SteyrTMP
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Is my organic chemistry teacher an idiot?
So I'm doing a quiz for my organic chemistry class, open book and online. There's a bunch of questions on there that lead me to believe that my
instructor has no understanding of 3d shapes - doing things like asking which conformation is more stable and showing the same conformation flipped
upside down. This is kinda forgivable, because after my gen. chem class with her I've come to accept that she's not the brightest cookie in the
toolshed.
But then I came across one question that just stopped me in my tracks.
Guys, I hope I'm going insane here, but 1-ethyl 4-methyl cyclohexane can't possibly have any stereoisomers, can it?
I understand that the axial-equitorial configurations can change, but those aren't stereoisomers, are they? Just different energy states.
Like I said, I hope I'm misunderstanding this because if she really just made a mistake as big as I think she did, I'm gonna have to take it to the
head of the department - she's a terrible teacher, but I at least thought she was a competent chemist. I'm beginning to doubt she could even pass the
ACS final herself.
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SteyrTMP
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Here's the question in full, copied and pasted:
Identify the correct stereoisomer and the most stable conformation of the following compound. (bottom left)
(THIS IS THE PICTURE FROM THE BOTTOM LEFT)
(AND HERE ARE THE ANSWERS)
A) A B) B C) C D) D
[Edited on 19-9-2011 by SteyrTMP]
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bbartlog
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I don't believe that it can. Anyway, if the case is as you say then I think the problem is that your teacher doesn't realize her own blind spot. She
should surely be able to obtain test materials that she doesn't have to compose herself. Of course a full grasp of the material would be better but
some teachers are not going to have that.
[Edited on 19-9-2011 by bbartlog]Actually, on looking at the images you post I do see a possibility for left and right handed versions of the
molecule. Interesting how the chair shape and the orientation of the groups interact. Maybe we are not the smart ones :-)
[Edited on 19-9-2011 by bbartlog]
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SteyrTMP
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Well, she relies on the (terribly written) textbook for all the homework questions - there have been a few errors that students have found, but
nothing too monumental. It's really starting to bug me though, because this isn't a rare occurrence, just the one that made me twitch the hardest.
This is her second semester teaching at my school (third if you count summer) and when I had her in gen chem I it was terrible - I was always having
to bail her out when she tried to demonstrate simple concepts (like calculating a kEq) and screwed it up horribly, confusing the entire class until I
got up there and (purely from knowledge I had gleaned from the book) showed everyone how to do it. I explained it in my head as maybe this just wasn't
the sort of chemistry she did (she's an environmental chemistry major) or that maybe she had lab assistants to do it for her.
She's not a mean teacher or an elitist or anything, in fact she's a very nice person, but her teaching abilities are terrible and I'm now starting to
doubt her knowledge as well. I really hope I'm wrong, but at this rate I will be surprised if more than 3 or 4 people out of the class of 20 pass the
ACS exam. Eventually the department will do something about it, but not until 5 or 6 more classes flunk (possibly more, just because chemistry classes
have such a low pass rates even when you do have a good teacher)
I dunno man, it's not my place to pass judgement on who's a good chemist and who's not, but it's getting bad enough that I'm thinking it might be a
good idea to at least bring this to the attention of the chemistry department.
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SteyrTMP
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Quote: Originally posted by bbartlog  |
[Edited on 19-9-2011 by bbartlog]Actually, on looking at the images you post I do see a possibility for left and right handed versions of the
molecule. Interesting how the chair shape and the orientation of the groups interact. Maybe we are not the smart ones :-)
[Edited on 19-9-2011 by bbartlog] |
Really? Could you point this out to me? And are you taking the projection in the first picture into account?
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SteyrTMP
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Excerpt from the book:
"We have seen that alkanes are not locked into a single conformation. rotation around the central carbon-carbon bond in butane occurs rapidly,
interconverting anti and gauche conformations. Cyclohexane, too, is conformationally mobile. Through a process known as ring inversion, or chair-chair
interconversion, one chair conformation is converted to another chair.
...
The most important result of ring inversion is that any substituent that is axial in the original chair conformation becomes equatorial in the
ring-inverted form and vice versa"
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SteyrTMP
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As I understand, A and D would be the same, just inverted, as would B and C. But I don't think any of those would be stereoisomers - bond angles
rotating and rings inverting are just different energy states - not stereoisomers.
EDIT: Oh wait, I think I may have just answered my own question... Can anyone confirm that A and C/B and D are valid stereoisomers?
[Edited on 19-9-2011 by SteyrTMP]
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dennisfrancisblewettiii
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Sometimes teachers are idiots.
