Rules of the Day
Click here for a copy of my lecture notes from today's lecture
2. 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".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.
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.
J. 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.
K. 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.
O. THEORY: When there are two sets of adjacent H atoms, the number of peaks multiply. For example, a CH2 group with a CH2 group and a CH3 group on either side should show 3 x 4 = 12 splittings! You can say this group is a "triplet of quartets" (or a "quartet of triplets").
P. WHAT YOU WILL SEE IN REALITY : For alkyl groups complex splittings simplify because coupling constants ("J") are all about the same. In practice, if there are n adjacent H atoms, equivalent or not, you will see n+1 peaks. This is an approximation, but almost always true on spectra taken with all but the most sophisticated NMR spectrometers.
HERE IS HOW WE SUGGEST THAT YOU STUDY
1) DO THINGS, do not just reread things when you study. Generate things and have specific tasks to accomplish. That is the most efficient and effiective way to learn!
A) Use your class note summaries as a primary source of information when preparing for exams. Throughout the semester, you should prepare summaries, in your own words, of your lecture notes in the form of outlines you generate. DO NOT just reread your lecture notes. See "3) Be the best student you can be all semester" below for more details. You need to be doing this during the entire semester.
B) Watch lectures again and take notes to review what you do not understand. Use the Rules Of The Day as a guide to help you find which sections of old lectures you need to rewatch. Consider rewatching them at increased speed to save time. Previous classes say this really works.
C) Fill in blank mechanism sheets. Click here to get copies of blank mechanism sheets from the entire semester. This is the best way to learn mechanisms.
D) Fill in complete roadmaps from memory. Roadmaps put all the reactions you will learn in context. You will need to be able to fill one in from memory to make sure you know all the reactions, and how they work together during synthesis. Click here to get a blank roadmap template for all ther reactions we will learn this semester. Click here for a filled-in roadmap that covers all reactions through Chapter 9.
E) Practice working reactions backwards to help with synthesis. Study with a friend, and both of you should do the same thing. Write down a starting molecule, then carry out a reaction on that starting material. Write the product. Now using your roadmap as a guide, write as many different reactions as you need so that you cover the reactions in the roadmap. Show your friend only the products, and only look at your friend's products. Both of you then guess the starting molecules and reagents for each product. This is the best way to learn how to work backwords during synthesis problems.
2) AFTER you are finished with the above tasks, work through the old exams as your final preparation.
A) You need to know the material first, but research shows that practice exams are the MOST EFFECTIVE test preparation of all. We posted all of my previous exams (with and without answers) dating back to 2006 to help with your exam preparation. PRACTICE, PRACTICE, PRACTICE!! One caveat here is that the course material and emphasis has changed over the years, and it will continue to evolve. The most recent exams should be the most similar to the level and types of questions you will see.
Videos to help you prepare for the final. These videos walk you through all of the answers to the 2018 Final Exam:
Pages 1 and 2 from the 2018 Final
Pages 15 and 16 from the 2018 Final
Tips on How to Study for Synthesis Questions
Problem 23A Synthesis Question from Page 17 of the 2018 Final
Problem 23B Synthesis Question from Page 17 of the 2018 Final
Problem 23C Synthesis Question from Page 18 of the 2018 Final
Problem 23D Synthesis Question from Page 19 of the 2018 Final
Problem 23E Synthesis Question and Problem 24 from Pages 20 and 21 of the 2018 Final
HOMEWORK:
Read: All of Chapter 13! Yep you need to read all of this chapter, but you have an entire week!
There are no quizzes for the rest of the semester
We are here, the end of the semester. Your last homework problem set covers NMR and you will not turn it in. It is designed to help you prepare for the final Click here to get a copy. Click here for the answers.
More Practice Problems for the Final - Do Not Turn In Problems Answers