Organic Chemistry Exam Show-and-Tell

November 13th, 2013

ChemBark's Orgo BunnyWhen I was in college, there was a professor who was notorious among the grad students for stopping them to show off his organic chemistry exam questions. Unlucky TAs on their way to the bog would get tied up for what seemed like hours going through drafts of exams in the hallway.

But now I think I know how he felt. There’s a certain degree of satisfaction associated with writing a problem that makes students integrate their knowledge of multiple subjects within a given set of consecutive chapters in whatever textbook you’re using.

Instead of stopping TAs in the hallway at work, I figured I could move the practice into the 21st century by using the blog. I’ll post any gems I’m proud of in this thread. Feel free to share your favorites as well.

To start…

Exam 1, Problem 3-2. (8 points) Draw the most (Brønsted–Lowry) acidic, optically-active isomer of C6H10.

Click here for the answer. The chapters for this exam included acids and bases, isomerism, functional groups, alkanes, and stereochemistry.


50 Responses to “Organic Chemistry Exam Show-and-Tell”

  1. A recent PhD Says:

    Technically your question is flawed, there are at least two molecules equally acidic (enantiomers), therefore asking for “The most acidic” is wrong.

  2. Alex G Says:

    OSNAP

  3. The Iron Chemist Says:

    What percentage of the students got the correct answer?

  4. Graet Chem Says:

    Enantiomers aside, I’d be interested in how many students got it correct too and what level you were pitching it at.

    It seems to me like one of those exam questions you can just ‘get’ if you had the factoid about the relative acidity of alkanes, alkenes and alkynes tucked in your brain.

  5. Paul Bracher Says:

    Yes, either enantiomer was accepted, as well as not specifying stereochemistry.

    About 6 of 37 students got full credit, IIRC.

  6. Tiger Chemistry Says:

    I, personally, would have probably given a little helper question (ie “what is the degree of unsaturation in this molecule”) for a piddly sum (eg 1 mark, maybe a half) to get all the kids thinking about how they can vary hybridization (and with it, acidity). Of course, if this was a challenge/bonus/weeder question it’s fair to leave them to their own devices.

  7. NQ Says:

    ‘Factoid’ about acidity of sp/sp2/sp3 C-H bonds? Or a necessary part of one’s chemical understanding and something that can be figured out with clear logic?

  8. Paul Bracher Says:

    @Tiger Chemistry & NQ: The practice exams I wrote had detailed explanations of how to approach problems where you’re given a molecular formula…i.e., start by calculating the degree of unsaturation. If they did so but got the wrong answer, I gave some partial credit.

    We also spend a lot of time talking about trends in acidity, including hybridization effects.

  9. Older and Wiser Chem Prof Says:

    The question is a good one – my only concern is that the two elements of the question are entirely independent, so they act as a double filter. Maybe test these concepts independently or come up with examples where stereochemistry and acidity are more essentially intertwined.

  10. Bryan Says:

    A slightly more sadistic prof would ask the students to provide the IUPAC name instead of simply letting them draw the structure.

  11. Graet Chem Says:

    @NQ – yes, a factoid. I’ve passed enough exams to know that committing such things to memory is a lot more of an efficient way to get marks than sitting there trying to work things out from first principles. The practice of Chemistry should not be a simple exercise in memory, but many of the exams are (they being entirely artificial time pressured constructs after all).

  12. Gui Says:

    I liked your question! tumbs up! :)

    My favorite, which are the one I still remember from my exam, are:

    – Why is azulene polar (the “neutral” strucuture was drawn in the question)
    and
    – Draw cis-decaline in newman projection

    I will think on my own questions latter!

  13. had Says:

    There has been a lot of positive feedback about this question. I do not agree. I think this could be improved mightily.
    I like the aspect of having to understand and compute the degree of unsaturation and I like that they need to understand to incorporate a chiral center, but I think the Lewis acidity part and the “find the best” are aspects that miss the mark.
    First, organic chemistry should be about applying your acquired knowledge in a creative way — that’s why all of us are in organic chemistry. Therefore I think you shouldn’t ask the student to find “the one” answer, but rather to find any answer that satisfies the criteria specified. That is a much more satisfying exercise for everyone involved.
    And as for the lewis acidity part — why? You will just end up with a bunch of students who now remember than alkynes are lewis acidic. Is that a useful thing to teach about alkynes? I would say no, if I could have students remember one thing about alkynes, I would want to reinforce their lewis basic nature — which is more often than not how they behave in reactions. So this part about lewis acidity is technically correct, but will only hurt your students in the long run.

