Structural orientation of CO3 (2-)

[quicksilver123]

So I need to find the structural orientation of CO₃^2-

The answer is that its trigonal planar, forming a double bond with one of the oxygen atoms.

However, why is this so?

Why does one of the oxygen atoms form a double bond instead of a single bond as the other atoms do?

See attachment for diagram of answer.

 

[ChiralWaltz]

1) Resonance effect.

2) Octet Rule.

Be sure you understand there are 3 separate resonance structures, each with a pi bond connecting an oxygen to the carbon. Each time you flip the pi bond onto the next oxygen, you are left with -2 charge on the 2 remaining oxygen atoms. Draw the lone electron pairs into the 2nd diagram if you are having trouble.

 

[quicksilver123]

I don't know what a resonance structure is and I don't know what a pi bond is.

I just started this course.

I guess the octet rule makes sense because the double bond would allow the extra two electrons (via the single double bond) to fill a valence shell of carbon, thereby stabilizing it.

 

[ChiralWaltz]

Pi bonds are double bonds, sigma are single bonds and look up tau bonds if you enjoy chemistry.

Resonance structures allow us to have multiple structures of a "single" molecule being expressed simultaneously. Some are better than others and we try to examine the best ones. These are the significant contributors. They are dictated by various electronic effects, charge distribution and over all stability. There is more on this topic and it is incredibly important. Please do some reading into resonance structures!

Draw the Lewis Dot structure for diagram 2 with the lone electron pairs. Always draw lone electron pairs and charges. Label your oxygen atoms 1,2, and 3. The oxygen atoms with sigma bonds are going to have a negative charge of -1. Draw this structure 2 more times, rotating the the pi (double) bond around to each one of the oxygen atoms in three separate structures with a sigma on the other 2 oxygens.

http://cosm.georgiasouthern.edu/chemistry/general/molecule/resonan.htm

 

[quicksilver123]

I'm on my phone right now so I can't do additional research at the moment - but I'll have to do so later on.

Sigma bonds = single bonds ... Easy enough

Pi bonds = double bonds... Ok

Tau bonds = ... Banana bonds? (lol. Skimmed over wiki, wil do better research later.)

So as far as resonance structures.. They're basically isomers?

You'll have to forgive me if I'm being a bit thick, I haven't really touched chemistry in about five years.

What charge will the pi bonded oxygen have, if the sigma bonded oxygns are given a charge of -1?
Also, how are these charges related to bonding in a specific sense? (dumb question, I know)

 

[ChiralWaltz]

Carbonate Resonance Structures

Not isomers!
The pi bond rotates at a speed great between the carbon and oxygen atoms. These three structures are considered to be in a simultaneous existence and a single molecule.

Formal charge = [# of valence electrons] – [electrons in lone pairs + 1/2 the number of bonding electrons]

 

[quicksilver123]

Wait. Does this have anything to do with schrodinger's idea of superpositions of states?
[it probably doesn't, but you said all three states would be considered to exist silmiltaneously for the molecule, so I have to ask. Or are they simply considered to exist in all three states at once because the pi bond switches at such high speeds? If so, is there experimental evidence to corroborate? ]

 

[ChiralWaltz]

If the cat was passing between life and death to the point you could not tell what state it is in, the yes. You have the right concept but I think it taking a snap shot of carbonate in only one of those resonance structures would be close to impossible.

Experimental Evidence-
We have resonance to help us create and utilize reaction mechanisms. The way we justify these structures is by various means of measurement. Inferred spectroscopy, Nuclear magnetic resonance, x-ray crystallography, thin layer gas chromatography and others I may not know. I haven't had the privilege to play these machines but have a general idea of the results they produce.

Carbonate's analysis would demonstrate all 3 bonds are of equal length. We know pi bonds are stronger than sigma bonds and therefore shorter. The reason we drawn them in their resonance states is to help us create reasonable reaction mechanisms. When a larger molecule starts branching, multiple reactions sites become available. Being able to identify these points is important in writing the mechanism.

 

[quicksilver123]

Okay, I'm interested in what you said about multiple reaction sites.

If this molecule were to undergo a reaction, what would determine which resonance structure is used for said reaction?
Is it simple probability?
You said something about some states being better than others - does this mean that one is more likely to observe this state?

If the pi bond is stronger and shorter than the two sigmas, yet all three bonds are observed to be the same length, how can we verify vespr theory's assertion that this bond even exists?

Or is all of this a type of quantum mechanical effect?

 

[Yanick]

Resonance structures are our way of representing a dynamic system using static diagrams. The resonance structures are not isomers of separate structures at all, they all represent the molecule albeit not in the same amount. Analogous to why the atomic weights of elements are not integer multiples of the number of protons/neutrons. It is because they reflect the weighted average of all of the naturally occurring isotopes. The point therefore of using the three resonance structures is to show that in fact carbonate does not have two single bonded oxygens and one double bonded oxygen but instead has approximately three 1.5-bonded oxygens.

To address the issue of which sites in a larger molecule are reactive. You won't notice it on something like carbonate because all of the oxygens are the same, but in a large molecule the reactive functional groups all have unique properties dictated by themselves (you may have a halogen in one place and an alcohol in another) and their environment. This makes for situations where certain resonance structures contribute more to the overall structure of the molecule. This behavior of course leads the molecules reacting in certain ways over others. As you learn more Chemistry you will learn a lot of the models and theories which allow you to judge which structures are better then others and hence contribute more to what the molecule really is like.

 

[ChiralWaltz]

If this molecule were to undergo a reaction, what would determine which resonance structure is used for said reaction?
Good question. We can take any of the three initial resonance structures to react with, as they are all the same. By picking up the molecule and flip flopping or spinning them, we can achieve any of the 3 resonance structures. This is because carbonate is Trigonal planar and rather symmetrical. If we had some models in your hand, this would be easier to explain.

Is it simple probability?
In an overall sense, yes. Right now, the ability for an atom to stabilize a charge for the reaction is most significant. Electronegativity contributes greatly in this aspect.

You said something about some states being better than others - does this mean that one is more likely to observe this state?

One would observe the Lewis Structure (man made) using electron pushing arrows to demonstrate the flow of electrons for the reaction mechanism. Important considerations are the solution the reaction is taking place in, concentrations, temperatures and possible side reactions. One may use different chemicals to increase or decrease the probability of reaction with a specific desired resonance structure.

If the pi bond is stronger and shorter than the two sigmas, yet all three bonds are observed to be the same length, how can we verify vespr theory's assertion that this bond even exists?
Pi bonds are shorter and stronger than sigmas. This is what all 3 structures would look like together.
http://www.biologie.uni-hamburg.de/b-online/library/Newton/Chy251_253/Lectures/Resonance/ResonanceHybrid.GIF
Think of the oxygen atoms having a -2/3 charge and the bonds being equal lengths.
I'm not entirely sure of your approach to the vespr question. Experiments are usually confirmed by laboratory analysis through the machines mentioned earlier.

Or is all of this a type of quantum mechanical effect?
We can choose to view and discuss reactions with various levels of depth and quantum chemistry is certainly one of them.

 

[quicksilver123]

Thanks for the responses. I've only just started this course but I hope it explores these concepts in depth... They are really interesting and have huge implications on my simplified chemistry knowledge.