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: None today
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).
HOMEWORK:
Read: Sections 2.6, 2.7, 3.1, 3.2 in the ebook textbook. This text is part of the Longhorn Textbook access program.
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 5% of your final grade.