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Chemistry = electron reconfig?

  1. Dec 8, 2006 #1
    Is all chemistry ultimately about electrons in atoms and their configurations? If there is an electron reconfiguration then chemistry is involved a new chemical entity comes into existence.

    So take 2,4-dimethylpentane. It has two types which cannot be superimposed onto each other. One where both methyle groups are in the same direction. The other one where one methyl group is on top the other down the bottom.

    However, one can be made into the other without an electron reconfiguration so these two molecules are not isomers of each other even though they are not superimposible?
  2. jcsd
  3. Dec 8, 2006 #2
    Ask yourself this: What exactly is isomerism? How can something not be superimposed onto each other if they are not isomers? Do not mix up the definition of isomerism and the different types of it.
  4. Dec 8, 2006 #3
    Isomers are when two or more species have the same formula but different properties.

    There are two main types of isomerism: Structural isomerism and stereroismersim

    My example is about stereroismersim. They are clearly not superimposible. But they have the same structural formulae, that is 2,4-dimethylpentane although that is okay since we are not dealing with structural isomersim. So you suggest they must be isomers of each other? Hence have different properties? But one can be made into the other without an electron reconfiguration.
    Last edited: Dec 8, 2006
  5. Dec 8, 2006 #4
    No it doesn't. There is only 1 type of 2,4-dimethylpentane. Just because it looks different on paper, doesn't mean it is different in nature. Remember certain types of chemical bonds are NOT static, they are constantly rotating.
  6. Dec 8, 2006 #5
    That is a good point which I have thought as well. I just realised that the only type of (potential) isomerism for 2,4-dimethylpentane is geometric (cis, trans) isomerism. But it will always be trans although one of the longer carbon chains on each side will have a C group up and the other down. But that is irrelevant as you say since the bonds are constantly moving. It goes back to the fundalmental reason that "one can be made into the other without an electron reconfiguration" dosen't it.
  7. Dec 8, 2006 #6
    there is absolutely no difference between the two structures I have drawn. You are labeling carbons (A and B I assume) and calling them "Cis" and "trans" when carbons A B C are all the same.

    Looking at the newman projection of the c2-c3 carbons you can see that there are only two configurations that are likely to exist, which I have drawn. It is possible for the methyls to line up directly with the isopropyl on the front carbon, but this is very energenically unstable. These aren't isomers, however. They are all the same molecule, the newman projection just shows you a snapshot of the c2-c3 carbons while they rotate at an instant in time.

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    Last edited: Dec 8, 2006
  8. Dec 9, 2006 #7
    Thanks for the diagrams.

    In my previous post I was trying to say that the two configurations which you have clearly drawn in picture 1 are both trans (wrt third carbon as centre). Is this a correct way of stating it? Or have I misused the language because whenever there is a trans there must also be a cis (otherwise don't use cis, trans lanaguage)? In this case there is no cis so I shouldn't be using the word trans?
    Last edited: Dec 9, 2006
  9. Dec 9, 2006 #8


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    I hope you are using the term 'configuration' in its standard meaning (i.e., the list of occupation numbers of the different partially or fully occupied orbitals). Most parts of Chemistry do, in fact, boil down to electron configurations, but
    (i) that is not the way that most of these parts of chemistry are dealt with (for just the same reason that most sociology boils down to chemical reactions in a human being yet no one can hope to understand why for instance, the Shia and the Sunni hate each other, by balancing equations), and
    (ii) there are some areas in chemistry (e.g., NMR chemistry) that do not even boil down to this.

    Chemistry is a tool to study and model phenomena. Chemistry is not a phenomenon or fundamental principle of any kind. Chemistry, as a science, is a completely different thing than the term used colloquially in phrases like "there's no chemistry between us." What do you mean by saying chemistry is involved in an electron reconfiguration? And what is a "reconfiguration"?


    "Types"? What are 'types'? If you do not use standard terminology, it's hard to have a coherent discussion. Do you mean isomers?

    2,4-dimethylpentane has no isomers. All bonds are single-bonds and permit free rotation. There are no cis- trans- isomers for this compound.

    What do you mean by "top" and "bottom", when no three co-terminal bonds are coplanar? In fact, there are an infinite number of rotations where, viewed from some external point, you will see the two methyl groups take the highest and lowest points in the molecule.

    But I believe there is one orientation each which makes those two C-CH3 bonds parallel or antiparallel (perhaps that's what you mean). These two rotational orientations, in general, need not put the two methyl groups closest or farthest apart from each other.

    They are not considered isomers for the reason that free rotation allows any given molecule to sample "both" orientations. Furthermore, isomers are never superimposable. If two spatial configurations are superimposable over each other, they are the same molecule. How would you distinguish one from the other?
    Last edited: Dec 9, 2006
  10. Dec 9, 2006 #9
    Geometric isomerism only applies to double bonds or higher. So no isomerism can occur with this molecule. Free rotation explains why they can be both parallel and anti parallel as both of you have pointed out.
  11. Dec 9, 2006 #10
    Why do you say (i) (electrons and chemistry are directly related whereas social science is not)? How is most of chemistry dealt with? You mean people usually do chemistry without thinking about electron reconfigurations (the way you defined it). It seems if chemists don't think about this than a large part of the 'why' in chemistry is lost?
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