Rules of the Day CH320M/328M

1. To summarize what we learned last lecture, in the NMR spectrum of a molecule, a signal appears at a characterisic chemical shift (listed in the units ppm), the number of signals tells you how many different types of H atoms are in the molecule, themical shift, the location of which tells you what kind of H atoms gives rise to the signal (i.e. what hybridization state, what functional group etc.) and the relative area of each signal tells you how many equivalent hydrogens in the molecule give rise to the 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.

2. The distance between peaks split in this way is called the coupling constant ("J").

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.

3. For alkenes or ring structures such as cyclopropanes the splitting does not simplify (no bond rotation) and you see full multiplicative splitting ("doublet of doublets", etc.) Click here to go to Pictures of the Day for today in which the NMR spectra for an alkene and a cyclic structure are explained.

Q. Non-equivalent H atoms on the same C atom can split each other (called geminal coupling), for example on alkenes or small rings. This coupling usually has very small coupling constants, so is difficult to see on some spectra. Note that EQUIVALENT H ATOMS on the same C atom do NOT split each other.

R. Deuterium atoms do not show up in 1H-NMR spectra, so deuerated solvents are used to dissolve NMR samples.

S. The H atoms of relatively acidic functional groups (alcohols, carboxylic acids, amines) exchange rapidly, so they often do not split adjacent protons, and they can be replaced (signal disappears) with deuterium by adding a drop of D2O to the NMR sample.

T. H-bonding changes the location of a signal for H-bonding groups in a concentration dependent manner explaining why -OH and -NH2 group signals can vary so much in location.

U. The splitting of a -CH2- group adjacent to a chiral center will be "messed up", that is split into many peaks. This is useful for identifying chiral centers in molecules.

V. When solving NMR spectra problems 1) Determine number and relative integrations of signals predicted for a given structure 2) Make sure the splitting pattern matches with the spectrum for each signal and 3) If the number and relative integrations as well as splitting patterns match with the spectra, compare expected chemical shifts with those of the signals in spectra.

7. The popular medical diagnostic technique of magnetic resonance imaging (MRI) is based on the same principles as NMR, namely the flipping (i.e. resonance) of nuclear spins of H atoms by radio frequency irradiation when a patient is placed in a strong magnetic field. Magnetic field gradients are used to gain imaging information, and rotation of the gradient around the center of the object gives imaging in an entire plane (i.e. slice inside patient). In an MRI image, you are looking at individual slices that when stacked make up the three-dimensional image of relative amounts of H atoms, especially the H atoms from water and fat, in the different tissues. [Memorize the preceding passage, as it will be worth 14 points on the first page of the Final] Click here for a handout on MRI. IF YOU ARE STRUGGLING TO CATCH THE WAVE IN THIS CLASS, DO NOT GIVE UP, IT IS NOT TOO LATE!! YOU NEED TO DO THE FOLLOWING!!

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

Page 3 from the 2018 Final

Pages 4 and 5 from the 2018 Final

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Page 10 from the 2018 Final