N&PD Moderators: Skorpio
Difference between isomers and resonance structures
Anon0631
Bluelighter
Am I right in saying that these two images of n,n-MET are different resonance structures and not different isomers?
Can someone explain why?
sekio
Bluelight Crew
They are 2 views of the same molecule.
bloodshed344
Bluelighter
No, they're exactly the same. Either that or my knowledge about this stuff is completely off.
23536
Bluelight Crew
As stated above, they're neither. Think of the amine N as the vertex of a propeller and you'll see that they're the same. Better yet, think of the N as the pelvis of a person whose right leg has been amputated below the knee. During his physical therapy he has pushed his hips into the air and is rotating them.
endotropic
Bluelight Crew
A conformational isomer just means that you have the exact same molecule, with the bonds rotated to different angles (a different conformation).
bloodshed344
Bluelighter
Yeah, I know what it means. But that's not what is, as I thought 23536 explained it. Look at the very first thread I made on here, ha.
Crashing
Bluelighter
They're exactly the same, just denoted differently. CH3 = H3C that is one carbon and 3 hydrogens. The methyl group is just written different order on paper in one of the pictures. GENERALLY you write carbon first for this exact reason (not to confuse). They aren't rotated in reality but on a model they would be.
SeenSoFar
Bluelighter
My understand of the matter (although I could be wrong, please correct me if I am) is that although both involve roation around one particular bond, some bonds will rotate freely, with the compound oscillating between the two states, as is the case with the compound in question in this thread. You cannot isolate one from the other because they're always swapping between one and the other. Conformational isomers are also produced by rotating functional groups around single bonds, but the results are somewhat stable, with any given sample containing a percentage of each conformational isomer of the compound. These may also switch from one conformation to another easily. Easily, but not FREELY as in the previous case, to the point that one conformation may be isolated from another in some circumstances. An example would be the different conformations of cyclohexane. Yes? No? Total misunderstanding?
Crashing
Bluelighter
The methyl rings are in perpetual rotation around the nitrogen, so it doesn't matter how it's drawn.
SeenSoFar
Bluelighter
Yes exactly, thank you for putting my long-winded explanation into simple terms.
As is said: if drawn this way isomerism is neglected or ignored. That is indeed because the nitrogen and everything hanging on it (i.e. the ethylmethylamine group) can rotate, making the drawn molecules identical.
For there to be different isomers you need at least one chiral center. A chiral center is an atom (abstracted as a point in the structural formula) that has so many different groups / atoms hanging from it that this gives you different ways to arrange or order them, different enough that rotations do not show that they are identical.
Nitrogen as you find it in organic molecules could never be a chiral center unless it is quaternary (charged + and having not 3 but 4 bonds / groups / substitution).
An example of isomers is dextroamphetamine and levoamphetamine:
The dotted line is a normal bond like the other lines, but it is not 3-dimensionally flat in the plane, but instead sticking out to the back (away from you). That's because when carbons have 4 single bonded substitution groups they are spread evenly across 3-D space yielding a tetrahedral shape. The center where you see 4 lines meeting represents the chiral carbon atom. All 4 groups bound to that carbon are different from each other:
- H
- CH3
- Benzyl (the phenylring part)
- NH2
They are called optical isomers because they polarize the direction of light in a different way. Dextroamphetamine is dextrorotatory and its physical property is that it turns light to the right (dextro = right), and levo = left.
Resonance structures arise in different situations.
Basically, there are molecules that have electronic distributions that are continuously 'changing', in fact at such a mind-bending speed that we can consider them delocalized instead: neither in one place or the other but spread over the 2 places (I guess quantum weirdness rears is head here if I'm correct).
If we look at the hexagonal benzene-ring or phenyl-ring in the amphetamine molecule structures, we see a hexagon representing 6 carbon atoms (with hydrogens hanging off them which we will ignore since they are irrelevant here), and we see 3 bonds between the carbons that are a single line (single bonds) and 3 double lines (double bonds). (shown clearly here)
Between every carbon there is always at least one bond, but the three double bonds are formed by so-called π-electrons (pi-electrons) and they are spread over the whole ring. Because of that it doesn't matter if we draw the phenylrings with double bonds this way or upside down, it is a matter of convenience and it isn't particularly correct either way. More correct is to draw a dotted line all across like this:
This dotted line is different from the one we saw before because before it was a wedge-shaped one and this one is a simple one. This time it means that there is half a bond's worth of electrons at every edge between vertices.
Resonance structures are only drawn if the resonance is relevant to a certain reaction or some involved property. In so-called conjugated systems the ambivalent nature of (double) π-bonds can make it seem like bonds can jump.
To show a relevant example, here I attempt to show how psilocin might oxidize via base-catalysis:
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endotropic
Bluelight Crew
I don't understand what the disagreement is here, what 23536 explained was a description of a conformational isomer (not a structural isomeror positional isomer mind you).
This is all getting a bit silly, I think everyone here understands the difference between the two molecules.
bloodshed344
Bluelighter
Eh, everyone except you maybe? They're just different views of the same molecule. The ethyl and methyl attached to the nitrogen freely rotate constantly so there's no difference between the two molecules, they're the same molecule but with a different view. Is this what you were trying to express? Edit: Okay looked at the link for conformational isomer, thank you.
bloodshed344
Bluelighter
I edited my post before you made this post, pay attention
Like I told him, I looked up conformational isomer. Thank you, endotropic.
endotropic
Bluelight Crew
I edited my post before you made this post, pay attention
Like I told him, I looked up conformational isomer. Thank you, endotropic.
You got it, I had a feeling that was where the confusion was.
