Chem 201 Archive – Alan Shusterman

Month: November 2010

  • When is a 'glycol' not a glycol?

    In class today, I gave you a special name for 1,2-diols: glycols. Since a 1,2-diol requires a minimum of two carbon atoms, the simplest compound in this class is ethylene glycol, HOCH2CH2OH.

    It turns out that one glycol has been in the news a lot recently. During lunch I skimmed through a recent article on hair straighteners that appeared in C&ENews, the weekly news magazine of the American Chemical Society. The article had this to say:

    “Straightening techniques such as Brazilian Blowout originated in Brazil, where they’re called escova progressiva
    (progressive blow-dry). They provide a few months’ worth of
    straightening before wearing off. That staying power comes from fusing
    additional amino acids from keratin to the hair fiber. A cross-linking
    reagent, such as a solution of formaldehyde gas in water that can go by
    the name of formalin or methylene glycol, accomplishes the fusion.”

    Methylene glycol, HOCH2OH, contains only one carbon so it is a 1,1-diol. Because this name seems to break the rule that I had stated in class (glycol = 1,2-diol = vic-diol), I decided to do a little more leg work. First, I checked our textbook. Loudon, p. 323, states that glycols contain two hydroxyl groups on adjacent carbons. The Encyclopaedia Britannica gives a looser definition, saying that a glycol is any diol in which the hydroxyl groups are attached to different carbons. Common sense application of these definitions seems to suggest that “methylene glycol” is a misuse of the term “glycol” because this one-carbon compound cannot contain “adjacent” or “different” carbons. This probably won’t keep people from using this term, however, because it sounds very scientific and it is fairly well entrenched.

    So why was I reading an article about hair straighteners and why is one “glycol” so newsworthy?
    (more…)

  • Hg-free Exams? Not quite

    Homework assignment #6 contained an important announcement, but many students who worked on the homework either ignored the announcement or didn’t fully understand it (and many students may not have even seen the announcement given the small number of completed assignments that were turned in).

    Here’s the announcement quoted in its entirety and verbatim:

    Reagent Alert: I will not cover, and I am not going to test you on, the chemistry of Hg(OAc)2 + H2O, then NaBH4, but Loudon uses this reagent extensively in practice problems. Every time you run into this reagent, simply substitute another reagent in its place: H2O + cat. H2SO4.

    And here’s what I saw on HW #6:

    • occasionally replacement of the entire oxymercuration-reduction reagent with H2O + cat. H2SO4 (correct behavior)
    • more often, replacement of parts of the oxymercuration-reduction reagent with H2O + cat. H2SO4. This is incorrect. Do not draw hybrid reagents like “H2O + cat. H2SO4 then NaBH4”. In fact, do not resort to oxymercuration-reduction in this class at all.

    But that’s not all. This reagent-alert does not mean that you should never draw Hg-containing reagents. The activity for class #33 – Alkyne addition reactions: hydrohalogenation and hydration describes the use of salts containing Hg+2 as a method for promoting hydration of alkynes.

    Bottom-line: draw H2O + cat. H2SO4 to hydrate alkenes (Markovnikov) and draw H2O + Hg+2 + H2SO4 to hydrate alkynes (Markovnikov).

  • How to finish the acetylferrocene lab

    We do not have any lab work scheduled for this week or beyond. The only
    thing that needs to be accomplished by most students is printing the NMR
    spectrum of acetylferrocene. This can be done during the lab periods
    this week, but you can also do it on your own.

    Instructions for printing the NMR spectrum have been added to the Week
    #3     section of the acetylferrocene procedure (see lab manual). Please
    note that two pages are required.

    The lab assistants will be available in the lab from 1-1:30 each day
    this week. If no one comes to lab by 1:30, they will go into the
    computer room to help out there. If no 201 students seek their help in
    the computer room by 2:00, the lab assistants will be free to leave.
    After that, all NMR printing will have to be unassisted.

    Instructions for writing the acetylferrocene lab report have also been
    updated. I will give one final lecture on NMR spectroscopy that also
    pertains to the lab report. This will be the Thursday evening *after*
    Thanksgiving, Dec 2. The lab reports are due Wednesday, Dec 8.

  • Anticrastination anyone?

    Procrastination is a problem. I have trouble dealing with it. Everyone that I know suffers from it. (Actually, “suffers” is the wrong word; “complains” would be more accurate. Procrastination can be a good thing, but if you find yourself complaining about not getting around to things, you have a problem.)

    How do I procrastinate? Two ways. First, I don’t do things that I need to do. Second, I siphon my time into other things that don’t help at all. So what can I do? Fight back. Become an Anticrastinator.

    Today’s At Reed describes two computer programs, Freedom and Anti-Social (no joke!) that can help you anticrastinate more effectively. These programs won’t make you do the stuff you ought to do, but they can keep you away highly distracting time siphons.

    You might also be interested in signing up for one of the anticrastination workshops offered by Student Services. The complete schedule of academic workshops is posted here; RSVP encouraged, but drop-ins are welcome.

  • New Homework Signals New Direction

    I have just posted a new homework assignment: HW #6. You can download it from the homework page.

    This assignment signals a switch from an emphasis on modeling to an emphasis on chemical transformations. It is essential at this point that you adjust your study techniques so that you learn about transformations from several angles:

    • knowing what to expect from certain reactions (fill in the product)
    • knowing how to bring certain transformations about (fill in the reagent)
    • knowing how to produce desired materials (fill in the starting material + plan a synthesis)
    • knowing how to rationalize why things are the way they are (draw a plausible mechanism).

    I hope that the new direction in homework assignments will encourage you in this.

    A Study Guide and Solutions Manual essay that is particularly worthwhile: 5.2 How to Study Organic Reactions (p. 70). I hope you will incorporate the suggestions that this essay makes into your study habits. Some other valuable essays:

    • 5.3 Solving Structure Problems (p. 74)
    • 11.1 Learning New Reactions from Earlier Reactions (p. 211)
    • 11.2 Common Intermediates from Different Starting Materials (p. 211).

  • Watch the TLC video

    Last Thursday night I started showing a video demonstrating the technique of thin-layer chromatography. I also pointed out where to find this link. Let me point you at it again:

    Lab techniques (MIT videos): TLC (thin-layer chromatography) (16 min)
    (located in Off Campus section of our Links page)

    You might also like the video and animation provided by the Interactive Lab Primer. I haven’t watched it so you tell me which one, MIT or Interactive Lab, should go into the lab manual.

    Please watch one video or the other before lab this week. Students who finish their columns by 4 PM will be expected to do some TLC analyses before leaving the lab. Stragglers will need to watch the videos again before next week’s lab.

  • Learn about the last EDG on your own

    I have posted all of the slides from today’s lecture (see Syllabus or here). I did not finish my description of electron-donating groups (EDG), but I had saved the easiest material for last so I would like you to look over slides 15-18 on your own. The next exam covers all of the material through this class. The exam will not cover material from Friday’s upcoming class.

    Here is a verbal description of what you should get from slides 15-18: (more…)