Why is AlCl3 a lewis acid ?

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Why is AlCl3 a lewis acid ?

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Answer 1 · 2015-11-12T09:10:02

Due to the electronegativity differences between $Cl$ $"Cl"$ ( $3.16$ $3.16$ ) and $Al$ $"Al"$ ( $1.61$ $1.61$ ), $Cl$ $"Cl"$ makes a good electron-withdrawing group in ${AlCl}_{3}$ $"AlCl"_3$ .

Furthermore, $Al$ $"Al"$ must access both its $2 s$ $2s$ and its three $2 p$ $2p$ orbitals to bond, so it uses $s p^{3}$ $sp^3$ hybridization (one $2 s$ $2s$ and three $2 p$ $2p$ orbitals), giving it four bonding orbitals (one of which is empty as ${AlCl}_{3}$ $"AlCl"_3$ ). This allows it to form a fourth bond and acquire a tetrahedral structure as ${AlCl}_{4}^{-}$ $"AlCl"_4^(-)$ .

With one empty orbital and three electron-withdrawing $Cl$ $"Cl"$ atoms attached, the compound is thus an electron-acceptor at the $Al$ $"Al"$ center. By definition, that is a Lewis Acid.

Here is an example of Friedel-Crafts Acylation that shows the Lewis Acid behavior of $Al$ $"Al"$ in ${AlCl}_{3}$ $"AlCl"_3$ :

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Why is AlCl3 a lewis acid ?

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