ORGANIC - BROWN 8E CH NUCLEAR MAGNETIC RESONANCE.

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2 CONCEPT: 1 H NUCLEAR MAGNETIC RESONANCE- GENERAL FEATURES 1 H (Proton) NMR is a powerful instrumental method that identifies protons in slightly different electronic environments by inducing tiny magnetic fields in the electrons around the nucleus. General Spectrum: is the standard reference point for NMR Electrons protons from the effects of NMR The further downfield, the more the proton There are 4 types of information we can gain from NMR spectra. Four Types of Information 1. Total Number of Signals Describes how many different types of hydrogens are present 2. Chemical Shift Describes how shielded or deshieldied the hydrogens are 3. Height of Signals (Integration) Describes the relative ratios of each type of hydrogen 4. Spin-Splitting (Multiplicity) Describes how close or far the different hydrogens are to each other Page 2

3 CONCEPT: 1 H NMR TOTAL NUMBER OF SIGNALS There are as many signals on each spectrum as there are unique, non-equivalent protons. Equivalent protons are defined as protons that have the same prospective on the molecule For now, let s assume that hydrogens bound to the are equivalent Symmetry will reduce the total number of signals EXAMPLE: How many different types of protons (signals) are there on each molecule? Page 3

4 PRACTICE: How many types of electrically unique protons (peaks) are there in the following molecule? PRACTICE: How many types of electrically unique protons (peaks) are there in the following molecule? Page 4

5 PRACTICE: How many types of electrically unique protons (peaks) are there in the following molecule? PRACTICE: How many types of electrically unique protons (peaks) are there in the following molecule? Page 5

6 PRACTICE: How many types of electrically unique protons (peaks) are there in the following molecule? PRACTICE: How many types of electrically unique protons (peaks) are there in the following molecule? Page 6

7 CONCEPT: 1 H NMR PROTON RELATIONSHIPS Hydrogens attached to the same carbon actually do have different relationships based on their chirality. The Q-Test is used to determine the specific type of chirality of each hydrogen. a. Homotopic Protons Q-Test DOES NOT yield new chiral center Protons are always homotopic and are considered (They share a signal) In general, the three hydrogens on -CH3 groups will always be homotopic b. Enantiotopic Protons Q-Test DOES yield new chiral center. No original chiral centers = protons are still (They share a signal) c. Diastereotopic Protons Q-Test DOES yield new chiral center. 1+ original chiral centers = protons are now (Each proton gets its own signal) Page 7

8 EXAMPLE: How many signals will each molecule possess in 1 H NMR? Page 8

9 PRACTICE: Identify the indicated set of protons as unrelated, homotopic, enantiotopic, or diastereotopic. PRACTICE: Identify the indicated set of protons as unrelated, homotopic, enantiotopic, or diastereotopic. PRACTICE: Identify the indicated set of protons as unrelated, homotopic, enantiotopic, or diastereotopic. Page 9

10 CONCEPT: 1 H NMR CHEMICAL SHIFTS The chemical shift indicates the exact electrochemical environment that each proton is experiencing. In general, electronegative groups will pull electrons away from nuclei, deshielding them Shifts increase (move downfield) as protons become more deshielded C H 1 2 C = C C C 2.5 Benzene 6 8 Z C H 2 4 Aldehyde, -CHO 9-10 OH, NH 1 5 Carboxylic Acid, -COOH Your professor will determine how many chemical shifts you should memorize. We ll go over them just in case. EXAMPLE: Order the following five protons from most deshielded to most shielded Page 10

11 PRACTICE: Which of the labeled protons absorbs energy most upfield in the 1 H NMR? D O A H C E B PRACTICE: Which of the labeled hydrogens will be most de-shielded? O O O O O O O A B C D E PRACTICE: Which compound possesses a hydrogen with the highest chemical shift for its 1 H NMR signal? F F F F F F A B C D Page 11

12 CONCEPT: 1 H NMR PROTON RELATIONSHIPS d. E / Z Diastereoisomerism Q-Test DOES yield new trigonal center on terminal double bonds Protons are always diastereotopic and are (Each proton gets its own signal) EXAMPLE: How many peaks will each molecule possess in 1 H NMR? Page 12

13 CONCEPT: 1 H NMR SPIN-SPLITTING WITHOUT J-VALUES Also known as spin-spin coupling, or J-coupling, this describes the distances between different protons. Note: This topic can be taught with or without J-values. Check with your professor to determine how much detail you should learn. For now, we will start with the simplest explanation, (should suffice for 90% of professors), which is without J-values. Adjacent, - protons will split each other s magnetic response to the NMR We use the rule to determine how many splits we will achieve Pascal s Triangle predicts the shape of the splits we will get EXAMPLE: How will the following protons be split? Page 13

14 PRACTICE: Predict the splitting pattern (multiplicity) for the following molecule: PRACTICE: Predict the splitting pattern (multiplicity) for the following molecule: Page 14

15 PRACTICE: Which of the following compounds gives a 1 H NMR spectrum consisting of only a singlet, a triplet, and a pentet? a) CH3OCH2CH2CH2CH2OH b) CH3OCH2CH2OCH2CH3 c) CH3OCH2CH2CH2OCH3 d) CH3OCH2CH2OCH3 Page 15

16 CONCEPT: 1 H NMR SPIN-SPLITTING WITH J-VALUES AND TREE DIAGRAMS Coupling-Constants, also known as J-values, describe the amount of interaction that a proton will have on another. Here are some examples of common coupling-constants (measured in Hz): Pascal s Triangle only helps to predict the shapes of splits when all of the J-values are assumed to be the same. When multiple J-values are involved, tree diagrams are needed to predict the shapes of the splits. Drawing Simple Tree Diagrams: First, let s use tree diagrams to help us understand why Pascal s Triangle and the n + 1 Rule make sense. Each split represents the J-value in Hz of a single proton. What does n + 1 predict here? ANSWER Page 16

