Nearly all of the lab reports have been graded so you can expect to see graded lab reports turning up in your inbox over the next week or so.
The lab reports contain three grades: notebook, lab work, report. 99+% of the lab work grades were satisfactory, but this was not the case for notebook and report grades.
Because unsatisfactory grades on notebooks/reports were fairly frequent, and because it is natural to assume the worst concerning such grades, I want to clear up any concerns that you might have about unsatisfactory marks.
This lab experiment lasts 3 weeks. Students finished varying amounts of lab work during the first week so planning the remaining work is a little complicated. I’ve written up a two-page set of instructions to guide you. Please read this well ahead of your lab day and prepare accordingly.
General anesthetics, the chemicals that temporarily ‘put you under,’ have transformed surgery, but doctors and scientists are still learning how these chemicals work. Continue reading
I saw this article (C&ENews, 12 Oct 2015, p. 38) about the molecules that make Halloween pumpkins look and taste and smell the way they do:
There seems to be a little confusion about how to interpret your experimental IR spectra and how to use Spartan’s calculated spectrum. I hope the following will help clarify things a little (I recall that I had to interpret about 30-40 IR spectra as a student over a couple of years before this began to feel routine):
After the game (Renn Fayre ’09)
You can buy a new molecular model kit from the Chemistry Stockroom ($20), but you have other choices (learn more here), and used model kits are often available at reduced prices from students who have no more use for them. Before you buy a used kit, make sure it contains most of the pieces you need. Here’s a parts list + photo of a new kit.
I have (and will) post some extra “challenge” problems from time to time. Let me explain their purpose so that you can work these problems into your schedule in an appropriate way.
The most frequently made comment by Chem 201 students is that they learn to study organic chemistry as the semester rolls along. Please read that again. They don’t say, ‘they learn o chem’ because that’s a given. What they say instead is, they ‘learn to study o chem.’ Nearly everyone who takes 201 spends some time struggling to discover and cultivate beneficial study habits.
The reading of Exp’t 4 (salicylic acid) lab reports informed me that many 201 students struggled with the complexities of the proton NMR spectrum. The NMR of salicylic acid is complicated, no doubt about it, but it can be understood and it provides a great introduction to the types of coupling patterns seen in organic compounds.
Here are two readings that I strongly recommend:
- Combination Coupling Patterns in Salicylic Acid’s 1H NMR Spectrum (by me)
- Sorrell, “13.4 Spin Couplings in More Complex Systems” (p. 420)
Sorrell’s treatment is excellent, but it might help to read my essay first because it explains a spectrum about which you have already thought a great deal.
The web is loaded with videos explaining optical activity and demonstrating polarimeters of various types (see Sorrell, Fig. 4.3, p. 114). Here are two SHORT clever videos from David Whyte (and his cheeky daughter?):
- Home made polarimeter. Modern computer displays rely on liquid crystals and the light emitted by an LCD is polarized. David displays a yellow rectangle on his laptop screen so the yellow light emitted by the screen is a good approximation of polarized monochromatic (single wavelength) light. Next he loads some syrup (sucrose + water) into a transparent can on top of the screen. Finally, he films the screen+syrup setup through some polarized sunglasses and rotates the glasses back and forth. Things to notice:
- When he films the screen+syrup setup without the glasses in place, everything looks yellow and equally bright. You can’t see any effect of sucrose on the angle of polarization because your eyes are not sensitive to polarization angles.
- The first time he puts the sunglasses between the camera and the setup (0:31), the syrup goes black while the screen stays bright. Clearly, the syrup is rotating the plane of polarization, i.e., the light coming out of the syrup has a different angle from the rest of the screen. The syrup looks black because the light coming out of the syrup is rotated 90º from the sunglasses’ polarization.
- Three seconds later (0:34) he has rotated the sunglasses so that the screen goes black and the syrup is light. Again, this proves that the syrup is rotating the plane of the polarized light coming out of the screen. If the syrup were not optically active, the syrup and the screen would all go black simultaneously.
- Home made polarimeter, #2 by David Whyte. The previous video showed that syrup (sucrose) rotates plane-polarized light. This video shows that the angle is correlated with the path length of the solution. The same basic setup is used: computer screen (light source + polarizer #1) + sample + sunglasses (polarizer #2), but there are four samples: W (pure water), 1 (short layer of syrup), 2 (medium layer of syrup), 3 (tall layer of syrup). Also, in this video he holds the sunglasses steady and rotates the screen. Things to notice:
- Pure water, W, is achiral and optically inactive. Notice that the color and brightness of W matches that of the surrounding screen. Water has no effect on the angle of polarization.
- Syrup is optically active, but the amount of optical activity (the amount the sucrose sample rotates the plane of polarization) depends on the amount of sucrose in the light beam. The longest (tallest) sample, 3, deviates the most from pure water, i.e., it has the greatest effect on the plane of polarization. The shortest sample, 1, behaves almost identically to pure water. 2, as you might expect, falls in between.
- This video looks at a full 360º rotation of the screen polarization relative to the sunglasses. The screen should go dark twice during a full rotation: once when the light hitting the sunglasses is polarized 90º from the sunglasses’ polarization, and again when the light is polarized 270º from the sunglasses’ polarization. See if you can spot these two “go black” spots for each sample. (Hint: use the arrow that Whyte has drawn on the screen to read the angle; you should see a sample go black a second time when the arrow has reversed its direction from the first time the sample went black).