Acyl cation, electrostatic potential Friedel-Crafts acylation reaction

Friedel-Crafts acylation involves electrophilic attack by acyl cation (CHsCO ) on the ring, and the ring s electronic character should indicate its susceptibility to attack. Compare electrostatic potential maps of ferrocene and acetylferrocene. Which molecule contains the most electron-rich ring Which acylation reaction should be faster Does an acetyl substituent enhance or diminish ring reactivity What should be the major product when ferrocene is combined with one equivalent of acetic anhydride ... 

Figure 16.9 Mechanism of the Friedel-Crafts acylation reaction. The electrophile is a resonance-stabilized acyl cation, whose electrostatic potential map indicates that carbon is the most positive atom (blue).

The key reactive intermediates in Friedel-Crafts acylations are acylium cations. These spedes can be formed by the reaction of acyl halides with aluminum chloride. The Lewis add initially coordinates to the carbonyl oxygen because of resonance (see Exadse 2-11). This complex is in equilibrium with an isomer in which the aluminum chloride is bound to the halogen. Dissociation then prodnces the acylium ion, which is stabilized by resonance and, unlike alkyl cations, is not prone to rearrangements. As shown in the electrostatic potential map of the acetyl cation in the margin, most of the positive charge (blue) resides on the carbonyl carbon. 

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