Halogenation and the Role of Lewis Acids

Consider the compound AlBrs. The central atom in this structure is aluminum. Aluminum is in Column 3A of the periodic table, and therefore, it has three valence electrons. It uses each of these electrons to form a bond, which is why we see three bonds to the aluminum atom in AlBrs  

But you should notice that the aluminum atom does not have an octet If you count the electrons aronnd the aluminum atom, there are only six electrons. That means that aluminum has one empty orbital. That empty orbital is able to accept electrons. In fact it will exhibit a tendency to accept electrons because that would give aluminum an octet of electrons  

Therefore, we call AlBrj a Lewis acid. To put it simply, Lewis acids are just compounds that can accept electrons. Another common Lewis acid is FeBrs  

Now let s consider what happens when Br2 is treated with a Lewis add. The Lewis acid can accept electrons from Br2i 

The resulting complex can then serve as a source of Br+, like this  

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