CHEM 2425 (Organic Chemistry I)
EXPERIMENT 1
NMR Spectroscopic Analysis of Butanol Isomers
Introduction:
Isomers
Some molecular formulas are complex enough that more than one arrangement of atoms
is possible. Molecules with the same formula, but different structures are called isomers.
For instance, butanol, an important alcohol, has a molecular formula of C4H9OH with 4
structure isomers. Each isomer has its own unique boiling point and solubility.
The straight chain isomer with –OH group in the end of carbon chain is 1-butanol:
H3C CH2 CH2 CH2
OH
If the branched hydroxyl group –OH appears in the 2nd carbon or secondary carbon, we
call it 2-butanol:
H3C CH2 CH
CH3
OH
A single methyl branch with the –OH in the terminal carbon is called iso-butanol:
H3C CH CH3
CH2
OH
Two methyl branches with the –OH in the internal carbon or tertiary carbon is called
t-butanol:
CH3
H3C C
CH3
OH
Here we use NMR (Nuclear Magnetic Resonance) Spectroscopy to differentiate the four
isomers. NMR is the most widely used spectroscopic tool in chemistry. It can detect the
hydrogen and carbon framework and is the ultimate instrument in analytic chemistry.
NMR is closely related to the clinical MRI. When certain nuclei such as 1H, 13C, 19F…
are placed in a magnetic field, some are aligned with the field (low energy state); some
are aligned against the field (high energy state). If the nuclei are disturbed by the applied
electromagnetic (E/M) radiation, some nuclei are excited from low to high state by
absorption of E/M energy. The absorption occurs at different frequencies corresponding
by an atom’s environment.
A typical NMR spectrum is a plot of absorption of energy versus frequency during
resonance. After Fourier Transform, the frequency changes to “chemical shift” with unit
of parts per million (ppm) and is plotted on the x-axis.
1
H NMR
1. Number of Peaks
Each signal peak represents a single proton 1H or a group of equivalent protons. The
number of signals equals to the number of groups of nonequivalent protons.
In the case of t-butanol, all 9 protons in three methyl CH3 groups are equivalent; there
should be only one signal peak corresponding to these 9 protons and one single peak for
the proton in the OH group.
In isobutanol, there are four non-equivalent groups – four peaks.
In 1-butanol and 2-butanol, we should see 5 peaks.
2. Position of the Peaks – Chemical Shift
The horizontal position of the peaks are called chemical shift, it is determined by the
shielding and deshielding effect caused by the chemical environments of the protons.
The chemical shift is towards to the right, we call it upfield, it is due to the shielding
effect and the downfield shift is towards to the left and is due to the deshielding effect.
In the case of isobutanol, the single proton in OH group comes into resonance downfield
from the other protons. This is due to the electron-withdrawing effect of the oxygen atom.
The electron cloud that surround the 1H nucleus usually screens the nucleus from the
applied magnetic field, however the oxygen atom pulls away the electron cloud and
deshield the nucleus, so the nucleus resonates in a lower magnetic field, the position
appears at about 4.2 ppm.
3. Integrated Area – Number of Protons in a Group
If the area under the peak is calculated (integrated area), it should be proportional to the
number of protons contributed to the peak.
In the case of t-butanol, the ratio between the two integrated peaks is 9:1, corresponding
to the number of protons producing each peak.
4. Spin Coupling – Split of Peaks
Because nuclei are little magnets, they influence each other, change the energy, hence the
frequency of nearby nuclei as they resonate, cause the position shift a little from the
original place so the overall signal peaks can be split into a few more little peaks. This
phenomenon is called spin-spin interaction or spin coupling.
The splitting of the peak represents the number of adjacent hydrogen. A peak will be split
into n+1 peaks where n is the number of adjacent hydrogen.
In order to show the clear resolution, the chemical shift must be greater than the spin-spin
coupling, otherwise the NMR spectrum is going to be overlapped, this is the case for 1butanol and 2-butanol where the signals of –CH2 and -CH3 are stacked together around 1
ppm.
13
C NMR
1. Number of Peaks
Each signal peak represents a single proton 13C or a group of equivalent carbons. The
number of signals equals to the number of groups of nonequivalent carbons.
In the case of t-butanol, all 3 carbons in three methyl CH3 groups are equivalent; there
should be one signal peak corresponding to these 3 carbons and one single peak for the
carbon connecting to the OH group.
