Acyl cation, electrostatic potential

Acyl azide, amines from, 935 Acyl carrier protein, function of, 1 140 Acyl cation, electrostatic potential map of, 558... 

The electrophilic site of an acyl cation is its acyl carbon. An electrostatic potential map of the acyl cation from propanoyl chloride (Figure 12.8) illustrates nicely the concentration of positive charge at the acyl carbon, as shown by the blue color. The mechanism of the reaction between this cation and benzene is analogous to that of other electrophilic reagents (Figure 12.9). 

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).

DNA sequencing and. 1113 Electrospray ionization (ESI) mass spectrometry, 417-418 Electrostatic potential map, 37 acetaldehyde, 688 acetamide, 791,922 acetate ion. 43. 53, 56, 757 acetic acid. 53. 55 acetic acid dimer, 755 acetic anhydride, 791 acetone, 55, 56. 78 acetone anion, 56 acetyl azide, 830 acetyl chloride, 791 acetylene. 262 acetylide anion, 271 acid anhydride, 791 acid chloride, 791 acyl cation, 558 adenine, 1104 alanine, 1017 alanine zwitterion, 1017 alcohol. 75 alkene, 74, 147 alkyl halide, 75 alkyne. 74... 

The electrostatic potential map of propanoyl cation in Figure 12.4 illustrates the posihve character of the acyl carbon, and it is this carbon that is the reactive site in electrophilic aromahc subsdtution (Mechanism 12.5). 

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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