Name the rx.

[SaintJude]

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Anyone really clear (and good at explaining orgo 😛) please explain why each choice is wrong and right. P.S. Thank you!! Struggling with oChem

A. a free radical substitution (ok obvious, no UV light is involved)
B. an electrophilic aromatic substitution
C. an electrophilic addition
D. a nucleophilic aromatic substitution

 

[chiddler]

So it's either B or D because it's aromatic.

We look at the reagent besides the aromatic ring; if it functions as an electrophile, then it is an electrophilic aromatic substitution. If it functions as a nucleophile, then it is a nucleophilic aromatic substitution.

Imagine the mechanism: a carbocation forms which makes the alkene an electrophile. Therefore it's an electrophilic aromatic substitution.

Am I correct? please tell me i'm right :<

 

[MedPR]

Ok here goes.

A. You know this is wrong, and why, so I'll ignore it.
B. I think this is right. Electrophilic aromatic substitutions are when a hydrogen (or something else) is replaced (substituted) by an electrophile. Whenever I see an alkene and phosphoric or sulfuric acid, I automatically think electrophilic addition. So initially I thought the answer to this was C (you may have seen my initial post pre-edit). However, after thinking about it, I realized that had it been addition, a double bond would've been lost from the aromatic, and not the alkene. Though, thinking about it now, that could've been the case and the product shown is just a resonance form that has been protonated by the acid.
C. I'm thinking this is wrong because the product would have an alkene and hexane ring, but as mentioned in the previous sentence, this could be the case.
D. Nucleophilic substitution involves a good leaving group. If this is any kind of substitution, a hydrogen was substituted, not a good leaving group.

 

[SaintJude]

So it's either B or D because it's aromatic.

It's B!

Chiddler:
Are you saying that aromatic compounds don't undergo electrophilic additions? B/c that would have saved MedPr (and me) some trouble

MedPr:
Are you saying that nucleophilic substitution always involves a good leaving group? Is it because the leaving group has to usually "leave by itself?" B/c that would have saved Chiddler and me some trouble as well.

 

[MedPR]

It's B!

Chiddler:
Are you saying that aromatic compounds don't undergo electrophilic additions? B/c that would have saved MedPr (and me) some trouble

MedPr:
Are you saying that nucleophilic substitution always involves a good leaving group? Is it because the leaving group has to usually "leave by itself?" B/c that would have saved Chiddler and me some trouble as well.

It doesn't necessarily leave by itself. In Sn1, the leaving group might be a bad leaving group (like OH), but if it gets protonated, it might become a good enough leaving group to be removed. It didn't really leave by itself, since it had to be protonated first. My understanding of a nucleophilic substitution is that a nucleophile (leaving group) is removed and replaced by the new group. In this case, a hydrogen was removed and I've never seen or heard of such a reaction being called a substitution.

Thinking again, there's no way this could be electrophilic addition. 2 reasons.

1. I don't think aromatics undergo electrophilic addition.
2. Even if they did, the first step is protonation of the aromatic by the acid catalyst. If the acid catalyst protonated m-cresol, it certainly wouldn't do it at the site where the isopropyl is located on the product. It would have protonated the hydroxyl.

 

[chiddler]

Pretty sure that aromatics do not have electrophilic addition.

Quote:
a &#960; bond is broken and two new &#963; bonds are formed.

and there's even a nice illustrated example.

A pi bond breaking means there will be loss of aromaticity; very unlikely and only possible with hydrogenation, high temps and pressure.

 

[SaintJude]

1. I don't think aromatics undergo electrophilic addition.

Well if aromatics, don't undergo electrophilic addition---then isn't it indeed true that the major mechanism that aromatics undergo is electrophilic substitution?

It looks like, unless conditions of hydrogenation, high temps and pressure are involved, aromatics will only undergo electrophilic substitution.

 

[ljc]

Well if aromatics, don't undergo electrophilic addition---then isn't it indeed true that the major mechanism that aromatics undergo is electrophilic substitution?

It looks like, unless conditions of hydrogenation, high temps and pressure are involved, aromatics will only undergo electrophilic substitution.

This is pretty much correct. They don't undergo addition because that would break the aromaticity. And they rarely undergo nucleophilic substitution because the only nucleophile an aromatic has is its double bonds, and it's not going to give one of them up to attack an electrophile because again, the aromaticity will be broken. So the vast majority of the time, you see aromatics undergo electrophilic substitution.