Learning Objectives Use high resolution n.m.r spectrum of simple molecules (carbon, hydrogen & oxygen) to predict The different types of proton present

Learning ObjectivesLearning Objectives

Use high resolution n.m.r spectrum of simple molecules (carbon, hydrogen & oxygen) to predict

The different types of proton present

The relative numbers of each type of proton

The number of protons adjacent to a given proton

Possible structures

Given a simple molecule predict features of the n.m.r spectrum.

Describe the use of D2O to identify –OH groups

Nuclear Magnetic Nuclear Magnetic Resonance SpectroscopyResonance Spectroscopy

NuclearNuclear Magnetic Magnetic Resonance SpectroscopyResonance Spectroscopy

NuclearNuclear MagneticMagnetic Resonance SpectroscopyResonance Spectroscopy

HH

NuclearNuclear MagneticMagnetic Resonance SpectroscopyResonance Spectroscopy

Nuclear MagneticNuclear Magnetic Resonance Resonance SpectroscopySpectroscopy

NuclearNuclear MagneticMagnetic ResonanceResonance SpectroscopySpectroscopy

NuclearNuclear MagneticMagnetic ResonanceResonance SpectroscopySpectroscopy

Absorption peak corresponds to the radio frequency absorbed

Only nuclei with an odd number of Only nuclei with an odd number of nucleons (neutrons and protons) possess nucleons (neutrons and protons) possess a magnetic spina magnetic spin

11H (proton nmr)H (proton nmr)1313CC

The vast majority of proton NMR The vast majority of proton NMR spectroscopy is performed on liquids.spectroscopy is performed on liquids.

You have a solid sample, what do you You have a solid sample, what do you dissolve it in? dissolve it in?

structure

spectrum

Proton NMR SpectraProton NMR Spectra

A

B

S

O

R

T

I

O

N

CHEMICAL SHIFT

Electrons around the nucleus shield it from the applied magnetic field.

Different radio-frequencies are aborbed depending on the environment of the proton.

CHEMICAL SHIFT is a measure of the magnetic field experienced by protons in different environments.

CHEMICAL SHIFT is measured in ppm relative to TMS, Si(CH3)4

CHEMICAL SHIFT tells us about the types of protons present

A

B

S

O

R

T

I

O

N

CHEMICAL SHIFT

ABSORPTIONS

The area under each peak is directly proportional to the number of protons responsible for the absorption

These areas are most often presented as integration traces on the spectrum

Your TurnYour Turn

Our TurnOur Turn

Different types of proton?

Relative numbers of each type of proton?

CHEMICAL CHEMICAL SHIFT ppmSHIFT ppm

Type of Type of protonproton

Number of Number of protonsprotons

1.01.0 R-CHR-CH33 33

3.53.5 O-CHO-CH22-R-R 22

4.94.9 R-O-HR-O-H 11

CH3CH2OH

CH3CH2OH

CH3CH2OH

CH3CH2OH

structure

spectrum

CH3CH2OH

Expect 3 different types of proton

Expect peaks in the following ranges

0.7-1.6ppm (CH3)

3.3-4.3ppm (CH2-O)

3.5-5.5ppm (OH)

Expect integration 3:2:1

Predict

3 different types of proton

Number protons is in ratio 1:2:3

Assign possible types of proton to chemical shifts obtained






Possible structures



The number of absorption peaks tells us the number of different types of protons.

The chemical shift helps us identify the type of proton.

The integration values tells us the relative number of protons.

Nuclear Magnetic Nuclear Magnetic Resonance SpectroscopyResonance Spectroscopy

Part 2Part 2






Possible structures



CHEMICAL CHEMICAL SHIFT ppmSHIFT ppm

Type of Type of protonproton

Number of Number of protonsprotons

1.01.0 R-CHR-CH33 33

3.53.5 O-CHO-CH22-R-R 22

4.94.9 R-O-HR-O-H 11

CH3CH2OH

CH3CH2OH

CH3CH2OH

CH3CH2OH

why does this happen?

Is it any use?

CH3CH2OH

C C OWhat name do we give to this effect?

spin spin

coupling

CH3CH2OH

C C OH

H

H

H

H

H

triplet

As a result of spin-spin coupling effects the signals are multiplets

What causes this triplet splitting pattern?

The spin-spin coupling effects from the protons on the adjacent carbon.

The n+1 rule

For n adjacent protons

We have n+1 peaks in the multiplet

2 adjacent protons

Gives a triplet

quartet

The n+1 rule

For n adjacent protons

We have n+1 peaks in the multiplet

We have quartet

How many protons on the adjacent carbon?

why does signal splitting this happen?

Is it any use?

Spin-spin coupling

Yes.

It tells us the number of protons attached to the adjacent carbon.

multipletmultiplet Protons on Protons on adjacent carbonadjacent carbon

singletsinglet nonenone

doubletdoublet One (C-H)One (C-H)

triplettriplet Two (CHTwo (CH22))

quadrupletquadruplet Three (CHThree (CH33))

Identifying O-H protonsIdentifying O-H protons

O-H protons can absorb at different O-H protons can absorb at different chemical shifts, dependant uponchemical shifts, dependant uponSolvent usedSolvent usedConcentration of solventConcentration of solvent

As a result it is difficult to identify O-H As a result it is difficult to identify O-H protons from chemical shiftsprotons from chemical shifts

Trick DTrick D22O – sometimes called a “DO – sometimes called a “D22O O

shake”shake”

In CCl4

In D2O

Identifying O-H protonsIdentifying O-H protons

To identify O-H protons, To identify O-H protons, run the sample in a suitable solventrun the sample in a suitable solventRe-run the sample in DRe-run the sample in D22OO

Compare spectraCompare spectra If the signal for the O-H proton dissapears If the signal for the O-H proton dissapears

in Din D22OOEvidence Evidence






Possible structures








Possible structures








Possible structures








Possible structures



Your TurnYour Turn

Old Exam QuestionsOld Exam Questions

[8]June2007 Q7d

Jan2007

June2008 Q4d

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Learning Objectives Use high resolution n.m.r spectrum of simple molecules (carbon, hydrogen & oxygen) to predict The different types of proton present