[kaslee]

for example we'd want to do chlorination on para or meta xylene. which would yield the product at a faster rate?

initially, i thought it was para-xylene since there can only be one regiochemical outcome for the product.. but meh that wasn't really thought out

then i drew out the sigma complex for para and meta xylene and saw that the sigma complex for meta-xylene had more stability since 2 out of 3 of the resonance structures had the positive charge adjacent to the methyl group (i.e. there are more tertiary carbocations than secondary carbocations). i was thinking that since the sigma complex had additional stability, the activation energy would be lower, hence the a faster reaction

BUT then again, i searched up online the stability of all isomers of xylene (ortho, meta, para) and found the meta-xylene is most stable. If meta-xylene is most stable, does that mean it won't readily react as para-xylene?

now im really just confused between the two. was any of my guesses close to correct? can someone offer a better explanation? thanks

[rolnor]

In m-xylene the 2 and 4-position is activated by 2 methyl-groups and should be more reactive.

[Enthalpy]

Is the chlorination expected to happen at the methyls?

[rolnor]

No, that was not my thought, the methyls are activating the aromatic ring and are orto-para directing. But its possible to chlorinate the methyls, that would be free radical substitution, yes?

[wildfyr]

Two unrelated reactions here Enthalpy. One is electrophilic aromatic substituion. It uses a diatomic halogen and a lewis acid such as FeCl3, and reacts via a two electron mechanism to form an aryl-halogen bond.

The other is radical halogenation. It uses a radical source (or UV light) and a diatomic halogen to react on an activated alkyl group of some kind (such as benzyl or α-carbonyl)

[Enthalpy]

Maybe Kaslee could detail more which reaction he considers with "chlorination on para or meta xylene". I have vaguely in mind that hydrogen atoms bind strongly with aromatic carbons and weakly with carbons next to an aromatic ring.

[wildfyr]

Yes, but a better technical way to describe it would be that an aromatic C-H bond is much stronger than a benzylic C-H bond. Or even to say that the benzylic hydrogens are more acidic than aromatic C-Hs (by a couple tens of orders of magnitude pKa).

This topic is titled electrophilic aromatic substitution, but you're right, his description is a little fuzzy.

[rolnor]

But what about the question, wich is faster, para or meta?

[wildfyr]

We are trying to be sure if OP is asking about EAS not benzylic bromination.

[rolnor]

But if you read the question, it seems like electrophilic aromatic substitution?

[Enthalpy]

Oops, I had missed the title. My bad!