Rules of the Day
Click here for a copy of my lecture notes from today's lecture
Click here for a copy of the handout used in class today
Featured Golden Rules of Chemistry: Five- and Six-membered Rings are the Most Stable
1. Energy and stability are related to each other, and they are both RELATIVE terms in organic chemistry, i.e. they are not absolute, but rather only make sense in the context of comparisons. A molecule with higher energy is less stable, and a molecule with lower energy is more stable. Strain of any kind raises energy and decreases stability in a molecule.
2. The C-C sigma bonds of alkanes rotate rapidly at room temperature, giving rise to conformational isomers.3. The two extreme conformations in ethane are staggered (more stable) and eclipsed (less stable). Staggered is more stable due to minimized torsional strain (interactions between filled and unfilled orbitals). *** The notion that alkanes prefer to be staggered due to torsional strain is the key concept in molecular conformational analysis.***
5. Averaged over time, alkanes exist in the preferred "zig-zag" conformation, but on the picosecond (10-12 sec) timescale they are rapidly vibrating, and these vibrations randomly lead to rotations. Watch the animations of ethane, butane, and octane to see how molecules move.
6. Unsubstituted cycloalkane rings pucker to minimize angle and torsional strain as much as possible. Cyclopropane, bent cyclobutane, and envelope cyclopentane all have significant angle and/or torsional strain, but chair cyclohexane does not have appreciable amounts of either.
7. Unsubstituted cyclohexane adopts a chair conformation because it has minimum angle, torsional and steric strain.
8. There are two kinds of hydrogens on chair cyclohexane, axial (pointing up and down) and equatorial (pointing out and away from the cyclohexane ring).
9. The two possible chair forms of cyclohexane can convertinto each other rapidly at room temperature via boat like intermediates. During this interconversion the axial positions are switched to equatorial, and vice versa.
10. Groups larger than H prefer to be equatorialbecause when they are axial there is steric strain (also called non-bonded interaction strain or 1,3 diaxial interactions) with the other axial groups.
11. Larger groups on chair cyclohexanes have a greater preference for being equatorial, since large groups cause greater atom "crunching" (non-bonded interaction strain).
HOMEWORK:
Read: Sections 2.6, 2.7, 3.1 - 3.7 in the ebook textbook. This text is part of the Longhorn Textbook access program.
After reading sections 3.1 - 3.7, click on the link called "Breakout Learning" on the Canvas page margin and follow the directions. You will need to complete the assignment by this coming Sunday evening (Sept. 20) at 10 PM. You have the entire week! And this is going to prepare you for the hardest subject we will encounter so you all crush it! Succesful completion of the Breakout Learning discussion will earn you 0.5 TScore or raw percenbtage points on next week's midterm. But my bet is it will help you earn far more "real" points than that on the midterm itself because you will understand stereochemistry! Click here for the Breakout Learning Step by Step onboarding guide. In addition, here is a 1-min video of the student experience. Alternatively, this 4-min best practices video shows students all of the steps in the onboarding guide and gives them a sense of how to get the most out of their Breakout discussion.
Take the Daily Quiz 6 before 10 PM tomorrow. Click here to access the quiz. These quizzes are designed to review the important material from today's lecture. Together, they will count as 3% of your final grade.
Continue working on the Homework Problem Set 3, due at 10 PM on Wednesday, September 16. Note there are Aktiv Learning and Gradescope Questions, and you MUST DO BOTH. Click here to access to the Aktiv Learning electronic homework system.
Click here to access Aktiv learning Homework Problem Set 3
Click here to access Gradescope Homework Problem Set 3