29 Chromatography and Spectroscopy

Question 1

What is chromatography used for?

Answer 1

Separates individual components from mixture of substances.

Question 2

Thin layer chromatography

Answer 2

Uses plastic sheet/glass coat with solid adsorbent surface e.g. silica.

• Draw pencil line near bottom of TLC plate.
• Spot some sample on base line using capillary tube.
• Place TLC in beaker with thin layer of solvent and cover with watch glass.
• Allow solvent to rise, them remove and mark the solvent front.
• Use UV lamp/chemical spray to see spots.

Question 3

Equation for rf value

Answer 3

Distance moved by component/distance moved by solvent front.

Question 4

What are the phases of gas chromatography?

Answer 4

Stationary phase: liquid adsorbed onto inert support

Mobile phase: inert carrier gas e.g. neon.

Question 5

What is retention time?

Answer 5

Time taken for each component to travel through capillary column.

Question 6

Describe the process of gas chromatography.

Answer 6

• Mixture carried to capillary column by carrier gas.
• More soluble solvents are slower in liquid phase.
• Components reach detector at different times depending on interactions with stationary phase.

Question 7

What does the peak integration of a gas chromatograph show?

Answer 7

Concentration of components in sample.

Question 8

How can you test for the presence of alkene?

Answer 8

Bromine water.

Question 9

How can you test for the presence of a haloalkane?

Answer 9

Silver nitrate, ethanol, heat.

Question 10

How can you test for a carbonyl group?

Answer 10

2,4-DNP test.

Question 11

How can you test for an aldehyde?

Answer 11

Tollen’s reagent, warm.

Question 12

How can you test for a primary/secondary alcohol or an aldehyde?

Answer 12

Acidified potassium dichromate ions, warm.

Question 13

How can you test for a carboxylic acid?

Answer 13

Aqueous sodium carbonate.

Question 14

How does NMR spectroscopy work?

Answer 14

Strong magnetic field and radiofrequency radiation used to flip spins of H-1 or C-13.
At certain frequency, resonance occurs.

Question 15

What is chemical shift 𝛿?

Answer 15

Electrons surrounding nucleus cause a shift in energy and the rf radiation required for NMR to take place.

Question 16

What is TMS used for in NMR?

Answer 16

It is used as a standard reference, chemical shift = 0 pm.

Question 17

How do you test for a phenol?

Answer 17

Weak acidity but no reaction with carbonates.

Question 18

What is a deuterated solvent?

Answer 18

All H-1 atoms are replaced by H-2 atoms.

Question 19

What information does a carbon-13 NMR spec give?

Answer 19

Number of different carbon environments from number of peaks.
Types of carbon environments from chemical shift.

Question 20

What is the effect of symmetrical carbon atoms on the NMR spec?

Answer 20

They contribute to the same shift.

Question 21

What information does a proton NMR spec give?

Answer 21

Number of different proton environments from number of peaks.
Types of proton environments from chemical shift.
Relative numbers of each type of proton from integration traces.
Number of non-equivalent protons next to a proton from splitting pattern.

Question 22

What is the splitting pattern in a proton NMR caused by?

Answer 22

Proton spin state interacting with spin states of other proton.

Question 23

What is the rule for the number of peaks in a splitting pattern?

Answer 23

n + 1

n is number of protons attached to adjacent carbon atom.

Question 24

What do the peaks caused by OH and NH protons usually look like on a proton NMR spec?

Answer 24

Broad peaks at any chemical shift.

Question 25

How can OH and NH peaks be identified?

Answer 25

Deuterium replaces OH and NH protons with D atoms , so OH or NH peaks disappear.

Question 26

Combined techniques

Answer 26

Elemental composition and % by mass - empirical formula.
Mass spectra, M + 1 peak - molecular mass and formula, fragments of molecules.
IR spec - identify bonds and functional groups.
NMR spec, chemical shifts and splitting patters - determine number and types of C/H environments.