  14. Ian Says:

    I would disagree with had; the fact that alkynes are Bronsted acidic is incredibly important–think about the Sonagashira coupling, for example. I think that it is an excellent example of how the hybridization of an atom can strongly affect its ability to stabilize a charge.

  15. Paul Bracher Says:

    Also, the Bronsted acidity of alkynes is very important in terms of generating nucleophiles for SN2 reactions for making C-C bonds (e.g., by opening epoxides).

    We spent a lot of class time on the acidity trend of sp3 < sp2 < sp.

    The question was challenging, but given what we covered in class and what was on the practice exams, I think it was fair. I write some questions to give those on the right side of the mean the chance to shine. This was one of them. I was very pleased several people got it 100% right, and that several more had decent approaches to solving the problem.

  16. Gui Says:

    Nest time you can allways ask C5H8.

    I am not sure I know the correct answer!

  17. had Says:

    Sorry guys, my bad. I read you question as “Lewis acidity” rather than “Bronsted acidity” and thought were trying to have them memorize minutia regarding he relative LUMO energies. I revise my comment to reflect that the acidity portion is legit. However I’m holding my ground on the creative part. What I loved about orgo I & II was that there was no single correct answer to an organic synthesis problem — there were as many answers as I could think of and the more I knew the more answers I could come up with and then evaluate against each other. If I was taught to limit my thought processes to only find the single “best” answer, and that no other answers were worthwhile, I would probably have said “f*** it” and and found another subject that was more fun.

  18. Sadist chem Says:

    Phenanantroline based mouth makes me uncomfortable. Why don’t you put some retrosynthesis in the first exam? Just to create panic.

  19. Steve Chamberland Says:

    Wonderful discussion everyone. I’d like to throw in a question. Many of my students voice that my concept-based exams take too long to finish in just a fifty-minute class session. I’ve tried to shorten them, but then it’s just down to fewer than 10 questions. I can’t give take-home exams because I feel many will collaborate even if I tell them not to. Besides, it’s just not fair to those who are honest and do their own work.

    Has anyone considered giving their sophomore-level organic chemistry exams in the evenings, or on Saturdays, for example, as a way to eliminate the time pressure? When I took a grad-level organic mechanisms course as a senior undergrad, he gave us from 7-10 or 8-11pm to do the exam. No one complained. We respected the professor’s authority and just showed up when we were told to do so. At my current institution, I’ve proposed holding exams outside of class time, but the prospect of outright mutiny from my students deters me. I’m not sure many would be willing to go the extra mile. Even so, someone would have to work, provide childcare, or do something else at virtually any alternative time I could propose. What do y’all think, is it a stupid idea altogether, or does it depend on how you sell it?

  20. Paul Bracher Says:

    @Steve:

    We hold our exams at night. My organic exams are 6:30-8:30 pm. The students don’t question it, because that’s the way it’s always been. If professors want to give exams in class, that is their prerogative, but I like the night system for the reasons you describe.

  21. Nick K Says:

    Feeling very smug this morning – I got it in 15 seconds!

  22. Graet Chem Says:

    It’s interesting that US colleges don’t appear to have an ‘exam diet’ organised at the university level like here in the UK. Exams are run at set times of the year in designated time-slots and venues organised across all departments and faculties (the most formal of these are the Oxford exam schools).

    It was fairly common for me to be sitting chemistry papers in a large hall with several different examinations taking place. Then again, we generally don’t do ‘mid-terms’ or class-based tests in the UK only formal examinations directly after christmas and at the end of the academic year (in some institutions such as Oxford and Cambridge, only the latter).

  23. Tiger Chemistry Says:

    @Steve:

    For organic courses at my undergraduate institution (large public university) we almost always had midterms in the evening. Generally a night could be found where >95% (of a ~150 class) could attend; the rub is that you will almost certainly have to write a make-up exam (something that you still probably have to do most years for in-class exams) for the few that absolutely can’t make it.

    @Graet Chem

    I’ve got to say that I am not a huge fan of the exams-after-Christmas style: “let us teach you everything and then give you weeks of holidays to forget all of it.” I’d much rather have Christmas to nurse my (academic) wounds than to agonize over impending exams.

  24. Mike K Says:

    Reminds me of the classic: what is the smallest (by MW) chiral hydrocarbon? There are two equally good non-enatiomeric answers.