In isobutanol, there are three non-equivalent groups – three peaks.
In 1-butanol and 2-butanol, we should see 4 peaks.
2. Position of the Peaks – Chemical Shift
The horizontal position of the peaks are called chemical shift, it is determined by the
shielding and deshielding effect caused by the chemical environments of the carbons.
The demonstrations of 1H and 13C spectra are shown as following.
1-butanol:
H3C CH2 CH2 CH2
1
H NMR
13
C NMR
OH
2-butanol:
H3C CH2 CH
OH
1
H NMR
13
13C
C NMR
CH3
isobutanol:
H3C CH CH3
CH2
OH
1
H NMR
13
C NMR
t-butanol:
CH3
H3C C
OH
1
H NMR
13
C NMR
CH3
Materials and Apparatus:
Four NMR tubes, each one is filled with one type of butanol isomers (in DMSO-d6)
60MHz NMR spectrometer
Procedure:
1H
NMR Spectrum:
(For Instructors: The system needs to be setup, following the steps of A, B, C in page
6 of the EFT 60MHz operation manual. If you choose to set up by yourself, please
arrive 30 minutes prior to your class. Or call the office and let the lab assistants set
up for you.)
1.
2.
3.
4.
5.
Divide the entire class to four groups.
Each group pick up one Unknown sample
Turn on the Air Pump
Turn the black Knob on top of the magnet, take out the existing sample.
Remove the existing sample (the water sample) from the sample holder; place it
on the sample rack.
Use a paper napkin to hold the white plastic part of the sample holder, never
use your hand to touch it.
6. Place your unknown sample into the sample holder. Again Use a Paper Napkin to
hold the white part of the sample holder.
7. Turn the black knob to make the sample spinning. You may have to do it a couple
of times to ensure that the sample tube is spinning. Use a penlight if necessary to
look down the sample holder.
8. From the attached computer, switch to the PNMR program by clicking the
PNMR icon.
9. Only the 1st group type a command: acq
acq is a combination command of shim the magnet and acquire data.
The other groups just type: zg
zg is to acquire data.
10. When the dialog box appear in the screen, the spectra is obtained, switch to the
NUTS program by click the icon of NUTS
11. In NUTS program, type Ctrl F1 (control f1)
12. When the dialog menu appears on the screen, type your unknown number and
your group name, click OK
13. Your spectrum is printed out, do the corresponding analysis or comparison with
existing spectra to determine the unknown structure.
14. Rotate to next students group, repeat steps from 4 to 13.
15. When all the groups acquire the spectra, remove the last sample from the
magnet and replace it with the water sample (the original sample), stop the
spin, turn off the air pump.
You don’t have to log off the computer.
13C
NMR Spectrum:
1. steps 1-8, the same as the 1H NMR spectrum.
2. Select 13C observe: if the prompt is H1> not C13>, switch to C13>; type nu C13
and hit Enter key:
H1> nu C13 <Enter>
3. “shim” the magnet and set the parameters:
C13> shim
4. Set the parameters:
C13> ns 8 (number of scan : 8 times)
C13> rg 16 (Relay Gain: 16)
5. Acquire data:
C13> zg
6. Band Width: when the dialog box appear in the screen, enter 0.5 for the band
width
7. The spectra is obtained, switch to the NUTS program by click the icon of NUTS
8. In NUTS program, type Ctrl F3 (control f3)
9. When the dialog menu appears on the screen, type your unknown number and
your group name, click OK
10. Your spectrum is printed out, do the corresponding analysis or comparison with
existing spectra to determine the unknown structure.
11. Rotate to next students group, repeat steps from 2 to 10.
12. When all the groups acquire the spectra, remove the last sample from the
magnet and replace it with the water sample (the original sample), stop the
spin, turn off the air pump.
You don’t have to log off the computer.
Report Form:
NMR Spectrum of your butanol: (sketch or attach actual spectrum). Draw your butanol
on the spectrum and identify the peaks.
Post-lab questions:
1. Point out two identifying features from your spectrum that rule out other butanols.
2. What are the difference between resonance structures and isomers? Give examples.
3. Sketch the 1H and 13C NMR spectrum for acetone (CH3COCH3). Identify the peak(s).
4. There are exactly two isomers of the molecule C2H4F2. Draw both of them.