Ortho/Para and Meta Groups on Ring - which takes precedence?

[Aletheia]

If you have a meta and ortho/para director on, for ex, a benzene ring, how do you decide whether it's meta or ortho para directing? And, what if the position you want to put it on is taken up by another group? How do you decide which one is the "R" group and number the ortho, meta, para positions accordingly? Thanks in advance! Much appreciated.

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[BennieBlanco]

If you have a meta and ortho/para director on, for ex, a benzene ring, how do you decide whether it's meta or ortho para directing? And, what if the position you want to put it on is taken up by another group? How do you decide which one is the "R" group and number the ortho, meta, para positions accordingly? Thanks in advance! Much appreciated.

Sometimes they direct in the same place.

If they don't, chose the least sterically hindered first.

Beyond that, probably not MCAT material.

They won't give a question that has ambiguity, if you have 2 group directing to multiple sites then you will get a mix of products.

I had the same Q's when I took organic, then you just realize that they test on thinigs that have an unequivocal answer.

 

[PiBond]

If you have a meta and ortho/para director on, for ex, a benzene ring, how do you decide whether it's meta or ortho para directing? And, what if the position you want to put it on is taken up by another group? How do you decide which one is the "R" group and number the ortho, meta, para positions accordingly? Thanks in advance! Much appreciated.

Ortho/para wins out when two groups have different directing properties. You generally won't have a situation where the position you're directed to is already taken up but another group. Hypothetically, if you had 3 meta-directing (aka electron withdrawing groups) on benzene then the benzene ring would mostly likely lose its ability to act as a nucleophile. This last part is not fair-game for the MCAT though.

 

[Handinhand]

Are aromatics even on the MCAT?

Either way, if a meta director is ortho or para to the ortho/para director, they are going to doubly "activate" the same area. I say "activate", because directors don't really activate in the sense that an electron donating group does. Really, a meta group directs a positive charge (through resonance) to the ortho and para position, thus deactivating those spots to an electrophilic attack, making a meta attack most likely.

Now, if the meta director is meta to the ortho/para director, you're probably going to get a mix of every position, because the meta directing deactivation will be offset by the donating groups donating of electrons to the same spot. Use common sense though, if you have an NO2 group and a CH3 group meta to that, the CH3's weak electron donating isn't going to offset the extreme withdrawing of the nitro group.

 

[TurkSurg]

Are aromatics even on the MCAT?

Either way, if a meta director is ortho or para to the ortho/para director, they are going to doubly "activate" the same area. I say "activate", because directors don't really activate in the sense that an electron donating group does. Really, a meta group directs a positive charge (through resonance) to the ortho and para position, thus deactivating those spots to an electrophilic attack, making a meta attack most likely.

Now, if the meta director is meta to the ortho/para director, you're probably going to get a mix of every position, because the meta directing deactivation will be offset by the donating groups donating of electrons to the same spot. Use common sense though, if you have an NO2 group and a CH3 group meta to that, the CH3's weak electron donating isn't going to offset the extreme withdrawing of the nitro group.

It is covered in TBR orgo so maybe...

 

[PiBond]

Just looked it up, confirmation that ortho/para win out.

Organic Chemistry: Brown, Foote, Iverson, 4th Edition, page 876. "..where there is a competition between ortho-para and meta directing groups, ortho-para win out."

This should help as well: http://iverson.cm.utexas.edu/courses/old/310N/spring2008/POTDSp06/POTD4-18-08.html

 

[Handinhand]

Just looked it up, confirmation that ortho/para win out.

Organic Chemistry: Brown, Foote, Iverson, 4th Edition, page 876. "..where there is a competition between ortho-para and meta directing groups, ortho-para win out."

This should help as well: http://iverson.cm.utexas.edu/courses/old/310N/spring2008/POTDSp06/POTD4-18-08.html

For the text book, this is right. In reality though, you're going to get as my old Ochem prof would say, "Dog's breakfast", or any and everything.

EDIT: And for the MCAT, follow the textbook.

 

[BennieBlanco]

It is covered in TBR orgo so maybe...

in what ch? it isn't in the hydrocarbon chapter.

I think this is beyond the MCAT.

Just understand what an EDG/EWG do.

Activator (most likely) > Deactivator when forced to chose b/t BUT do you really think the MCAT will test this without referring to it in a passage?

nope.

MCAT = simple concepts disguised in mumbo jumbo passages.

MCAT is NOT ambiguous concepts and random trivia.

 

[BennieBlanco]

Just looked it up, confirmation that ortho/para win out.

Organic Chemistry: Brown, Foote, Iverson, 4th Edition, page 876. "..where there is a competition between ortho-para and meta directing groups, ortho-para win out."

This should help as well: http://iverson.cm.utexas.edu/courses/old/310N/spring2008/POTDSp06/POTD4-18-08.html

Which makes sense, Ortho-para are actually doing something (throwing in electron density) rather than just pulling out electron density. I'm sure it still depends on what ortho/para directors vs meta directors. Such as two NO2- groups directing to one meta spot vs a methyl group directing to an ortho spot.

 

[BennieBlanco]

For the text book, this is right. In reality though, you're going to get as my old Ochem prof would say, "Dog's breakfast", or any and everything.

EDIT: And for the MCAT, follow the textbook.

Exactly... I think the academic world forgets that when we draw reactions it is just the most likely, by no means is it the CORRECT product.

 

[boaz]

The subject of aromatic chemistry is not on the MCAT.

 

[BennieBlanco]

The subject of aromatic chemistry is not on the MCAT.

lol. Thought so, Berkeley covers EVERYTHING and I didn't see it in their book.

 

[boaz]

It is unfortunate that people regularly post questions on aromatic reactions. There must be lots of people who, seeing these threads on this subject, hunker down and break their heads on this difficult and complicated subject, and which is not at all tested on the MCAT.

This forum is specifically for the MCAT. So, OP: if you need help with the MCAT we'd be glad to help, but this is not the right forum for homework help.

 

[obgyny]

It is unfortunate that people regularly post questions on aromatic reactions. There must be lots of people who, seeing these threads on this subject, hunker down and break their heads on this difficult and complicated subject, and which is not at all tested on the MCAT.

This forum is specifically for the MCAT. So, OP: if you need help with the MCAT we'd be glad to help, but this is not the right forum for homework help.

if aromaticity is not on the MCAT... why is there a whole section about it in TPR?

 

[loveoforganic]

Idk, but http://www.aamc.org/students/mcat/preparing/bstopics.pdf

nothing on aromaticity

 

[boaz]

if aromaticity is not on the MCAT... why is there a whole section about it in TPR?

In previous years the MCAT tested it. But they changed it a few years ago, as you can see in the link loveoforganic provided.

 

[obgyny]

In previous years the MCAT tested it. But they changed it a few years ago, as you can see in the link loveoforganic provided.

oh okay. hmm... my TPR book is the 2009 edition... luckily I've been checking through the AAMCAS content outline as I've been studying.

thanks for the replies, one less thing to study 🙂