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 1 H nmr spectrum? What information does a 1 H nmr spectrum give? What does the integration trace show?

Proton nmr In proton nmr spectroscopy it is the 1 H nucleus that is being examined. As nearly all hydrogen atoms are 1 H, it is easier to get a proton nmr spectrum than a 13 C 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 1 H nmr, values of δ range from 0-10ppm. In 1 H 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 CH 3 CH 2 OH 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 CH 3 CH 2 OH

δ/ppmType of hydrogen Number of H 1.0-CH CH 2 -O2 4.5-O-H1 Note 1: In 1 H 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 – CCl 4 (tetrachloromethane) CDCl 3 C 6 D 6 D 2 O Deuterated solvents where 1 H atoms are replaced by 2 H atoms (D atoms). This is expensive.

Tasks 1.Worksheet 2.Summary questions page Explain why CHCl 3 is not used as a solvent in proton nmr spectroscopy 4.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-ol b.Pentan-3-ol c.Butanone d.Butane

Spectra 1 Methyl propanoate Spectra 2 Propanoic acid