How do metal halides work?

Maybe this is referring to compounds like #"AlCl"_3# and #"FeBr"_3#.

• Generally, since the more electronegative halogens (#"Cl"# and #"Br"#) are more electronegative than the metal, the metal center becomes Lewis acidic.

• Furthermore, the halogens own a significant portion of the molecular orbitals, because their orbitals are far away in energy from the metal's orbitals. This allows a fourth halide on a negatively-charged metal halide to act as a nucleophile.

All of this is to the extent that we can expect normally strange things to occur.

FRIEDEL-CRAFTS ACYLATION

For example, #"Cl"# on an acyl chloride coming off as a #"Cl"^(-)# nucleophile, leaving behind an acylium ion in the Friedel-Crafts Acylation, which adds an acyl group onto a benzene ring:

Although, formation of the acylium ion is the slow step (the rate-determining step), so it's still difficult like we expect it to be.

FRIEDEL-CRAFTS ALKYLATION

A similar process occurs in the Friedel-Crafts Alkylation, which can use #"FeCl"_3# with #R-"Cl"# (or #"FeBr"_3# with #R-"Br"#) to add the alkyl group onto a benzene ring.

In this case, you form the normally-unstable primary carbocation, which clearly should be the slow step, as we almost never see a halide leaving group depart a primary alkyl halide and leave behind a primary carbocation.

You should see the similarities in metal halide behavior between these two reactions:

• The metal center is Lewis acidic, convincing normally unstable cations to occur more easily.
• The halogen owns a significant portion of the molecular orbitals, so the fourth halide can act as a nucleophile and come straight off a four-halogen metal halide.