17 CONCEPT: 1 H NMR SPIN-SPLITTING WITH J-VALUES AND TREE DIAGRAMS Drawing Complex Tree Diagrams: Now let s use an example where multiple J-values are involved. Always split in order of highest to lowest values. Before starting, what does the n + 1 Rule predict here? ANSWER EXAMPLE: Use a tree diagram to predict the splitting pattern of the bolded proton. Page 17

18 PRACTICE: Draw a tree diagram for H * in the structure below. F2CH * CH(CH3)2 JH*-F = 50 Hz JH*-H = 7 Hz Page 18

19 CONCEPT: 1 H NMR SPIN-SPLITTING COMMON PATTERNS Some splitting patterns are highly indicative of certain structures. We can get ahead by memorizing them. EXAMPLE: Which common 1 H NMR splitting pattern seen below could help us determine the molecular structure? Page 19

20 CONCEPT: 1 H NMR INTEGRATION Integration describes how many of each type of hydrogen are present, expressing this information as relative ratios. Uses the Area Under the Curve (AUC) to visually demonstrate which hydrogens are most prevalent. EXAMPLE: Draw the complete NMR spectrum: Page 20

21 PRACTICE: Which of the following molecules gives a 1 H NMR spectrum consisting of three peaks with integral ratio of 3:1:6? Page 21

22 PRACTICE: Draw the approximate positions that the following compound might show in its 1 H NMR absorptions? Page 22

23 PRACTICE: Draw the approximate positions that the following compound might show in its 1 H NMR absorptions? Page 23

24 CONCEPT: 13 C NMR GENERAL FEATURES 13 C NMR is a more limited type of nuclear magnetic resonance that identifies 13 C instead of 1 H. Due to low natural incidence of the 13 C isotope, is NOT observed. ( ) ( ) = All of the other principles from 1 H NMR apply, except that we must learn new shift values: C H 5-45 C = C C C Benzene Z C H Carbonyl EXAMPLE: How many 13 C signals would ethylbenzene give? EXAMPLE: Which compound(s) will give only one peak in both its 1 H and 13 C spectra? Page 24

25 CONCEPT: STRUCTURE DETERMINATION MOLECULAR SENTENCES The holy grail of this section is structure determination. You may be asked to produce a structure from scratch given only a MF, NMR Spectrum and IR Spectrum. Our goal is to build a strong molecular sentence by gathering clues, then propose drawings. How to build a molecular sentence: 1. Determine IHD. 2. Analyze NMR, IR and splitting patterns, integrations for major clues (i.e.). NMR = 9.1 ppm IR = 1710 cm -1 Triplet/Quartet 9.1 ppm (2H) 3. Calculate 1 H NMR Signal : Carbon Ratio. Ratio < ½ suggests symmetrical, whereas ratio > ½ suggests asymmetrical Never rule out a structure based on symmetry (you may not be able to visualize it) 4. State the number of 1 H NMR signals needed. --- DRAW POSSIBLE STRUCTURES Use a combination of Shifts, Integrations, and Splitting to confirm which structure is correct. EXAMPLE: Build a strong molecular sentence using the following data. MF: C4H6O2 IR: peak at 2950 cm -1 1 H NMR peak at 2700 cm (doublet, 4H) peak at 1720 cm (triplet, 2H) Page 25

26 PRACTICE: Propose a structure for the following compound that fits the following 1 H NMR data: Formula: C3H8O2 1 H NMR: 3.36 δ (6H, singlet) 4.57 δ (2H, singlet) Page 26

27 PRACTICE: Propose a structure for the following compound that fits the following 1 H NMR data: Formula: C2H4O2 1 H NMR: 2.1 δ (singlet, 1.2 cm) 11.5 δ (0.5 cm, D 2 O exchange) Page 27

28 PRACTICE: Propose a structure for the following compound that fits the following 1 H NMR data: Formula: C10H14 1 H NMR: 1.2 ppm (6H, doublet) 2.3 ppm (3H, singlet) 2.9 ppm (1H, septet) 7.0 ppm (4H, doublet) Page 28

29 PRACTICE: Propose a structure for the following compound, C7H12O2 with the given 13 C NMR spectral data: Broadband decoupled 13 C NMR: 19.1, 28.0, 70.5, 129.0, 129.8, δ DEPT-90: 28.0, δ DEPT-135: 19.1 δ ( ), 28.0 ( ), δ ( ), 70.5 δ ( ) & δ ( ) Page 29

30 PRACTICE: Propose a structure for the following compound, C5H10O with the given 13 C NMR spectral data: Fully Broadband decoupled 13 C NMR and DEPT: δ ( ); 55.0 δ ( ); 21.0 δ ( ) & 11.0 δ ( ). Page 30

31 PRACTICE: Provide the structure of the unknown compound from the given information. Formula: C4H10O IR: cm -1 1 H NMR: 0.9 ppm (6H, doublet) 1.8 ppm (1H, nonatet) 2.4 ppm (1H, singlet) 3.3 ppm (2H, doublet) Page 31

32 PRACTICE: Provide the structure of the unknown compound from the given information. Formula: C4H9N IR: 2950 cm -1, 3400 cm -1 1 H NMR: 1.0 ppm (4H, triplet) 2.1 ppm (4H, triplet) 3.2 ppm (1H, singlet) Page 32