  25. Paul Bracher Says:

    @Mike K: Are you “allowed” to use isotopes of hydrogen?

  26. Mike K Says:

    @Paul

    Well, lets specify that you’re not :)

  27. Nick K Says:

    Mike K: Is the answer 2,3-pentanediene (an allene)? If so, full marks for ingenuity! (7 seconds this time, BTW).

  28. Mike K Says:

    @Nick K

    Correct!
    But there is a second answer, equally good.

  29. Melanie Cooper Says:

    I agree that your question is “cute”, and I can see why people like it – but I would ask what is its the purpose? Assessment can serve two purposes – either it can be used to provide feedback and improve student performance, or as a sorting mechanism. Either you see yourself as a weeder (for which this question clearly works very well) or a gardener – helping students learn and improve. I would say that this question is not useful for helping students improve their understanding – it requires students to know two facts – but the way the question is written it is unlikely that you will be able to determine what a student’s problem is – and therefore it is unlikely that you can help them improve.
    In addition I would ask what are the learning goals for your course? What are the big ideas and how do you want students to use them? Do your learning goals require that students use their knowledge to explain a phenomenon, use scientific principles to predict outcomes, used evidence and data to make an argument? If all you are doing is asking students to memorize and recognize then even though the question is “rigorous” you are not doing them any favors.

  30. Nick K Says:

    Mike K: 2-methyl methylenecyclopropane. (20 seconds this time)

  31. Mike K Says:

    @Nick K

    You’ve opened my eyes! Apparently there is 3 solutions :)
    The third one would be 1,3-dimethylcyclopropene, very similar.

  32. OldGuy Says:

    @Melanie Cooper. I think Paul’s question definitely asks the students to use scientific principles to predict an outcome. It’s certainly not a memorise and recognise question. And surely you don’t have to be a gardener OR a weeder. A bit of both is usually required and it sounds like Paul did his gardening in the lead up to this exam, which does the weeding (or sorting).

  33. Paul Bracher Says:

    @Melanie:

    I really don’t see how you could look at this question as one based primarily on memorization. Is knowing that hydrogens bonded to sp-hybridized carbons are more acidic than those bonded to sp2 and sp3 carbons a question of memorization, or one of understanding how s and p orbitals differ? I think it is the latter, although you can certainly “get there” by memorization. Also, I don’t emphasize memorization in this course. I let students bring two handwritten sheets of notes into the exam just so that they don’t feel like they should spend their time memorizing.

    Finally, I don’t see the roles of “gardening” and “weeding” (your words) as being mutually exclusive. You do both as a professor, and individual assignments can serve both roles. Part of my job is to evaluate students and assign them grades. My principal mechanism for doing so is through performance on exams. How else should one evaluate/”weed” students in organic chemistry? The vast majority of “gardening” happens in class, through homework assignments, one-on-one in office hours, and through practice exams with detailed keys.

    Finally, this question was 8 points on a 100-point exam. Surely you can see how such a question could be of value. Alternately, I would love to see what you regard as a good (or ideal) exam for a sophomore course in organic chemistry. If you e-mail it to me, I’ll post it.

  34. Bob Sacamano Says:

    @Mike K:

    There are at least four C5 solutions, you can add 3-methylcyclobut-1-ene to the list.

  35. Mike K Says:

    @Bob

    Shit, this is embarrasing, i thought I had it right. But it’s good to know there’s more :)

    If we ignore the chemical reality and solve a purely structural problem, then 3-methyl-cyclobut-2-yne is the best answer…

  36. anon Says:

    Nope. 1,2-cyclobutadiene would be the correct answer in fantasy land:

    http://stuartcantrill.com/2011/09/09/the-smallest-chiral-hydrocarbon/

  37. Nick K Says:

    Mike K et al: Very elegant solutions indeed! Why didn’t I think of 1,3-dimethylcyclopropene or 3-methylcyclobutene? The latter is particularly elegant, IMO.

  38. wolfie Says:

    well, give ‘Em all their grades.

    Remember, they are your clients.

  39. Untenured Prof Says:

    @Paul B

    Paul, I’m fascinated that you let the students bring in two sheets of “notes”. I’m at a loss to understand how this can work, what are your class averages? (Wait, FERPA probably doesn’t allow you to tell us that…)

    Do you ask, as a general rule, extraordinarily difficult questions that the students hadn’t seen in class and they must apply their knowledge (scribbled on a sheet of paper) to solve them?

