ORGANIC - BRUICE 8E CH. 14 - NMR SPECTROSCOPYlightcat-files.s3.amazonaws.com/...organic...14-nmr-spectroscopy-14968.pdf · CH. 14 - NMR SPECTROSCOPY Page 23. CONCEPT: STRUCTURE DETERMINATION

! www.clutchprep.com

!

ORGANIC - BRUICE 8E

CH. 14 - 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

ORGANIC - BRUICE 8E


Page 2

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?

ORGANIC - BRUICE 8E


Page 3

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


ORGANIC - BRUICE 8E


Page 4



ORGANIC - BRUICE 8E


Page 5



ORGANIC - BRUICE 8E


Page 6

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)

ORGANIC - BRUICE 8E


Page 7

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

ORGANIC - BRUICE 8E


Page 8

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



ORGANIC - BRUICE 8E


Page 9

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

ORGANIC - BRUICE 8E


Page 10

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

ORGANIC - BRUICE 8E


Page 11

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?

ORGANIC - BRUICE 8E


Page 12

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

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

ORGANIC - BRUICE 8E


Page 13

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

ORGANIC - BRUICE 8E


Page 14

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

ORGANIC - BRUICE 8E


Page 15

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.

ORGANIC - BRUICE 8E


Page 16

PRACTICE: Draw a tree diagram for H* in the structure below.

F2CH*CH(CH3)2 JH*-F = 50 Hz JH*-H = 7 Hz

ORGANIC - BRUICE 8E


Page 17

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?

ORGANIC - BRUICE 8E


Page 18

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:

ORGANIC - BRUICE 8E


Page 19

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

ORGANIC - BRUICE 8E


Page 20

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

ORGANIC - BRUICE 8E


Page 21

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

ORGANIC - BRUICE 8E


Page 22

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?

ORGANIC - BRUICE 8E


Page 23

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)

ORGANIC - BRUICE 8E


Page 24

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)

ORGANIC - BRUICE 8E


Page 25


Formula: C2H4O2 1H NMR: 2.1 δ (singlet, 1.2 cm)

11.5 δ (0.5 cm, D2O exchange)

ORGANIC - BRUICE 8E


Page 26


Formula: C10H14 1H NMR: 1.2 ppm (6H, doublet)

2.3 ppm (3H, singlet) 2.9 ppm (1H, septet) 7.0 ppm (4H, doublet)

ORGANIC - BRUICE 8E


Page 27

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 (↓)

ORGANIC - BRUICE 8E


Page 28

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 (↑ ).

ORGANIC - BRUICE 8E


Page 29

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)

ORGANIC - BRUICE 8E


Page 30

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)

ORGANIC - BRUICE 8E


Page 31

Documents

ORGANIC - BRUICE 8E CH. 14 - NMR SPECTROSCOPYlightcat-files.s3.amazonaws.com/...organic...14-nmr-spectroscopy-14968.pdf · CH. 14 - NMR SPECTROSCOPY Page 23. CONCEPT: STRUCTURE DETERMINATION