what can you remember from last lesson? 1.suggest how pentan-1-ol and pentan-3- ol could be...
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What can you remember from last lesson?
1. Suggest how pentan-1-ol and pentan-3-ol could be distinguished from their carbon-13 nmr spectra.
2. Suggest how pentan-3-ol and pentan-3-one could be distinguished from their carbon-13 nmr spectra.
3. Suggest how chloromethane and methanal could be distinguished from their carbon-13 nmr spectra.
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Proton nmr
Aims• What is a 1H nmr spectrum?• What information does a 1H nmr
spectrum give?• What does the integration trace
show?
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Proton nmr
• In proton nmr spectroscopy it is the 1H nucleus that is being examined. As nearly all hydrogen atoms are 1H, it is easier to get a proton nmr spectrum than a 13C spectrum.
• As with Carbon-13 atoms, the Hydrogen-1 atoms are surrounded by electrons which partly shield them from the applied magnetic field.
• The amount of shielding, and hence the energy gap ∆E, depends on the electron density surrounding the nucleus and varies for different hydrogen nuclei within a molecule.
• The greater the electron density, the smaller the chemical shift. In 1H nmr, values of δ range from 0-10ppm.
• In 1H nmr, hydrogen atoms in different environments give different chemical shift values.
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• All the hydrogen atoms in methane are in the same chemical environment
• There is only one chemical shift value
How many peaks will appear in the proton nmr of methane?
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How many different chemical environments are
there in methanol?
How many peaks will appear on the nmr spectrum?
Which peak will be the highest?
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Ethanol CH3CH2OH
How many different chemical environments are there in ethanol?
How many peaks will appear in the nmr spectrum?
Which peak will be the highest?
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Ethanol CH3CH2OH
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δ/ppm Type of hydrogen
Number of H
1.0 -CH3 3
3.5 -CH2-O 2
4.5 -O-H 1
Note 1: In 1H nmr, the height, or more specifically, the area under each peak is significant. The areas under the peaks are proportional to the number of hydrogen atoms causing the peaks. Note 2: As it can be difficult to evaluate the area under each peak by eye, the nmr instrument produces a line called the integration trace. The relative heights of the steps of this trace give the relative number of each type of hydrogen.
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Tetramethylsilane (TMS)
By definition, the δ value of TMS is zero. This is the chosen standard because – • It gives a single intense peak as there are 12 chemically equivalent
protons (hydrogens)• It gives a signal that resonates upfield (to the right) from almost all other
organic hydrogen resonances as the 12 hydrogens are highly shielded• Non-toxic and inert• Low boiling point (26.5˚C) so can be easily removed from the sample
The δ values of chemical shifts are measured by reference to a standard- the chemical shift of the hydrogen atoms in the compound TMS. The chemical shift of these hydrogen atoms is zero. A little TMS, which is liquid is added to samples before their nmr spectrum are run, and gives a peak at δ value of exactly zero ppm to calibrate the spectrum.
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Proton n.m.r spectra are recorded in solution. The sample to be examined (a few mg) is dissolved in a proton free solvent to avoid unwanted absorptions.
Typical solvents include – • CCl4 (tetrachloromethane)
• CDCl3• C6D6
• D2O
Deuterated solvents where 1H atoms are replaced by 2H atoms (D atoms). This is expensive.
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Tasks1. Worksheet2. Summary questions page 1473. Explain why CHCl3 is not used as a solvent in
proton nmr spectroscopy4. For the compounds below give the number of
chemical environments, the integration ratio and the chemical shift each peak will appear at:
a. Pentan-2-olb. Pentan-3-olc. Butanoned. Butane
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Spectra 1
Methyl propanoate
Spectra 2
Propanoic acid