In carbon-13 NMR, what do the number of peaks

represent?

The number of chemically different carbon atoms

present

What are the two main factors to look for in the analysis of carbon-13 NMR spectra?

The number of peaks present & the chemical shift of the

peak

Which two features are present in proton NMR spectra, but not in carbon-13 NMR spectra?

The relative peak area (integration value) and the

splitting of the peak

What does the splitting of the proton NMR peak tell you?

The number of protons on the carbon (or other) atom

adjacent to a particular type of proton

Which solvent is used in proton NMR?

TMS (Tetramethylsilane)

What does the area under the peak represent in proton NMR?

The number of protons responsible for the peak

What splitting pattern will be shown by a group adjacent to a methyl (CH3) group?

A quartet

Combined spectral analysis

A compound containing 58.8% carbon, 9.8% hydrogen and 31.4% oxygen is subjected to mass spectrometry and found to give intense peaks at m/z = 43 and m/z = 71, in addition to a molecular ion peak at m/z = 102.

Infra-red analysis of the molecule showed a sharp peak at m/z = 1710 cm-1.

A proton nmr spectrum of the molecule yielded the following peaks:

Deduce the structure of the molecule and account for the formation of all the peaks the spectra.

Chemical shift Splitting Integration factor

0.8 Triplet 3

1.2 Sextet 2

2.3 Triplet 2

3.7 Singlet 3

Answer• Molecular formula = C5H10O2

• mass spectrum:peak at 43 from: CH3CH2CH2COOCH3

+. CH3CH2CH2+ + .COOCH3

peak at 71 from: CH3CH2CH2COOCH3+. CH3CH2CH2CO+ + .OCH3

• infra-red spectrum:peak at 1710 cm-1 indicates a carbonyl

• proton nmr spectrum:• peak at 0.8 is CH3- adjacent to -CH2-

• peak at 1.1 is -CH2- adjacent to CH3- and -CH2-

• peak at 2.3 is -CH2CO- adjacent to -CH2-

• peak at 3.7 is CH3O-

• so molecule is methyl butanoate, CH3CH2CH2COOCH3

Chromatography

Aims• Why can similar substances be

separated using chromatography?• What is column chromatography?• What is gas-liquid

chromatography?

Chromatography describes a whole family of separation techniques. They all depend on the principle that a mixture can be separated if it is dissolved in a solvent and then the resulting solution (mobile phase) moves over a solid (stationary phase).

The mobile phase carries the soluble components of the mixture with it. The more soluble the component in the mobile phase, the faster it moves with it. The stationary phase will hold back the components of the mixture that are attracted to it. The more attraction the component has for the stationary phase, the slower it moves with the mobile phase. If suitable mobile and stationary phases are chosen, a mixture of similar substances can be separated completely, because every component of the mixture has a unique balance between its affinity for the stationary and mobile phases.

Thin layer chromatography (TLC) Paper chromatography is an example of thin layer chromatography.Instead of paper, a glass plate with a layer of silica on its surface is often used as the stationary phase.

• Series of spots forms• Compare samples in

mixture with known substances.

• Measure Rf values.

Separation and identification

Column chromatography

This uses a powder such as silica, aluminium oxide, or a resin as the stationary phase. This is packed into a narrow tube and the solvent is added at the top. As the solvent (eluent) runs down the column, the components of the mixture move at different rates and can be collected separately in a flask at the bottom. A mixture of amino acids can be separated by this method.

Gas-Liquid Chromatography (GC)

• The stationary phase is a powder, coated with oil. It is packed into or coated onto the inside of a long (100m), thin (<1/2mm diameter) capillary tube.

• The mobile phase is an inert gas e.g nitrogen or helium. • The sample to be separated is injected and carried by the

gas. The mixture separates as the components will be carried by the gas / retained by the oil to different degrees.

• The components therefore leave the columns at different times after injection – they have different retention times.

Gas - Liquid Chromatography

Sample introduced by syringe.

Column separates components.(Heated in oven)

Detector monitors compounds emerging from outlet.

Recorder plots signals as a chromatogram.

What happens in practice?

• Compounds that have high affinity for mobile phase emerge first, (most volatile).

• Chromatogram charts recorder response against time.

• Each component - separate peak.• Retention time – characteristic of the

compound under given conditions.

The results are usually presented on a graph as a series of peaks. The area under each peak is proportional to the amount of that component.

The identification of a component is done by matching its retention time with that of a known substance under the

same conditions.