[jkenne25]

Hi all

I have a question in my latest paper and I cannot for the life of me understand it. Here goes:

There is a benzene ring with a chlorine attached to the top carbon. As far as I was aware, chlorine was a ortho/para directing group, but the question states otherwise.

It asks why the para isomer is the minor product, and to suggest what the major product would be when the chlorinated ring reacts with:

CH3OCH2CH2Cl and AlCl3

Am I completely missing the point here? Has it something to do with chlorine being involved in both the substituent and the directing group?

Any help whatsoever would be greatly appreciated.

John

[Dan]

There is a benzene ring with a chlorine attached to the top carbon.

Chlorobenzene? Nomenclature exists for a reason.

Is the question asking:

Why is the major product not para substituted?

or

Why is the major product not this:



These are two different questions, I suspect it is the second question that is being asked.

Can you post the question exactly as you have it in front of you?

[hblondie26]

The question states that an isomer (which is the minor product) is formed - it is chlorobenzene with CH2CH2OCH3 attached at the para position, but it says that actually another isomer will be the major product. Write the structure of the isomer you think is most likely to form and briefly explain how it is formed. I was thinking it is to do with the resonance forms and the carbocation rearranging to form a more stable tertiary cation - CH3CHOCH3? and it would be on the para position....or am I completely wrong?

PS> I have spent 7 hours on this today!
 Thanks in advance for any help

[Dan]

I was thinking it is to do with the resonance forms and the carbocation rearranging to form a more stable tertiary cation - CH3CHOCH3? and it would be on the para position....or am I completely wrong?

I think you're on the right track here. Based on carbocation rearrangement, what do you think is the more stable carbocation that can form and what product do you predict?

[hblondie26]

 I thought that it would be CH3CHOCH3 and the product would be chlorobenzene with CH3CHOCH3 in the para position. Wrong???

[orgopete]

Although rearrangement could lead to a plausible major product, I can also reason the oxygen electrons participating to prevent rearrangement. Rearrangement would create additional products. I'd consider a different product, especially without rearrangement. Hint, what kind of directing group is a chlorine?

[hblondie26]

So chlorine is O/p directing so would it simply be that the CH2CH2OCH3 would go to the ortho site? It almost seems too simple? Or am I over thinking it? When you say the oxygen electrons participating to prevent rearrangmenet what do you mean?


Would you also get a major product at the ortho site because there are 2 ortho sites therefore the ratio would be 2:1 ortho para respectively?

[jkenne25]

I thought that o/p directing groups always produced the para isomer in the greatest amount, which is the root of my confusion as the minor product is the para isomer.

The fact that there are two ortho sites facilitating a major ortho isomer makes sense, but would this then not be the case for all o/p directing groups? According to my organic chemistry textbook, o/p directing groups yield the para isomer as the greatest, then ortho and then meta as the least.

Why is this not the case here? This is what i cant understand.

Thanks to everyone replying btw!

[Dan]

Although rearrangement could lead to a plausible major product, I can also reason the oxygen electrons participating to prevent rearrangement.

Good point, I had overlooked the oxiranium ion as an alternative stabilised carbocation to the acyloxonium.

I was also under the impression that you'd get mostly substitution at the 4-position (where the -I effect is minimum, but the +M still operates) rather than the 2-position (-I is maximum, maybe sterics). But I suppose that if the 2-position is at least half as reactive as the 4-position, then statistically the 2-substituted product will still be the major one.

[hblondie26]

So would I be correct in thinking that a "good answer" would be the the oxygen atoms prevent carbocation rearrangment and therefore the major product would be ortho position clockwise to the chlorine atom?

But if thi is the case what about steric hindrance? Alkso the relevant chapter in our book speaks alot about carbocation rearrangement so would it not rearrange to tertiary more stable cation still on the para position?

Sorry for all the q's - its doing my head in! the more I read the more I convivne myself of another arguement!

[jkenne25]

I think I will need to give up with this. I cant seem to find any reasoning for an ortho product being a higher yield than the para isomer.

The rearranging to form a more stable cation seems plausible to me, but the question specifically asks to draw the isomer not the resonance structure.

 

[orgopete]

I don't recall where I had found a claim to preferred o/p reaction, but my recollection is that such a rule does not exist. I seem to recall that some reactions give primarily ortho (nitration of toluene) and some para. I assume this to be the case and then search for examples to confirm which I might find for a given reaction. You may indeed find that some reactions give high ratios of para products, but I'd be wary of generalizing from that example to all o/p directors.