Rules of the Day CH320M/328M

1. Epoxides are important because the ring strain within epoxides allows them to react with nucleophiles. This provides the motive for reactions of epoxides with nucleophiles.

A) When strong nucleophiles attack epoxides at neutral or basic pH, the LESS hindered carbon is attacked.

B) Epoxides react with nucleophiles under acid catalysis, the nucleophile attacks preferentially MORE substituted carbon atoms that possess greater positive charge in the positively-charged intermediate. Analogous to bromonium ion intermediates.

Click here to watch a video that explains epoxide reactions in detail

2. Old concepts refined: Better leaving groups are groups that make more stable anions or molecules, however what makes a good nucleophile is not that simple. The halogens are good leaving groups AND good nucleophiles. The other good nucleophiles you have seen are not good leaving groups.

3. The key paradigm of Organic Chemistry is that functional groups react the same in complex molecules as they do in simple ones. You have largely seen molecules with a single functional group in order to focus attention, but the same rules apply to complex molecules with multiple functional groups.

4. To understand NMR you need to know the following:

A. Physics: Moving charge generates a magnetic field, and a moving magnetic field causes charges to move in a conductor.

B. Atomic nuclei, like electrons, have a quantum mechanical property of "spin". Spin can be thought of as a small magnetic field around the nucleus created as if the positive charge of the nucleus were circulating.

C. NMR, nuclear magnetic resonance, is used to assign structures of organic molecules.

D. We care about the nuclei 1H and 13C since these are commonly found in organic molecules and they have spin quantum numbers of 1/2.

E. Nuclei with spin quantum number 1/2 are quantized in one of two orientations, "+1/2" (lower energy) or "-1/2"(higher energy) in the presence of an external magnetic field, that is, with and against the external field, respectively.

F. The difference in energy between the +1/2 and -1/2 nuclear spin states is proportional to the strength of the magnetic field felt by the nucleus.

5. In the NMR experiment, a sample is placed in a strong magnetic field, the sample is exposed to electromagnetic energy of the precise energy to be absorbed by hydrogen nuclei in the +1/2 nuclear spin state so they flip to the -1/2 nuclear spin state. (The energy of the absorbed electromagnetic energy corresponds exactly the energy difference between the +1/2 and -1/2 spin states.) The amount and energy of the absorbed electromagnetic radation are measured. 5. The process of absorbing energy and flipping nuclear spin from +1/2 to -1/2 is called "resonance". 6. To understand NMR you need to know the following:

G. Electron density is induced to circulate in a strong external magnetic field, which, in turn, produces a magnetic field that opposes the external magnetic field. This shields nuclei from the external magnetic field. The greater the electron density around a nucleus, the more shielded it is, and the lower the energy (frequency) of electromagnetic radiation required to flip its nuclear spin.

HOMEWORK:

Read: All of Chapter 13! Yep you need to read all of this chapter, but you have an entire week!

There is no quiz or homework during exam week or Thanksgiving!!