substituent X. This effect is examined in detail in Section 6.2.2. 

SN Reactions at the Carboxyl Carbon The Influence of Resonance Stabilization of the Reacting C=0 Double Bond on the Reactivity of the Acylating Agent... 

The 6th rank in terms of acylation reactivity that is attributed to the acyl imidazolides in Table 6.1 (entry 10) is also plausible. In the acyl imidazolides, the free electron pair of the acylated N atom is essentially unavailable for stabilization of the C=0 double bond by resonance because it is part of the -electron sextet, which makes the imidazole ring an aromatic compound. This is why acyl imidazolides, in contrast to normal amides (entry 2 in Table 6.1) can act as acylating agents. Nevertheless, acyl imidazolides do not have the same acylation capacity as acylpyridinium salts because the aromatic stabilization of five-mem-bered aromatic compounds—and thus of imidazole—is considerably smaller than that of six-membered aromatic systems (e. g., pyridine). This means that the resonance form of the acyl imidazolides printed red in Table 6.1 contributes to the stabilization of the C=0 double bond. For a similar reason, there is no resonance stabilization of the C=0 double bond in N-acylpyridinium salts in the corresponding resonance form, the aromatic sextet of the pyridine would be destroyed in exchange for a much less stable quinoid structure. 

Table 6.1. Acylating Agents in the Order of Decreasing Resonance Stabilization of the Attacked C=0 Double Bond ...

Resonance forms drawn black contribute to the overall stabilization of the acylating agent but not to the stabilization of the 0=0 double bond, which is attacked by the nucleophile. 

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