why does aluminium bromide dissolve in benzene but aluminium fluoride doesnt?

Question

is it because AlBr3 is a better lewis acid and forms an acid base adduct with benzene or something else?

Answer 1

Beautiful - when the asker answers his own question.

Yes, since Fluorine is so electronegative it holds onto its electrons and actually pulls electron density away from Aluminum, while in Bromine this is not the case.

AlBr3, is still a STRONG lewis acid and complexes with benzene - and during that process it solubilizes itself.

Answer 2

Liquid benzene is made up of covalently bonded molecules. The electronegativity of C is 2.5 and H is 2.1. The difference in electronegativities between C and H is 0.4 and between C-C atoms is 0.Therefore the bonds are all covalent. Benzene will dissolve covalent molecules very well and ionicly bonded compounds not well or at all.
The electronegativity of Al is 1.5 while Br is 2.8 and F is 4.0.
If the electronegativity difference across a bond is less than 1.7 it is considered to be covalent. If it is greater than 1.7 it is considered to be ionic in nature.
The difference in electronegativity for the
Al-Br bond is 1.3 and is therefore a covalent bond.
The electronegativity difference for Al-F is 2.5 and is therefore an ionic bond.
The rough rule is that Like dissolves like. Therefore covalent benzene will dissolve covalent aluminium bromide but will not dissolve ionic aluminium fluoride.

Answer 3

The answers you received sound very plausible. Let me throw a fly into the ointment. What about match up of molecular size?

Is AlBr3 closer in size to Benzene than AlF3. Remember in covalent solvation size match up becomes a powerful influence.

I phrased this as a question rather than an answer because I don’t know this for sure, but it is something for you to think about. Some situations might encourage deeper consideration, this sound like one.Great question.

Answer 4

Aluminium bromide has such a large covalent character that it is better to think of it as a covalent, non-polar molecule. Thus it is not surprising that it is soluble in non-polar benzene.

Aluminium fluoride is much more ionic in character.