Organic Chemistry: Stabillity of Nitrogen Containing Cyclic Compounds

In summary, pyridine and pyrrolidine react rapidly with dilute aqueous HCl to form the corresponding hydrochloride salts which are easily purified, isolated and stored in a charge. However, pyrrole, which is another nitrogen-containing heterocycle, does not form a hydrochloride salt under these conditions, explaining. Drawing the hypothetical protonated form of pyrrole, it is not aromatic. Protonating the nitrogen in pyrrole would result in the loss of its stability because it would eliminate all its possible resonance structures by inducing the + charge on the nitrogen while the others still are able to resonate.
  • #1
rgk13
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Homework Statement


Pyridine and pyrrolidine react rapidly with dilute aqueous HCl to form the corresponding hydrochloride salts which are easily purified, isolated and stored in a charge. However, pyrrole, which is another nitrogen-containing heterocycle, does not form a hydrochloride salt under these conditions, explain.

Homework Equations


Pyridine:
80px-Pyridine-2D-Skeletal.png


Pyrrolidine:
Pyrrolidine.png


Pyrrole:
100px-Pyrrole-2D-numbered.svg.png


The Attempt at a Solution



I am pretty sure that resonance plays a major role in the answer. Is it because by protonating the pyrrole with the HCl you create an aromatic compound that prefers to stay in that form?
 
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  • #2
You're right, resonance does have a role, but its role is a lot simpler than you're thinking.

Draw the hypothetical protonated form of pyrrole. Now ask yourself if it is aromatic. Remember: breaking aromaticity comes at a cost of ~35 kcal/mol, which is a lot of energy and often unfavorable!

What I'll also mention: the lone pair in pyrridine is not involved in the conjugated pi system (that p orbital is orthogonal to the other p orbitals, meaning it has zero interaction). In pyrrole, the lone pair/p orbital is involved in the conjugation.
 
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  • #3
so by protonating the pyrrole you would lose its stability because it would eliminate all its possible resonance structures by inducing the + charge on the nitrogen while the others still are able to resonate?
 
  • #4
rgk13 said:
so by protonating the pyrrole you would lose its stability because it would eliminate all its possible resonance structures by inducing the + charge on the nitrogen while the others still are able to resonate?

Eep, watch your terminology because that will be your pitfall! By protonating the nitrogen in pyrrole, its lone pair electrons are no longer delocalized across the ring in the pi molecular orbital--they would be confined to a sigma bond between the N and the new H. Note that in pyrrole, every atom is sp2 hybridized. If any of them are changed to sp3 hybridization, as protonating the N would do, then the aromaticity is broken (i.e., 4n + 2 pi electrons in the ring--Huckel rule!).

There is no charge on the nitrogen when it is protonated! It is electrically neutral (count the protons and electrons around it--neutral). It is only a formal charge, which has no relation to actual electrostatic charges.

Also, be careful with the word "resonating." The structures/bonds are not resonating between each other in any classical sense. It is both structures *at the same time.* Weird eh? Basically, the "true" structure, representing the real bond strengths, distances, and charge densities cannot be represented by these simple drawings. So, we draw a series of "contributing structures" to describe its actual chemical behavior. What it comes down to, especially in regard to conjugated pi molecular orbitals, is that the electrons are not confined between any two atoms of a bond, but are distributed across the entire molecular orbital!

Yes, I realize I'm using a hybrid of valence bond theory and MO theory here, but I'm doing so because *it works.* There are some people on this forum who only want to discuss things in terms of MO theory, but that's not my style :)
 
  • #5
Ahhh so essentially the HCl can react with the pyridine because the lone pair of electrons is available on the nitrogen and it can essentially interact with those electrons without effecting the aromaticity while in the second compound the lone pair is there and aromaticity is not a factor while the third structure, using the lone pair on the nitrogen would cause the "destruction" of the pi orbital system that makes it aromatic and that would be too energetically unfavorable.
 
  • #6
Bingo.

Forming or breaking aromaticity has a *huge* impact in the reactivity of molecules!

On a similar note: forming or breaking anti-aromaticity (i.e., particularly unstable cyclic compounds with 4n pi electrons) also has a huge impact on reactivity.
 
  • #7
Thank you very much!
 

FAQ: Organic Chemistry: Stabillity of Nitrogen Containing Cyclic Compounds

1. What is the significance of nitrogen-containing cyclic compounds in organic chemistry?

Nitrogen-containing cyclic compounds, also known as heterocycles, play a major role in organic chemistry due to their diverse structural and functional properties. These compounds are found in many natural products and pharmaceuticals, making them important targets for synthesis and study.

2. How does the stability of nitrogen-containing cyclic compounds compare to other types of organic compounds?

The stability of nitrogen-containing cyclic compounds can vary greatly depending on the specific structure and functional groups present. However, in general, these compounds tend to be more stable than their oxygen-containing counterparts due to the stronger bond formed between nitrogen and carbon.

3. What factors influence the stability of nitrogen-containing cyclic compounds?

The stability of nitrogen-containing cyclic compounds is influenced by a variety of factors, including the size and shape of the ring, the nature of the substituents attached to the ring, and the presence of any resonance or conjugation. Additionally, the electronic effects of nitrogen and its neighboring atoms can also impact stability.

4. How does the stability of nitrogen-containing cyclic compounds affect their reactivity?

The stability of nitrogen-containing cyclic compounds can greatly impact their reactivity. Generally, more stable compounds are less reactive and require more energy to undergo reactions, while less stable compounds tend to be more reactive and can undergo reactions more readily.

5. Can the stability of nitrogen-containing cyclic compounds be predicted based on their molecular structure?

While there are some general trends in stability for nitrogen-containing cyclic compounds, predicting the stability of a specific compound based on its molecular structure can be challenging. The stability of a compound is influenced by a combination of factors, and it is best to consult experimental data or use computational methods to accurately determine stability.

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