[met1111]

Hi everyone, Im brand new here but Ive searched everywhere for help on a question about aromatic substitution.




After AlCl3 rips off the chlorine, I'm unsure whether or not a hydride shift would occur and the pi electrons of the benzene ring would attack the carbon NEXT to the carbonyl group or no hydride shift occurs and the pi electrons of benzene attack the carbonyl carbon.

Any help or clues are appreciated.

Ps. Sorry for the rough sketch.

[Arctic-Nation]

With a Friedel-Crafts acylation, hydride shift never takes place. Just draw the product of that shift and you'll see why: you get an aldehyde with a positive charge on the alpha-carbon, which is arguably even more unstable than a carbonium ion (which has some resonance stabilization). Hydride shifts only happen when the resulting product is more stable than the starting reagent (like going from a primary carbocation to a secondary or tertiary one).

[met1111]

This should be the answer then, thanks for the help.

[Schrödinger]

Nothing new, but just adding a little bit more detail to Arctic-Nation's reply.

When you have an alpha Carbon carrying a positive charge, try stabilising it using the pi bond on Oxygen...You will end up destabilizing the molecule... O carries +ve charge, no octet, and it couldn't get any worse.


@met1111 : Looks like you've got side products

[met1111]

The side products, I think are unimportant in the question I was asked, but thanks again for the extra explanation. After going back through my notes I did realize that friedel crafts ALKylation can undergo rearrangement whereas ACYLation cant.

I do, however have another question if anyone is around today to answer it:



I know that the butyl group is an ortho/para director that activates the ring, but my question is around whether or not SO3H would actually add to the Para position or the more reactive benzyllic position. I'm leaning towards the benzyllic attack because it doesnt require for the ring's aromaticity to be broken and then rearomatize which would occur if it attacked the para position.

Thanks again.

[stewie griffin]

No you will not get attack at the benzylic position. The aromatic ring is serving as the nucleophile here so the carbocation that forms will have to be inside the ring.
You are correct in understanding that benzylic carbocations are one of the more stable carbocations... but you're overthinking it and/or confusing that with standard electrophilic aromatic substitution chemistry.
Think of it this way.. the benzylic position has no lone pairs or pi electrons, so it can't be a nucleophile in this reaction. It also isn't electron deficient so even if there were some nucleophile present, the benzylic position isn't electrophilic and wouldn't be attacked.

[met1111]

Perfect, thanks for the explanation, again it's very much appreciated. Definitely clears things up for me.

[Schrödinger]

The side products, I think are unimportant in the question I was asked, but thanks again for the extra explanation. After going back through my notes I did realize that friedel crafts ALKylation can undergo rearrangement whereas ACYLation cant.

lol, I was referring to that face you had drawn