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.

Proton nmr

Aims• What is a 1H nmr spectrum?• What information does a 1H nmr

spectrum give?• What does the integration trace

show?

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.

• 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?

How many different chemical environments are

there in methanol?

How many peaks will appear on the nmr spectrum?

Which peak will be the highest?

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?

Ethanol CH3CH2OH

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.

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.

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.

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

Spectra 1

Methyl propanoate

Spectra 2

Propanoic acid