General organic chemistry -- finding maximum electron density

[harini07]

Homework Statement

In pyrrole the electron density is maximum on which carbon atom?

so if the numbering starts from nitrogen, the carbon on the right side of nitrogen let it be numbered as 2 and the next one as 3 and so on... which of the following options will have the carbons of maximum electron density? a) 2,3 b) 3,4 c)2,4 d)2,5.

Homework Equations

electron density will be maximum on that carbon in which there is a maximum mesomeric or resonance effect. [/B]

The Attempt at a Solution

here the -ve sign on the carbon atoms represent that the electron is crowded there. so i could see that from the resonance structures all of the carbon atoms were electron crowded for some time among the resonants.. so which option will be the apt? and how to identify it?[/B]

 

[Ygggdrasil]

You've looked at the mesomeric effect. What about inductive effects?

 

[harini07]

You've looked at the mesomeric effect. What about inductive effects?

by inductive means +I effect and it is more on C2 and C5. so does +I effect contribute to electron density? then 2 and 5 will be right! also please make me clear on why do we look at inductive effect but not mesomeric effect though mesomeric contributes more to electron density than inductive effect.

 

[Ygggdrasil]

The N will act as an electron withdrawing group through the inductive effect, so carbons 2 and 5 will have less electron density than carbons 3 and 4.

As you correctly note, the mesomeric effect is stronger than the inductive effect. But, as you note, the mesomeric effect does not allow one to distinguish between the four carbons, so if the mesomeric effect indicates no difference beteween the carbons, you next need to consider the inductive effect.

 

[harini07]

The N will act as an electron withdrawing group through the inductive effect, so carbons 2 and 5 will have less electron density than carbons 3 and 4.

As you correctly note, the mesomeric effect is stronger than the inductive effect. But, as you note, the mesomeric effect does not allow one to distinguish between the four carbons, so if the mesomeric effect indicates no difference beteween the carbons, you next need to consider the inductive effect.

so answer is 2 and 5, right?

 

[harini07]

So the answer for this question is 3 and 4, right?

 

[harini07]

Here the answer is given as 2 and 5 contrary to what we have discussed.could this(2 and 5) be correct?

 

[Ygggdrasil]

I would have said that the electron density is highest on carbons 3 & 4. I do not understand the explanation given for carbons 2 & 5.

 

[harini07]

They are saying that the resonating structures are stable when pyrrole has -ve charge on C2 and C5 and the electrophilic substitution site is also the proximal carbons, thus 'they should be electron rich' is what they are saying.though I M highly confused by this explanation :/ can it be? Which will be the final answer?

 

[Ygggdrasil]

They are saying that the resonating structures are stable when pyrrole has -ve charge on C2 and C5 and the electrophilic substitution site is also the proximal carbons, thus 'they should be electron rich' is what they are saying.though I M highly confused by this explanation :/ can it be? Which will be the final answer?

I don't understand why they say resonance structures III and IV are more stable than II and V.

While it is true that electrophilic substitution occurs preferentially at carbons 2 and 5, this is generally rationalized by the stability of the product, and not based on the nucleophilicity of the carbons (for example, see the section on "Electrophilic substitution at pyrrole" from http://www.chemgapedia.de/vsengine/...yclen/fuenfaromat/fuenfring_aromat.vscml.html).

 

[harini07]

I don't understand why they say resonance structures III and IV are more stable than II and V.

While it is true that electrophilic substitution occurs preferentially at carbons 2 and 5, this is generally rationalized by the stability of the product, and not based on the nucleophilicity of the carbons (for example, see the section on "Electrophilic substitution at pyrrole" from http://www.chemgapedia.de/vsengine/...yclen/fuenfaromat/fuenfring_aromat.vscml.html).

The following structures are considered relatively stable:

1. Structures having filled octet a for second row elements (C, N, O, F) are stable.
2. Structures having minimum number of formal charges and maximum number of bonds.
3. Structure in which negative charge appears on the most electronegative atom.
4. Structure in which there is minimal charge separation while keeping the formal charges closer together. and by this 4th rule structure III and IV is more stable than II and V as there separation between the formal charges is lesser. Also i don't understand what they mean by "stable by sigma complexes" in the post link that you quoted. all i understand is, if electrophilic substitution occurs at C2 and C5, it's because the electron density is more on it than those of C3 and C4. (found from wiki: what does it mean to be electron rich?
   In chemistry, an electrophile is a reagent attracted to electrons. Electrophiles are positively charged or neutral species having vacant orbitals that are attracted to an electron rich centre. It participates in a chemical reaction by accepting an electronpair in order to bond to a nucleophile.) so doesn't this fact support the argument where C2 and C5 is electron richer. PS: i still can't arrive at the conclusion and I'm feeling like chemistry becomes complex and tricky at times like this leaving us without any specific or accurate answers. can it be, @Ygggdrasil ? Enlighten me
   

 

[Ygggdrasil]

4. Structure in which there is minimal charge separation while keeping the formal charges closer together. and by this 4th rule structure III and IV is more stable than II and V as there separation between the formal charges is lesser.

I have not heard of this rule before. A few of the resources I've looked up online do not list this as a rule either:
https://chem.libretexts.org/Core/Ph...hemical_Bonding/Valence_Bond_Theory/Resonance
http://web.chem.ucla.edu/~harding/tutorials/resonance/imp_res_str.html

Also i don't understand what they mean by "stable by sigma complexes" in the post link that you quoted. all i understand is, if electrophilic substitution occurs at C2 and C5, it's because the electron density is more on it than those of C3 and C4. (found from wiki: what does it mean to be electron rich?

I think when they're talking about sigma complexes, they're talking about formation of a new single bond (i.e. a sigma bond).

In chemistry, an electrophile is a reagent attracted to electrons. Electrophiles are positively charged or neutral species having vacant orbitals that are attracted to an electron rich centre. It participates in a chemical reaction by accepting an electronpair in order to bond to a nucleophile.) so doesn't this fact support the argument where C2 and C5 is electron richer.

While that could be one reason, there are other factors to consider as well. If reactions occur reversibly, then ultimately the thermodynamic stability of the product will determine which product is more abundant after the reaction. The link I cited makes this argument, saying that addition to C2 or C5 results in a more stable product.

PS: i still can't arrive at the conclusion and I'm feeling like chemistry becomes complex and tricky at times like this leaving us without any specific or accurate answers. can it be, @Ygggdrasil ? Enlighten me

A full answer to the question probably requires quantum mechanical modeling of the atom to calculate the actual electron density around each atom. I also don't agree with the answer given. The question is probably just poorly conceived.

 

[harini07]

I have not heard of this rule before. A few of the resources I've looked up online do not list this as a rule either:
https://chem.libretexts.org/Core/Ph...hemical_Bonding/Valence_Bond_Theory/Resonance
http://web.chem.ucla.edu/~harding/tutorials/resonance/imp_res_str.html
I think when they're talking about sigma complexes, they're talking about formation of a new single bond (i.e. a sigma bond).While that could be one reason, there are other factors to consider as well. If reactions occur reversibly, then ultimately the thermodynamic stability of the product will determine which product is more abundant after the reaction. The link I cited makes this argument, saying that addition to C2 or C5 results in a more stable product.
A full answer to the question probably requires quantum mechanical modeling of the atom to calculate the actual electron density around each atom. I also don't agree with the answer given. The question is probably just poorly conceived.

May be I should settle with the distal carbons as more electron densed then. :)