C29 - Chromatography and Spectroscopy

Question 1

What do all forms of chromatography consist of?

Answer 1

Mobile phase - moves. Normally a liquid or gas.

Stationary phase - doesn’t move. Normally a solid or a liquid supported on a solid.

Question 2

What does TLC stand for?

Answer 2

Thin layer chromatography

Question 3

What happens in TLC?

Answer 3

Thin layer chromatography

A TLC plate is used which is usually a plastic sheet or glass, coated with a thin layer of a solid adsorbent substance (usually silica).
The adsorbent is the stationary phase.
Different components in the mixture have different affinities for the adsorbent and bind with different strengths to the surface.

Question 4

What’s adsorption?

Answer 4

The adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the adsorbate on the surface of the adsorbent.

(The process by which the solid silica binds to and holds different substances in the mixture to it’s surface.

Question 5

How is the Rf (retention factor) value calculated?

Answer 5

Rf = distance moved by the component / distance moved by the solvent front

Question 6

What’s GC and what’s it used for?

Answer 6

Gas chromatography - used for separating and identifying volatile organic compounds in a mixture.

(The stationary phase is a high boiling liquid adsorbed onto an inert solid support. The mobile phase is an inert carrier gas e.g. helium or nitrogen.)

Question 7

What is retention time?

Answer 7

The time taken for each component to travel through the column, in the oven, and reach the detector in gas chromatography.

Question 8

How are compounds separated in GC?

Answer 8

They’re separated depending on their solubility (in the liquid stationary phase).
The more volatile, the shorter the retention time.

Question 9

How are gas chromatograms interpreted?

Answer 9

Retention times - used to identify the components present in the sample by comparing these to retention times for known components.

Peak integrations - (areas under each peak) can be used to determine the concentrations of components in the sample.

Question 10

What’s the procedure to identify the concentration of components in a sample, with gas chromatography?

Answer 10

1) Prepare standard solutions of known concentrations of the compound being investigated.
2) Obtain gas chromatograms for each standard solution.
3) Plot a calibration curve of peak area against concentration.
4) Obtain a gas chromatogram of the compound being investigated under the same conditions.
5) Use the calibration curve to measure the concentration of the compound.

Question 11

What’s chemical shift in spectroscopy measured in?

Answer 11

Parts per million
ppm

TMS (used as the standard/calibration) has a 0ppm shift.

Question 12

Why are deuterated solvents used?

Answer 12

The use hydrogen isotope deuterium D (H2).

This produces no NMR signal so wouldn’t produce a peak on the spectrum.

Question 13

What information does a carbon-13 NMR spectrum provide?

Answer 13

The number of different carbon environments (from the number of peaks)

The types of carbon environment present (from the chemical shift)

Question 14

How do the groups bonded to carbon atoms affect the number of peaks in the NMR spectrum?

Answer 14

C atoms bonded to different atoms/groups of atoms have different environments so will absorb at different chemical shifts.

However, if 2 C atoms are positioned symmetrically within a molecule, then they’re equivalent and have the same chemical environment.
They will absorb radiation at the same chemical shift and produce one peak.

Question 15

What information does a proton NMR spectrum provide?

Answer 15

The number of different proton environments (from the number of peaks)

The types of proton environment present (from the chemical shift)

Question 16

How does proton equivalence affect the NMR spectrum?

Answer 16

If 2+ protons are equivalent, they’ll absorb at the same chemical shift and produce one peak (increasing size of peak).

Protons of different types have different chemical environments and are non-equivalent. They absorb at different chemical shifts, producing separate peaks.

Question 17

What does an integration trace on a proton NMR spectrum show?

Answer 17

The area under each peak, showing the number of protons in that environment.

Question 18

What’s spin coupling?

Answer 18

The splitting of a main peak into sub peaks (in a proton NMR spectrum).

The number of sub peaks is one less than the number of adjacent protons.

Question 19

What’s the n+1 rule?

Answer 19

For a proton with n protons attaches to an adjacent carbon atom, the number of sub peaks in a splitting pattern is n+1.

(The number of sub peaks is one less than the number of adjacent protons).

Question 20

What does a singlet show, in a proton NMR spectrum?

Answer 20

A single peak, indicating no protons on adjacent atoms.

Question 21

What does a doublet show, in a proton NMR spectrum?

Answer 21

Two peaks, indicating an adjacent CH / a single proton.

Question 22

What does a triplet show, in a proton NMR spectrum?

Answer 22

Three peaks, indicating an adjacent CH2 / 2 adjacent protons.

Question 23

What does a quartet show, in a proton NMR spectrum?

Answer 23

Four peaks, indicating an adjacent CH3 / 3 adjacent protons.

Question 24

What does a multiplet show, in a proton NMR spectrum?

Answer 24

Multiple (more than 4) peaks, indicating multiple (possible unsymmetrical) adjacent protons.

Question 25

What structural feature would a quartet and triplet indicate, in a proton NMR spectrum?

Answer 25

CH2 - CH3

Question 26

What structural feature would a triplet and doublet indicate, in a proton NMR spectrum?

Answer 26

CH - CH2

Question 27

What structural feature would a triplet and triplet indicate, in a proton NMR spectrum?

Answer 27

CH2 - CH2

Question 28

What structural feature would a doublet and doublet indicate, in a proton NMR spectrum?

Answer 28

CH - CH

Question 29

What structural feature would a quartet and doublet indicate, in a proton NMR spectrum?

Answer 29

CH - CH3

Question 30

What structural feature would a heptet and doublet indicate, in a proton NMR spectrum?

Answer 30

CH - (CH3)2

Question 31

What is proton exchange?

Answer 31

A technique to identify -OH and -NH protons in a proton NMR spectrum.

A proton NMR is run as normal. A small amount of deuterium oxide, D2O, is added.
The mixture is shaken and a second spectrum is run.

Deuterium exchanges and replaces the OH and NH protons (e.g. CH3OH becomes CH3OD).
Deuterium doesn’t absorb in the chemical shift range so the OH peak would disappear.

Question 32

How does proton exchange work?

Answer 32

A proton NMR is run as normal.
A small amount of deuterium oxide, D2O, is added.
The mixture is shaken and a second spectrum is run.

Deuterium exchanges and replaces the OH and NH protons (e.g. CH3OH becomes CH3OD).

Deuterium doesn’t absorb in the chemical shift range so the OH or NH peak would disappear.
This indicates the presence of these groups (if peaks remain, it isn’t one of these groups).

Question 33

Name the process by which TLC separates compounds:

Answer 33

Adsorption

Question 34

How does GC-MS identify compounds?

Answer 34

Gas chromatography: separates compounds

Mass spectrometry: gives structural information / can compare against database

Question 35

How are compounds separated in paper chromatography?

Answer 35

By their solubility to the mobile phase.

Question 36

How are compounds separated in TLC?

Answer 36

By adsorption to the stationary phase.

Question 37

How are compounds separated in GC?

Answer 37

By their solubility to the stationary phase.