Today’s activity (ChemActivity 34R) used potential surfaces to assign charges to hydrogen atoms in ethylene and acetylene. The charge trend goes like this (I’ve added ethane for good measure):
(least +) H in ethane < H in ethylene < H in acetylene << H in water (most +)
This trend can be rationalized by thinking about the energies of the overlapping atomic orbitals.
The web is a great place to waste, uh, invest one’s spare time, assuming one has any time to spare. YouTube, Facebook, they all have their aficianados, but my cousin Arkee, a retired professor at Tel Aviv University, just alerted me to a totally cool, totally science geek-oriented web site: Trailblazing: Three and a half centuries of Royal Society publishing.
It’s simple to use. You zoom along the time line, click on a year, and see what pops up. Not only do you get beautiful images and well-written explanations about amazing scientific trivia (do you know what kind of animal blood was used in the first blood transfusion?), you also get links to the original Royal Society publications. The following abstract was written by Isaac Newton himself (click on it to expand the image).
A few reflections on the material from Monday morning’s lecture.
Epoxide ring-opening. First, epoxides can react with strong nucleophiles without adding acid. The strained ring makes the epoxide reactive. These reactions look like SN2 reactions.
Second, epoxides can react with much weaker nucleophiles by adding acid. Examples of weak nucleophiles include water, alcohols, carboxylic acids. The mechanisms of these reactions are a little strange. If one epoxide carbon is tertiary, the nucleophile adds there, but regardless of where the nucleophile adds, a backside attack occurs. So a little SN1 and a little SN2-like behavior all at the same time.
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Slides from today’s lecture can be viewed here.
I didn’t quite get through everything that I had promised and I’m not taking a straight path through the chapter. If you would like to know what is coming next, here’s a list of the topics in the sequence that I will be talking about them:
?? = depends on time constraints
The images and data table for the three SN2 transition states shown in lecture today can be viewed here.
First, if you would like to see the MO pictures that I used in class today, check out last year’s post on back side attack & HOMO-LUMO overlap (Oct 13, 2008).
There are also a couple of points that I want to add concerning reaction rates (kinetics):
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Pity (or envy) poor Alice. In one book she falls down a rabbit hole. When she gets to her feet, she finds bits of food that demand to be eaten (“Eat me!”). And in the next book she steps through a looking glass only to find everything reversed. What is a girl to do?
Well, one thing scientists have always thought is that Alice should not eat any food while she visits the looking-glass universe. All of the protein in Alice’s body is built from single enantiomers of chiral amino acids. This means these proteins, including her digestive enzymes, exist as single enantiomers, and they wouldn’t be able to digest the mirror-image proteins that get cooked in a looking-glass kitchen. Worse, if she did eat looking-glass food, she might get a terrible stomach ache, and would definitely starve. According to the traditional view, there just isn’t any biological value in looking-glass amino acids. A new study, however, turns this view on its head. If you would like to read about this, check out “Expanding Functionality Within the Looking-Glass Universe” (News Perspective, Science, 18 September 2009, 325, 1505-1506).
Slides from today’s lecture can be viewed here.
Correction: I was gently informed after lecture that I was wrong about the behavior of CH3CN (acetonitrile) and water. They do mix. I had said they don’t. In fact, mixtures of these solvents are routinely used as a solvent mixture for HPLC, a form of chromatography that is closely related to the procedure we will be using in lab next week. My mistake.
I didn’t use PowerPoint on Wednesday, but I did show potential maps of several simple molecules that represent the functional groups covered in chapter 8. Slides of these potential maps can be viewed here.
The slides for last Friday’s lecture, which covered the last part of Chapter 7 – the stereochemistry of chemical reactions – can be viewed here.