Chem 201 Archive – Alan Shusterman

Aaron Nilsen and I have been discussing how to make the transition from Chem 201 to Chem 202 (new schedule, new format, same book) as smooth as possible. One idea: share a short list of ‘things to remember from Chem 201’.

Aaron’s suggestions for things to review from Chem 201 can be downloaded here.

An essential aspect of SN1 and E1 reactions is the dissociation of the leaving group. These lecture slides from Day 16 (Oct 29-30) discussed the following phenomena and trends from an MO perspective:

• How can we describe polar dissociation of a covalent bond (heterolysis) using MO models?
• How does this MO model explain the reactivity of sp3 C-Cl vs. the lack of reactivity of sp2 C-Cl and sp C-Cl bonds?
• How can we explain the stabilization of carbocations by vinyl groups (conjugation) and alkyl groups (hyperconjugation) using MO models?
• How does this MO model explain the extreme acidity of carbocations?

I showed a YouTube video in class last Wednesday, but didn't have time to show it to the Thursday class. The video presents the mechanism of the chemical reaction catalyzed by lysozyme enzyme, a chemical reaction that is also discussed in Sorrell 5.5.

You can watch the video (under 2 min) here. You can also get a copy of the lecture slides for this and other lectures by going to the Syllabus page and scrolling to Day 11.

Some interesting things about the video:

• energy, binding, and ring strain – while the video doesn't offer a reaction coordinate diagram, it gives you information about energy if you know where to look. When the substrate and enzyme bind to each other, their (combined) free energy decreases because bonding is always favorable. But even though binding occurs, the substrate is forced into an awkward conformation. This destablizes the bonds in this ring, making them easier to break. Still, the increase in substrate strain energy is more than offset by the overall drop in energy caused by substrate-enzyme binding.
• acid-base chemistry & amino acid sidechains – the mechanism depicts proton transfers between the enzyme and the substrate. The reactive groups in the enzyme are amino acid sidechains. This is virtually always the case. Even though we may think of a single amino acid as containing a reactive amino group and a reactive carboxylic acid group, these groups have been converted into unreactive peptides (amides). An enzyme's catalytic properties are due to its amino acid sidechains.
• curvy arrows – the video draws curvy arrows for the proton transfer that are incorrect. One arrow correctly shows electrons in the A-H bond moving towards A. The other arrow incorrectly shows H moving towards B. Come to think of it, the video's voice doesn't say these are curvy arrows, but if they were ….
• electrophiles & nucleophiles – are these polar reactions? Do they form new bonds? Yes and yes. You should be able to identify electrophiles and nucleophiles for each step.

You can find a variety of things here as you prepare for next week's exam:

• How to construct an MO orbital mixing diagram (Day 5)
• Activity 6 – internal rotation – Selected answers
• Activity 7 – cycloalkane conformers – Selected answers
• Activity 8 – EZ alkenes & chirality – Selected answers
• Activity 9 – Diastereomers, RS configuration – Selected answers

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