    Thanks.

  40. Paul Bracher Says:

    @Untenured Prof:

    Just to give you an idea of my reasoning, I allow notes during exams because I would prefer that the students not get stressed out about memorizing stuff. Instead, I would like them to focus most of their attention when studying on understanding the fundamentals of the material and how to apply them (with practice problems). That’s not to say that professors who don’t allow notes are focusing their students’ attention on memorization. We all have our own preferences and approaches to teaching; I am just starting out and trying to find a good mix that works for me.

    I make the students hand-write their “cheat sheets” as a mechanism for getting them to think a little more deeply about the material. Since they have so much material and limited space to summarize it (two pages), the students have to think about how to sort, prioritize, and organize everything. Another benefit of requiring the notes to be handwritten is that we tend to remember things better that we’ve written down versus just read or heard.

    Also, while organic exams are not simulations of “real life”, any time the students need this material in the future, there won’t be anything to stop them from looking at notes or the Internet to jog their memories. I’d allow electronic devices if I were more confident I could police communication among students taking the exam. I think allowing notes also helps remove the advantage a cheater using crib notes would have if they went undetected by a proctor.

    I’ve given two exams so far. The high score has been a 90 (on both), the lowest a 20/23. The means have been around 55. I like the range, but I—and the students—would like to see the means move up.

    They are learning and so am I.

  41. Untenured Prof Says:

    @ Paul

    Thanks for the feedback and for trying new things with your class!

  42. horde Says:

    “we tend to remember things better that we’ve written down versus just read or heard”

    so true, and yet so little appreciated

  43. casual reader Says:

    Paul,

    I like the question very much, well done!

    I’d be curious though: do you give partial points (more than 0, less than 8) for answers that get parts of the question right? Like, do you give more than 0 points for only the correct triple-bond functionality? If so, how do you do it? I’m asking because, as much as I like questions like that, there are so many partially correct answers to the question which are however so difficult to grade …?

  44. Lola spencer Says:

    I have been struggling with several questions in my organic chemistry class maybe you can help?

  45. wolfie Says:

    And still looking for exams.

    What is your reality, Paul ? You need to teach acids and bases at St. Pauls.

  46. 1 Says:

    http://www.rescorp.org/grants-and-awards/cottrell-scholar-awards/awardee-profiles/banerjee-sarbajit

    “A major worry is that our introductory chemistry courses are failing to convey the excitement of discovery that underlies science, instead presenting science as a tortuous and tedious process involving concepts that must be memorized and equations that must be manipulated while being vigilant for the ever-lurking possibility of the ‘trick’ question,” he says. “What is lost all too easily is perspective of the broader goals of the scientific enterprise and the interconnectedness of scientific and technological disciplines that undergirds modern research.”

    It would be good to come up with tricky questions….however, the focus should be elsewhere as to how to teach them better than to test them better. Lot of criticism for OCHM is the lack of context for all the things that the students are expected to learn.

    Pedagogically, even the profs should be asked to make the lesson plans akin to teachers to teach the subject matter and put in to the criteria of Bloom’s taxonomy to make sure that the students are provided with the learning experiences to gain the knowledge, understanding and appreciation.

    Instead of sharing the tricky questions, can the best notes, slides, or the approaches to teach various concepts be shared? It would help most of the young faculty to focus less on teaching and teaching materials preparation and to focus more on the things required for research. I agree that the students pay lot of money and they need to be taught well. How ever, the best approach to teach them is to involve them in the research right from the sophomore year, there by the learn the things by seeing the context where OCHM is applicable.

    Learning would b more fun for the sake of learning than to learn for the test/exam sake.

  47. wolfie Says:

    How far has the semster proceeded, meanwhile ?

    Not, that I meant anything mean, how could I ?

  48. DS Says:

    3-Methylcyclopentadiene with the methyl hydrogens oriented to break the mirror symmetry.

  49. Nick K Says:

    DS: 3-Methylcyclopentadiene has free rotation about the methyl group, so it cannot be regarded as chiral. The gauche conformation of n-butane is chiral, but that doesn’t mean n-butane is a chiral compound.

  50. Lain Says:

    I came across this while trying to find the answer to this exact problem, My professor had it on an O-Chem 1 test, and I can say that I dont know of anyone who got it right, including me, and I am one of the best in this subject of my class…very difficult, especially when considering the time restraints and pressures of a non-multiple choice test


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