Exam #2 answers

Exam #2 went quite well. You can pick up your exams from the box outside my office and download selected answers.

I apologize for the long delay in returning this exam. You know the main reason why everything got held up, but I regret the delay just the same.

A couple of comments beyond those in the answer sheet:

• The first step in problem #1 is quite difficult to think about. The other steps are more straightforward and everyone dealt with them more reliably. You are still having some problems with labels. Cycloadditions get [m+n], sigmatropic rearrangements get [m,n], and electrocyclizations get DIS/CON (and maybe number of electrons).
• The first step in problem #2 is nearly impossible to think about. Well, I exaggerate. You can think about it, but it is too difficult for an exam problem. Sorry. Interesting note: most of the remaining steps did not need to be analyzed because they duplicated steps in problem #1, but most of you analyzed these steps just the same. So it goes.
• Only one person built all four transition state models correctly. Two people built two models correctly. Two people were stumped or misled by the models that they had (which weren’t transition state models). I was hoping that the last problem on HW #8 would prepare everyone adequately for this exam problem, but I guess not. Is there something I could have done differently that would have helped?

Exam #1 answers

The results for exam #1 were quite good. Everyone passed! :-)

Grading the exam was difficult because everyone produced and worked from different models (one person might think a molecule should be planar, another person might choose nonplanar, one might point an OH or vinyl group “out” while another person would point it “in”, and then there would be procedural differences too: one person might perform an Energy calculation and another might perform an Equilibrium geometry calculation). So it was a real grading nightmare, but not unexpected. More to the point, you mostly did the things I wanted you to do with your models.

We do have some points of disagreement, however, so please study the Exam #1 answers and the comments I attached to your exam (2 sheets) carefully.

Exam #1 – download here

Exam #1 consists of two problems and two pages. Once you download the exam, the ground rules apply (see below). Read the ground rules before you download the exam (download = “receive”). Don’t download the exam if you are not ready to “receive” it, i.e., if you are still chatting with classmates about the material (especially paper #5). If you are ready to begin, you can download the exam (revised Fri Mar 6).

Ground Rules

You may refer to your lecture notes, papers that we have read (including the papers mentioned in the exam), any handouts (paper or web) that I have provided, and previous homeworks. Do not consult anything or anyone else once you receive the exam. You will need access to the computer lab but if you wait until the last moment, I won’t be able to rescue you if a calculation dies or a fire alarm closes the building (take this as adequate encouragement not to wait until the last moment for the modeling portion). You may start on the exam when you receive it, and you may work on it as often and as much as you like. Please label your answers clearly so that I can connect them directly to my questions.

Due: Monday, March 9, 10 am, my office or mailbox

Exam #2 comments

A number of you have stopped by to talk about the exam’s problems, or perhaps I should say, problems you’re having with the exam.

In order to minimize any inequities that might arise from me giving information to some students and not others, here is a summary of major items that I have communicated:

Problem #1

1. I mis-lettered the parts of problem #1. It starts with part b instead of part a. Oh well. Please follow the lettering in the exam.
2. C-Li bonds are drawn as covalent in the figures. This is convenient for the artist (me), but for purposes of drawing and analyzing pericyclic reactions, you may find it more useful to view this as an ionic bond.
3. Other aspects of the formulas might also be adjusted to make them more convenient for drawing and analyzing pericyclic reactions. (Just one of several possible hints: there is more than one way to draw a benzene ring…)

Problem #2

1. I have given you incomplete information about stereochemistry. I don’t intend to change that. Do the best you can. Perhaps more than one outcome is possible. Perhaps not.

Overall

1. Although this is a take-home exam, it is not meant to be a marathon exam. It was constructed as a take-home so that I wouldn’t have to sacrifice class time for the exam, and you would be able to do some modeling. I really figured that the non-modeling portion of the exam could be completed in an hour or less.