Category: Post-lecture

The learning objectives for activity #7 include being able to interconvert free energy changes (ΔG or ΔPE), equilibrium constants (Keq) and the percentages of the two conformers at equilibrium. Unfortunately, I neglected to list practice problems for this topic and there are three in Sorrell that you can attempt (and probably even more if you look back at your Chem 102 text):

Sorrell Ex. 3.6, 3.7, 3.14

The 2012 Sept 17 issue of Chemistry & Engineering News ("C&ENews" is what we call it) begins with an article, Bond Order via Microscopy. A team of scientists at IBM's Zurich research center used a scanning probe microscope to investigate electric fields in a large flat aromatic molecule. The images they generate look surprisingly like the bond pattern in the molecule. If you want to get even closer to this project, check out their research publication and microscope images in the 2012 Sept 14 issue of Science magazine.

You can download the slides that I showed last Wed/Thur (Sept 12/13) here.

I didn't get to show all of the slides, but if I could have, I would have divided the talk into three sections:

• what you already know (from Chem 101)
• the creation of MOs by combining AOs (or HOs) and the importance of overlap
• examples of MOs

There are some really beautiful photos of bonding MOs, and even a few antibonding MOs, in the third section. Please look them over carefully. Make sure you can say the following about each MO:

• is it bonding or antibonding
• is it sigma or pi
• what atoms are involved
• what atomic (or hybrid) orbitals are involved

I have prepared a list of learning objectives and study tips for last Wed/Thur's (Sept 12/13) class. You can download them here.

Some corrections and clarifications for Learning Activity #2 follow. Let me know if you have other suggestions.

• #DU. A ring and a double-bond both count as 1 DU. Consider some three-carbon compounds. CH3CH2CH3 is propane, the saturated molecule. CH2=CHCH3 is C3H6. This molecule is one H2 short of saturation (1 DU). Cyclopropane, (CH2)3, contains only single bonds in a three-atom ring. Because its molecular formula is C3H6, it is also one H2 short of saturation (1 DU).
• Model 4. The definition of alkane might be reworded to make it clearer. Let's try alkane = saturated hydrocarbon.
• Fact 2.3 gives an incomplete definition of branched alkane. A branched alkane must contain at least one methine (CH) group OR one quaternary C.
• The right-hand structure in Fig. 2.5 is incorrect. Add another CH3 group to the left end of the molecule so that there are 8 C's in the chain.
• Re-word Q#19 to read, "Circle each group of C in Fig. 2.5 …". The idea is to draw a single circle around each alkyl substituent.
• Q#20-22 might lead to some confusion (Sorrell will straighten you out). An alkane is a molecule in which all of the C and H bonding patterns are obeyed. An alkyl group contains one C with only three bonds, a rule violation. Thus, methyl is derived from methane. Q#20-21: the suffix that methyl and ethyl share is 'yl', not 'ethyl'. Applying this in Q#22 leads to CH3CH2CH2 = propyl and so on.

Friday, Aug 31 update. Several students called my attention to an apparent contradiction in Model 6 and Fig. 2.6. The molecule on the left is named 2-ethyl-1,4-dimethylcyclohexane. This name appears to contradict the rule given in Model 6 that states, "C#1 is the ring carbon bonded to the substituent that comes first in the alphabet (prefixes like “di” and “sec-” are ignored)". The rule suggests 'ethyl' is located at C#1, but the name that was provided clearly assigns a methyl to C#1.

(more…)

Organic chemistry has a long history. This week we took a look at William Perkin, whose "unsuccessful" preparation of quinine spawned an entire industry, and Silas Cook, Reed '99, who recently supervised the first short "total" synthesis of artemisinin.

A closer look at the following web pages reveals that I reported some dates incorrectly in class: Queen Victoria wore a mauve-colored dress, but not until after the Royal Exhibition of 1851 was just a memory.

A nice book on the subject of Perkin, mauve, etc., is Mauve: How One Man Invented a Color that Changed the World. You can also learn quite a bit at these links:

• William Perkin, 1838-1907. The teenager who "dyed" Queen Victoria purple
• August Wilhelm von Hofmann, 1818-1892. The professor who took on a precocious 15-year-old assistant. Wikipedia also describes him as the first lecturer to make use of molecular models
• Quinine, an early treatment for malaria obtained from the cinchona tree
• Artemisinin, a "new" treatment for malaria obtained from sweet wormwood
• Mauveine, the dye that transformed commoners into royalty
• Silas Cook (Reed '99), Chemistry Department, U. Indiana. His Reed thesis title was: "The synthesis of 3,5-bis(carboranyloxy)benzaldehyde : the precursor to a novel boronated porphyrin for use in boron neutron-capture therapy"

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…)

Today’s class on electron-deficient atoms and Lewis acids was very informative for me. I want to thank all of the students who were willing to share their questions with me. Your questions are how I learn.

Here are a few post-lecture comments on today’s material that I hope will provide some additional food for thought.
(more…)