organic - bruice 7e ch. 15 - nmr...
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ORGANIC - BRUICE 7E
CH. 15 - NMR SPECTROSCOPY
CONCEPT: 1H NUCLEAR MAGNETIC RESONANCE- GENERAL FEATURES
1H (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
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CONCEPT: 1H 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?
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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?
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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?
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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?
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CONCEPT: 1H 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)
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CH. 15 - NMR SPECTROSCOPY
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EXAMPLE: How many signals will each molecule possess in 1H NMR?
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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.
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CONCEPT: 1H 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 4.5 – 6
C ≡ C 2.5 Benzene 6 – 8
Z – C – H 2 – 4 Aldehyde, -CHO 9-10
OH, NH 1 – 5 Carboxylic Acid , -COOH 10-13
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
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PRACTICE: Which of the labeled protons absorbs energy most upfield in the 1H NMR?
O
HA
B
C
D
E
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 1H NMR signal?
A B C D
F F
F
F
F
F
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CONCEPT: 1H 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?
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PRACTICE: Predict the splitting pattern (multiplicity) for the following molecule:
PRACTICE: Predict the splitting pattern (multiplicity) for the following molecule:
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PRACTICE: Which of the following compounds gives a 1H NMR spectrum consisting of only a singlet, a triplet, and a pentet?
a) CH3OCH2CH2CH2CH2OH
b) CH3OCH2CH2OCH2CH3
c) CH3OCH2CH2CH2OCH3
d) CH3OCH2CH2OCH3
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CONCEPT: 1H 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
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CONCEPT: 1H 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.
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PRACTICE: Draw a tree diagram for H* in the structure below.
F2CH*CH(CH3)2 JH*-F = 50 Hz JH*-H = 7 Hz
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CONCEPT: 1H NMR SPIN-SPLITTING – COMMON PATTERNS
Some splitting patterns are highly indicative of certain structures. We can get ahead by memorizing them.
EXAMPLE: Which common 1H NMR splitting pattern seen below could help us determine the molecular structure?
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CONCEPT: 1H 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:
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PRACTICE: Which of the following molecules gives a 1H NMR spectrum consisting of three peaks with integral ratio of 3:1:6?
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PRACTICE: Draw the approximate positions that the following compound might show in its 1H NMR absorptions?
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PRACTICE: Draw the approximate positions that the following compound might show in its 1H NMR absorptions?
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CONCEPT: 13C NMR – GENERAL FEATURES
13C NMR is a more limited type of nuclear magnetic resonance that identifies 13C instead of 1H.
● Due to low natural incidence of the 13C isotope, ______________ is NOT observed. ( ---------) (---------) =
● All of the other principles from 1H NMR apply, except that we must learn new shift values:
C – H 5 - 45 C = C 100 - 140
C ≡ C 65 - 100 Benzene 120 - 150
Z – C – H 30 - 80 Carbonyl 160 - 210
EXAMPLE: How many 13C signals would ethylbenzene give?
EXAMPLE: Which compound(s) will give only one peak in both its 1H and 13C spectra?
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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 1H 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 1H NMR signals needed.
--- DRAW POSSIBLE STRUCTURES ---
5. 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 1H NMR
peak at 2700 cm-1 - 2.2 (doublet, 4H)
peak at 1720 cm-1 - 9.4 (triplet, 2H)
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PRACTICE: Propose a structure for the following compound that fits the following 1H NMR data:
Formula: C3H8O2 1H NMR: 3.36 δ (6H, singlet)
4.57 δ (2H, singlet)
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PRACTICE: Propose a structure for the following compound that fits the following 1H NMR data:
Formula: C2H4O2 1H NMR: 2.1 δ (singlet, 1.2 cm)
11.5 δ (0.5 cm, D2O exchange)
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PRACTICE: Propose a structure for the following compound that fits the following 1H NMR data:
Formula: C10H14 1H NMR: 1.2 ppm (6H, doublet)
2.3 ppm (3H, singlet) 2.9 ppm (1H, septet) 7.0 ppm (4H, doublet)
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PRACTICE: Propose a structure for the following compound, C7H12O2 with the given 13C NMR spectral data:
Broadband decoupled 13C NMR: 19.1, 28.0, 70.5, 129.0, 129.8, 165.78 δ DEPT-90: 28.0, 129.8 δ DEPT-135: 19.1 δ (↑ ), 28.0 (↑ ) , 129.8 δ (↑ ) , 70.5 δ (↓) & 129.0 δ (↓)
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PRACTICE: Propose a structure for the following compound, C5H10O with the given 13C NMR spectral data:
Fully Broadband decoupled 13C NMR and DEPT: 206.0 δ (↑ ); 55.0 δ (↑ ); 21.0 δ (↓) & 11.0 δ (↑ ).
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PRACTICE: Provide the structure of the unknown compound from the given information.
Formula: C4H10O IR: 3200-3600 cm-1 1H NMR: 0.9 ppm (6H, doublet)
1.8 ppm (1H, nonatet) 2.4 ppm (1H, singlet) 3.3 ppm (2H, doublet)
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PRACTICE: Provide the structure of the unknown compound from the given information.
Formula: C4H9N IR: 2950 cm-1, 3400 cm-1 1H NMR: 1.0 ppm (4H, triplet)
2.1 ppm (4H, triplet)
3.2 ppm (1H, singlet)
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