Sciencemadness Discussion Board
Not logged in [Login ]
Go To Bottom

Printable Version  
Author: Subject: Azo group and Cinnamic acid substitution orientation
IanCaio
Hazard to Self
**




Posts: 52
Registered: 26-9-2012
Member Is Offline

Mood: No Mood

[*] posted on 9-2-2013 at 15:23
Azo group and Cinnamic acid substitution orientation


Hey guys,

In the book COPAE, Chapter IV, in the substitution order rule 1, "The orienting effect", it says that Azo groups (-N=N-) orient ortho/para positions and Cinnamic acid (-CH=CH-CO-) orient ortho/para aswell. However in the induction (the method I always use to determine activating and desactivating groups) I would have the following:

-N+=N--R (unless the R was a really electronegative element like Fluor or Oxygen)

-C+H=C-H-C+O-


And what I would expect from those groups is to induce the aromatic nucleos in a way it would yield meta substitutions. Are the charges there wrong or they are some kind of exception to the rule? I'm banging my head here to understand it.

The other exception they show is the -CCl3, but using the induction method it does yield meta, which is what the book says.

Any clues why does that happen? Is that method of determining the activating/desactivating groups wrong?

Thanks in advance :)
View user's profile View All Posts By User
crazyboy
National Hazard
****




Posts: 436
Registered: 31-1-2008
Member Is Offline

Mood: Marginally insane

[*] posted on 9-2-2013 at 15:38


Activation/deactivation can take place via resonance as well as inductive effects. Even though there may be electron withdrawing by inductive effect which would lead to a meta director, the pi electrons induce resonance which causes an electron donating effect which overpowers the inductive electron withdrawing leading to ortho/para substitution.

For example the -OH is electron donating and ortho/para directing overall because it is electron donating by resonance despite the slight electron withdrawing due to induction.




View user's profile View All Posts By User
IanCaio
Hazard to Self
**




Posts: 52
Registered: 26-9-2012
Member Is Offline

Mood: No Mood

[*] posted on 10-2-2013 at 17:01


I just realized that now, -OH despite being electron donating is a ortho/para director in Picric acid for example.. Using the inductive effects it should be meta director.

I guess I understood, but I'm finding a little hard to visualize how the resonance in the structure would lead to ortho/para directing. I'll try to look further for information about this.
View user's profile View All Posts By User
crazyboy
National Hazard
****




Posts: 436
Registered: 31-1-2008
Member Is Offline

Mood: Marginally insane

[*] posted on 10-2-2013 at 17:42


The top shows all resonance forms of intermediates resulting from ortho attack. The second and third show the same for meta and para attack respectively. The blue resonance structures are resonance structures possible only for ortho and para attack in this case. The additional resonance structure adds stability and makes these more favorable attacks in this instance.






View user's profile View All Posts By User
IanCaio
Hazard to Self
**




Posts: 52
Registered: 26-9-2012
Member Is Offline

Mood: No Mood

[*] posted on 11-2-2013 at 09:53


Thanks a lot crazyboy!

I now understand why those groups despite being electrons withdrawing are ortho/para directors. They probably just like the -OH group have some extra ressonance structures, giving them more stability on ortho and para positions.

So the best way to know if a group is an activating group or desactivating one is actually to memorize them? Guess it would be really complicated to imagine all the ressonance structures from all the groups.
View user's profile View All Posts By User
crazyboy
National Hazard
****




Posts: 436
Registered: 31-1-2008
Member Is Offline

Mood: Marginally insane

[*] posted on 11-2-2013 at 12:30


http://www.learnchem.net/orgo/aro2_nom.shtml

This list is pretty comprehensive. All deactivators with the exception of halogens are meta directors. All activators are ortho/para directors.




View user's profile View All Posts By User
IanCaio
Hazard to Self
**




Posts: 52
Registered: 26-9-2012
Member Is Offline

Mood: No Mood

[*] posted on 11-2-2013 at 14:17


Thanks for the link.

Sorry to insist on the subject although you already told how it works. But I guess I didn't have a good explanation on "Organic Chemistry I" about this, and the fact that I had to study by myself might have lead me to misinformation.

Until now I only used the induction to identify the meta and ortho/para directors group, as in the first attachment.

Thinking further I realized it doesn't make much sense the way those neighbors carbons are affected. I think that a negative charged carbon would actually make the neighbors slightly negative, the same for positive charged carbons, just like in the second attachment.

After your explanations (all very easily to understand) it seens to me that its actually more about the carbocation structure being stabilized than the inductive charges itself. Which would make more sense for those exceptions.

That inductive method I used is then completely wrong? Are the charges in the first and/or second attachments wrong? So the charge is actually not that important, and the intermediate structure stabilization is the key to find which groups are meta and ortho/para directors?

Sorry again to bother you with more questions, I'm just kind of confused, trying to fit all the information together and to get rid of the misleading ones. And thanks for all the help so far.

AtivantesDesativantes.JPG - 20kB

AtivantesDesativantes2.JPG - 13kB
View user's profile View All Posts By User
IanCaio
Hazard to Self
**




Posts: 52
Registered: 26-9-2012
Member Is Offline

Mood: No Mood

[*] posted on 13-2-2013 at 15:09


Just deconsider the last post, as I thought the first attachment is completely wrong. I'm feeling even a little stupid for having not realized that before.

Thanks for the help Crazyboy
View user's profile View All Posts By User

  Go To Top