Rules of the Day CH320M/328M

1. 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.

2. 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. Click here for a filled-in version of the first handout Click here for a filled-in version of the second handout

3. 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.

4. 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.

H. Hybridization state of carbon atoms attached to an H atom influences shielding in predictable ways by removing differing amounts of electron density around adjacent nuclei.

I. Electron density in pi bonds also has a large effect on H atom shielding because pi electrons are more free to circulate in an a magnetic field compared to electron density in sigma bonds. Geometry of the pi bond is important.

5. NMR spectra record the energy (plotted as frequency) necessary for the nuclei to be excited from the lower energy spin state to the higher energy spin statein the presence of a strong external magnetic field. Different atoms in a molecule take different amounts of energy to accomplish this, and the different energies can be correlated to structure of the molecule.

6. The location of a given signal with respect to a standard, TMS, is called chemical shift (delta) and this has the units ppm (parts per million). The more shielded the nucleus, the smaller the chemical shift. Different functional groups have characteristic chemical shifts.

L. Equivalent hydrogen atoms in a molecule give the same NMR signal. Equivalent hydrogen atoms in a molecule have an identical relationship to all the other atoms in the molecule, and are found on the same sp3 atom (bond rotation makes them equivalent) or entire groups are equivalent due to symmetry in the molecule (i.e. the six equivalent hydrogens on the two methyls of an isopropyl group). Determining how many equivalent hydrogens are in a molecule can be very tricky (Skull and Crossbones!) so PRACTICE.

M. The area under a given signal is proportional to the number of equivalent hydrogen atoms that give rise to that signal.

N. Adjacent nuclei have magnetic fields associated with their spins. The spins of equivalent adjacent nuclei can be either +1/2 or -1/2, and at room temperature they are found in about a 50:50 mixture at any given nucleus (very slight excess of lower energy +1/2). These can add to give n+1 different spin combinations in the proportions predicted by Pascal's triangle. Each different spin combination produces a different magetic field, which leads to n+1 splittings in the peaks of the NMR spectra of the adjacent (no more than three bonds away) nuclei.

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 during exam week or Thanksgiving

We are here, the end of the semester. Your last homework problem set is due Monday December 5, it covers NMR. Click here to get a copy. You will also find a synthesis practice problems and a practice for the final there as well. You will not turn these in, they are there to help you prepare for the final. Homework 9 is your last official homework of the semester.