Understanding Resonance Structures: Tips and Guidelines

In summary, the conversation discusses the difficulty in determining which resonance structures are significant and which can be disregarded. The lack of a general rule for this leads to confusion and the need for clarification. The article referenced provides some helpful information, but it also has some errors that need to be addressed. The debatable rule number 5 is discussed, and the example given is questioned regarding the number of unpaired electrons. In the case of oxygen, it is pointed out that molecular orbital considerations are not necessary to show the stability of triplet oxygen over singlet oxygen. This was demonstrated in a 1937 study by Wheland and Lennard-Jones.
  • #1
member 392791
Hello,

I am having difficulty discerning which resonance structures are considerable enough to include and which are so unimportant that they should be discarded. There seems to be no general rule on this that I can find, so I was wondering if such a rule existed or if someone can lead me in the right direction.
 
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  • #2
http://www.chem.ucla.edu/harding/tutorials/resonance/imp_res_str.html
 
  • #3
Thanks, that cleared a lot of things up (though they need to spell check and grammar check that article again)!
 
  • #4
Ygggdrasil said:
http://www.chem.ucla.edu/harding/tutorials/resonance/imp_res_str.html

Rule number 5 is debatable. In general, two one electron bonds are more stable than one two-electron bond due to electronic repulsion.
Especially, what do they mean with their example?
How can structure 1 have 3 unpaired electrons? Either the electrons combine to total spin 1/2, then the molecule is already described by structures II and III, or the electrons combine to spin 3/2, then structure I corresponds to an excited state of the molecule which won't mix in structures II and III.
In the case of oxygen you also don't need molecular orbital considerations to show that triplet oxygen is more stable than singlet oxygen. Especially two three electron bonds are more stable than one two electron bond and one four electron bond. This was already shown in 1937 by Wheland and Lennard-Jones http://pubs.rsc.org/en/content/articlepdf/1937/tf/tf9373301499
 
  • #5


Hello there,

I can understand your confusion about which resonance structures to include and which to discard. Resonance structures are important in understanding the distribution of electrons in molecules and can greatly impact the overall stability and reactivity of a molecule.

In general, the more stable a resonance structure is, the more it should be considered in the overall description of the molecule. Stability can be determined by factors such as the number of covalent bonds and formal charges on atoms. Resonance structures with more covalent bonds and fewer formal charges are typically more stable and should be given more weight in the overall description.

Additionally, resonance structures that contribute significantly to the delocalization of electrons should also be considered. This can be determined by looking at the formal charges on atoms and the distribution of lone pairs of electrons.

It is also important to consider the overall symmetry of the molecule when deciding which resonance structures to include. Resonance structures that contribute to a more symmetrical distribution of electrons are generally more important.

In summary, there is no set rule for determining which resonance structures to include, but factors such as stability, electron delocalization, and symmetry can help guide your decision. It is also important to consider the overall context and purpose of your research when deciding which resonance structures are most relevant.

I hope this helps guide you in the right direction. Keep exploring and learning about resonance structures, as it is a crucial concept in organic chemistry. Best of luck!
 

1. What is a resonance structure?

A resonance structure is a representation of a molecule or ion that can be drawn in multiple ways with the same arrangement of atoms. This occurs when there is delocalization of electrons within the molecule or ion.

2. Why are resonance structures important?

Resonance structures help to explain the stability and reactivity of molecules and ions. They also provide a more accurate representation of the distribution of electrons within a molecule or ion.

3. How do I determine which resonance structure is the most stable?

The most stable resonance structure is the one that has the fewest formal charges and the most electronegative atoms with negative charges. It is also important to consider the octet rule and the overall charge of the molecule or ion.

4. Can resonance structures be hybridized?

No, resonance structures are simply different ways of representing the same molecule or ion. The actual molecule or ion is a hybrid of all the resonance structures.

5. How do I draw resonance structures?

To draw resonance structures, identify all the possible arrangements of double bonds and lone pairs within the molecule or ion. Then, draw all the possible structures, making sure to follow the rules of valence electrons and formal charges. Remember, resonance structures can only differ in the placement of electrons, not atoms.

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