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Ch_322b_15.05 - has two contributing resonance structures(2...

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Friedel-Crafts Acylation Acylation is the introduction of an acyl group, R-C- , into a structure. Two important acyl groups are: O = The Friedel-Crafts acylation reaction attaches an acyl group to an arene. For benzene, a Lewis acid catalyst is required to generate the electrophile from an acyl halide reactant. + RCCl O = AlCl 3 C-R O = + HCl acid chloride a phenyl ketone CH 3 C- O = C O = acetyl benzoyl
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Preparation of Acid Chlorides The acid chlorides are readily prepared from the corresponding carboxylic acids with thionyl chloride (SOCl 2 ) , or phosphorous pentachloride (PCl 5 ) . Examples CH 3 COH O = acetic acid + SOCl 2 thionyl chloride 80 o C CH 3 C Cl O = SO 2 + HCl + acetyl chloride (80-90%) COH O = benzoic acid + PCl 5 phosphorous pentachloride C Cl O = benzoyl chloride (~90%) + POCl 3 + HCl phosphorous oxychloride
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A Mechanism for the Friedel-Crafts Acylation Reaction (1) Generation of the Electrophile R-C-Cl: :O: = : : + AlCl 3 Lewis acid Lewis base R-C=O : : + R-C O: + acylium ion + AlCl 4 - R-C-Cl :O: = : : AlCl 3 - + R-C-Cl :O: = : : AlCl 3 - + An acylium ion is generally thought to be the electrophilic
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Unformatted text preview: has two contributing resonance structures. (2) Electrophilic Attack (3) Deprotonation and Re-aromatization H C = :O: R + etc. arenium ion H C :O: R + + :Cl-AlCl 3 :-C-R O + HCl + AlCl 3 Complexation of the Ketone Product The ketone forms a Lewis complex with the AlCl 3 that persists in the reaction mixture until the work up with water. Accordingly, one full equivalent of AlCl 3 is required for the reaction. This complexation deactivates the aromatic ring towards further electrophilic attack (discussed later), so only monsubstitution occurs. C-R :O: = + AlCl 3 Lewis base Lewis acid C=O R : AlCl 3-+ Lewis complex work up H 2 O C-R :O: + Al(OH) 3 + HCl Quiz 15.05 Draw the contributing resonance structures of the acylium ion produced in the reaction below. R-C-Cl O = + AlCl 3 : : R-C=O :: + R-C O : +...
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