why is water not compatible with lewis acids in a friedel-crafts alkylation [closed]








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I know that it destroys the lewis acid character but that doesn't tell me anything.




The catalyst used for a Friedel-Crafts reaction is anhydrous aluminum chloride $\ce{AlCl3}$. But this chloride reacts extremely violently with water. Try to put maybe 0.1 g $\ce{AlCl3}$ in a test tube, and add one or two drops of water. You'll soon observe an extremely violent and exothermic reaction. It produces a huge amount of gaseous $\ce{HCl}$ and the tube becomes so hot that you cannot hold it any more with your fingers. The equation is $$\ce{AlCl3 + 3H2O -> Al(OH)3 + 3 HCl}$$ If you continue adding water, the gaseous $\ce{HCl}$ is soon redissolved into this water, giving a solution containing now $\ce{H3O+}$ and $Cl^-$ ; and this solution redissolves $\ce{Al(OH)3}$ in the reaction : $$\ce{Al(OH)3 + 3 H3O+ -> [Al(H2O)6]^{3+}}$$ The surprising consequence of this operation is that evaporation of this solution produces $\ce{AlCl3·6H2O}$ or $\ce{[Al(H2O)6]Cl3}$. And this compound has no activity for a Friedel-Crafts reaction. Friedel-Crafts needs anhydrous $\ce{AlCl3}$


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