Heterogenous Catalyst Problem

I need help with these two similar questions:
https://www.thestudentroom.co.uk/album.php?albumid=24266&attachmentid=935416
https://www.thestudentroom.co.uk/album.php?albumid=24266&attachmentid=935418

The answer to the first question is C and the answer to the second one is also C.
the second answer is : it acts as a heterogeneous catalyst by providing active sites on which the
reaction occurs.
So, similarly why the first answer is not D: ''iron is a transition metal and acts as a heterogeneous catalyst in the reaction.''
I'm stuck.

Reply 1

charco

Study Forum Helper

Original post by tahmidbro

You are being tested on the mechanism of Friedel Crafts electrophilic substitution.

The first answer is C, because of the mechanism of the Lewis acid catalysis.

There is not simple absorption of the reactants on the catalyst surface. The chlorine reacts with iron to form iron(III) chloride, this then interacts with more chlorine to form the Lewis acid FeCl₄^----Cl⁺ which then acts as the electrophile.

The benzene ring electrons are attracted to the Cl⁺ , which starts the electrophilic substitution.

In summary, the catalytic behaviour of iron in the Friedel Crafts substitution is not heterogeneous.