ORGANIC - EGE 5E CH NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY

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2 CONCEPT: PURPOSE OF ANALYTICAL TECHNIQUES Classical Methods (Wet Chemistry): Chemists needed to run dozens of chemical reactions to determine the type of molecules in a compound. EXAMPLE: Tollen s Test Instrumental Methods (Dry Chemistry): Expensive scientific instruments investigate the properties of molecules. EXAMPLE: 1 H NMR Page 2

3 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 3

4 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 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 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 7

8 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 8

9 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 9

10 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 10

11 PRACTICE: Which of the following molecules gives a 1 H NMR spectrum consisting of three peaks with integral ratio of 3:1:6? Page 11

12 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 12

13 PRACTICE: Draw the approximate positions that the following compound might show in its 1 H NMR absorptions? Page 13

14 PRACTICE: Draw the approximate positions that the following compound might show in its 1 H NMR absorptions? Page 14