Often, it is not the case that they are idiots. The real case is that they are so busy, that they begin to neglect their duties as a teacher (or
proctor/exam-prepper if you want to go that route), and the materials they provide become sub-par. In research institutions, it's because they're more
busy doing their research and trying to get tenure/grants/what-have-you. These situations make me want to just !@#$@#$^. Those people are evil,
selfish deconstructionists with simple-minded goals.
Anyway, make sure you're very certain about what materials you're given before you decide they are sub-par, idiotic, and start calling a professor an
idiot: Most of the time, they're simply being neglectful (yet it's so damn hard to sue them for such).
]-==-=--=
Also, A=D; B=C (iir my sterochem correctly)
If you don't have a model kit yet, buy one. It works pretty well for organic I; but it tends to be someone inefficient in orgo II except for
particular reactions that require 3d thinking during bond breakage and ring formation.
I believe the bonds in that first picture are coming toward the person, thus visually the answer would be A, but I believe D would also be suitable,
because A=D. But A would be more visually correct.
It could be C, because you're looking DOWN onto the molecule, and the bonds pointing toward you are pointing up.
from a classical view if your reference point is the first image, I'm guessing the answer is C. But anyone else here can chime in.
Anyway, this seems like a typical sterochem problem. I think the answer should be found in a text like Carey....
It is in eighth edition on page 120. Your molecule in the first picture is cis, because of the wedge bonds. With a chair drawing, it translates into a
cis-1,4 molecule, which has one bond axial and the other equatorial.
Your answer is B.
They may appear similar, but the isopropyl (right, folks?) group being axial will cause molecular strain, as such you want groups that cause strain to
be equatorial. In reality, if a molecule started having some strain, it would ring-flip.
B is similar to C, except in energy.
If C was occurring, it would ring flip, thus it would then be B.
If you were doing a reaction and got C and B, you'd find most of your molecules in B, because it's more stable.
Anyway, you dropped the ball on this one. It took me a couple of times to read it, but I would have had something like this down in about 10 minutes
or less a year ago. But if you're prof/teacher didn't help you, nor you had available resources that discussed this, when then screw the teaching
plan.
Then again, I've been told you can say "screw you" to the educator when they given you a case such as this. Reason? Because they need to mark which
bonds are axial and equatorial. I suspect it's a notation that has been coming up as of late in education, and many professors want students to mark
which bonds are equatorial and axial, because it matters when discussing stability and strain. But from an older stand-point, you should be able to
look at what you've been given and determine which are axial and equatorial.
Some people will find my statements trivial, though. But I've come across more people who argue that bonds need to be labeled. But people who know how
the tetrahedral sp3 works and where things are pointing can make sense of what they are looking at.
In my opinion, unless someone has a transhumanist/truth-seeker agenda that is rational to maximize the production-possibility frontier, they're an
idiot to me. That's my view. Thus, most professors are either selfish, simple-minded fools or very conservative pricks.
Do your best; you're in hell atm. Be able to recall and abstract from the stuff you come across. You seem to have neglected that aspect (i'm
assuming). And if you continue to neglect that aspect, you will not do well. Sadly, most professors refuse to teach the reality of that; they're not
cognitive scientists, so don't expect them to explain how to learn organic chemistry or the mentality needed. Also, ignore the dude on the web who
keeps preaching arrow pushing (ignore that fool).
I assure you, though, that there are times when professors screw up, neglect their students, and become reckless. Hopefully you will have been
watching your ass all along and know how to get on their ass. Because if they're screwing up, and you can prove it (and I've had times where I could),
you can easily gather the class body to petition. But this isn't one of those times, I believe.
In the times I could have done something, the professors became very nice to me very quickly. But I'm often unforgiving.
Bust your ass. Know your shit. Be able to think abstractly.
p.s.
You need to know your sterochem for orgo II, because it'll show up on about 1/6th to 1/5th of the junk you'll have to deal with then. And that's still
a valuable amount.
[Edited on 19-9-2011 by Genecks]
[Edited on 19-9-2011 by Genecks]
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fledarmus
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No stereoisomers - there is a plane of symmetry running down the center of your molecule, so the mirror image of any of the conformers would be
identical.
What the answer seems to be showing is that there are two regioisomers, cis and trans about the ring, and two conformers of each regioisomer formed by
flipping the chair conformations of the ring. Of these, the most stable product would be when both substituents are equatorial, which would be one
conformer of the trans regioisomer, and the least stable would be when both substituents are axial, which would be the other conformer of the trans
regioisomer. Of the two cis conformers, the most stable would be when the larger group (the isopropyl) is axial and the other (methyl) is equatorial.
The order of stability - A>B>C>D as shown.
The only real problem with the question is that these are not stereoisomers; the rest of the question is valid.
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magnus454
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That's bad, I thought my step-daughter's math question was a pretty good text book foul up. (8+3) x(4+7)=55??? Of course not, she actually did it the
right way, and got it marked wrong, the book had the wrong answer. The correct answer is 132, so I ran the equation through wolframalpha and printed
the page for her to take to school, she received the corrected grade.
History is repeating itself.
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DJF90
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All of the question is valid - the error here lies in the students understanding. I suggest you might pay better attention in class and maybe you'll
learn something. Failing that, find yourself a good introductory book or lecture handout on ring conformations. Evans has a good set on his group
website at harvard.
There are two srereoisomers of the compound; cis and trans. The image you're given is the cis stereochemistry, so you're looking for two groups on the
same side of the molecule, i.e. B and C. Now you have to work out which of these two conformational isomers is the more stable. Due to 1,3-diaxial
interactions a monoalkyl cyclohexane will have the alkyl group equatorial. In this case, you have to work out which group (Me or i-Pr) has the largest
preference for the equatorial position, as this stereoisomer cannot have both groups equatorial at the same time (whereas the trans isomer will have
both groups equatorial). The isopropyl group "wins" in this case, so the most stable conformer is B.
Now that wasn't so hard now, was it.
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Nicodem
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| Quote: Originally posted by SteyrTMP | | Guys, I hope I'm going insane here, but 1-ethyl 4-methyl cyclohexane can't possibly have any stereoisomers, can it?
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And why not? The stereoisomerism of 1,2-, 1,3- and 1,4-disubstituted cyclohexanes is obvious, as you can have the substituents in either
trans or cis relationship and they are not interconvertible by a conformational change (provided the substitutuents are not equal).
I suggest you to UTFSE, because this same question was answered just a few months ago.
| Quote: | | I understand that the axial-equitorial configurations can change, but those aren't stereoisomers, are they? Just different energy states.
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Conformers and rotamers can be stereoisomers, depending on the angles and substituents. Energy state has nothing to do with defining the existence or
non-existence of stereoisomerism - it is a concept used in thermodynamics and as such its connection with stereoisomerism is in that different
diastereomers have different stabilities.
[Edited on 19/9/2011 by Nicodem]
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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SteyrTMP
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Ok, thanks everyone, I think I'm starting to understand this. My first confusion was that I confused the cyclohexane with a substituted benzene ring,
which I believe could not have any stereoisomers (except in the isopropyl group, which was not really a part of the question). The second bit of
confusion is that the book seems to treat rotamers and conformers not as different stereoisomers, but as energy states - more confused by a comment
made by the professor about how only double bonded groups have fixed rotation.
So to make sure I get this, the rotation of a single bond is not fixed, but even so the different rotations that the singe bond can take are
considered different isomers, even though they can all be found in the same molecule at different times? Like, if a ring flips it's considered a
different isomer?
I'm glad my teacher is right. Sorry for not UTFSE, but I really wouldn't know how to put my question in boolean search terms.
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Chordate
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| Quote: Originally posted by SteyrTMP |
So to make sure I get this, the rotation of a single bond is not fixed, but even so the different rotations that the singe bond can take are
considered different isomers, even though they can all be found in the same molecule at different times? Like, if a ring flips it's considered a
different isomer?
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If a ring flips, its considered a conformational isomer. If the atoms attached have different absolute configuration its considered a
configurational isomer or stereoisomer.
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DJF90
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Nicodem, sorry to nitpick but the 1,2-, 1,3- and 1,4-disubstituted cyclohexanes are regioisomers, not stereoisomers, but each regioisomer will have
stereoisomers associated with it.
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Nicodem
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DJF90, that is exactly what I said.
| Quote: Originally posted by SteyrTMP | | So to make sure I get this, the rotation of a single bond is not fixed, but even so the different rotations that the singe bond can take are
considered different isomers, even though they can all be found in the same molecule at different times? Like, if a ring flips it's considered a
different isomer? |
The IUPAC definition of isomers is: "One of several species (or molecular entities) that have the same atomic composition (molecular formula) but
different line formulae or different stereochemical formulae and hence different physical and/or chemical properties."
As you see it does not talk about isolable compounds, but "species (or molecular entities)". The border between isolable and nonisolable compounds is
impossible to draw anyway, even when you define the physical conditions. In any case rotamers and conformers are just as clearly isomers as any other.
The fact that they interconvert is not a matter of the definition. Thus rotamers can happen to be in stereoisomeric relationship, or in a more rare
occasion, they can be in an enantiomeric relationship.
| Quote: | | Sorry for not UTFSE, but I really wouldn't know how to put my question in boolean search terms. |
If you use "stereoisomers" as a keyword you get:
https://www.sciencemadness.org/whisper/viewthread.php?tid=15...
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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DJF90
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Sorry Nicodem, was reading on a mobile device and thought you said the 1,2-, 1,3- and 1,4- were stereoisomers. My bad..
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