Chapter 29 - Chromatography and Spectroscopy

Question 1

What are the stationary and mobile phases of chromatography?

Answer 1

Stationary - no movement, normally a solid or liquid supported on a solid
Mobile phase - does move, normally a liquid or gas

Question 2

What is TLC?

Answer 2

Thin layer chromatography - DISSOLVED SUBSTANCES
Uses a TLC plate (plastic sheet or glass) coated with a thin layer of a solid adsorbent substance (usually silica) - this is the stationary phase.
Separation is achieved by the relative adsorptions of substances with the stationary phase

Question 3

What is adsorption?

Answer 3

The process by which the solid silicas holds the different substances in the mixture to it’s surface

Question 4

What process separates components in a mixture in both gas and thin layer chromatography?

Answer 4

Adsorption

Question 5

What is Rf, how is it calculated?

Answer 5

Retention factor
Rf = distance moved by component/distance moved by solvent front

Question 6

What are the stationary and mobile phases of gas chromatography?

Answer 6

Stationary = high boiling liquid adsorbed onto an inert solid support in column
Mobile = inert carrier gas like helium or neon

Question 7

How are mixtures separated in gas chromatography?

Answer 7

Dependent on their relative solubility in the liquid stationary phase, meaning they reach the detector at different times - retention time is the time taken for each component to travel through the column

Question 8

What 2 key pieces of information can be gained for gas chromatograms?

Answer 8

• retention time, which can be compared to known component retention times
• peak integrations (area under each peak) used to determine concentrations of components in the sample

Question 9

What is retention time?

Answer 9

The time between injection and emergence/detection of a component

Question 10

How can you determine the concentration of a component in a sample (procedure)?

Answer 10

1. prepare standard solutions of known conc 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 same conditions
5. use calibration curve to measure concentration of the compound

Question 11

How can you test for an alkene functional group?

Answer 11

Add bromine water drop wise, bromine water is decolourised from orange to colourless

Question 12

How can you test for a haloalkane?

Answer 12

Add silver nitrate and ethanol and warm to 50 degrees in a water bath
Chloro = white precip
Bromo = cream precip
Iodo = yellow precip

Question 13

How can you test for a carbonyl functional group?

Answer 13

Add 2,4 - DNP and an orange precipitate will form

Question 14

How can you test for an aldehyde functional group?

Answer 14

Add Tollen’s reagent and warm, silver mirror will form

Question 15

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

Answer 15

Add acidified potassium dichromate (VI) and warm in a water bath. There will be a colour change from orange to green

Question 16

How can you test for a carboxylic acid functional group?

Answer 16

Add aqueous sodium carbonate, observe effervescence

Question 17

What is NMR spectroscopy?

Answer 17

Nuclear magnetic resonance spectroscopy

Question 18

What is resonance?

Answer 18

Property of nucleus to absorb energy when in the right combination of a strong magnetic field and radio frequency radiation and flip rapidly between electron’s 2 spin states

Question 19

What is the frequency shift in NMR measured in?

Answer 19

Chemical shift 𝛿 in units of parts per million

Question 20

What is TMS used for in NMR?

Answer 20

TMS = tetramethylsilane, used as a standard reference chemical against which all chemical shifts are measured

Question 21

How is an NMR spectrum produced?

Answer 21

Sample dissolved in solvent and placed in a narrow NMR sample tube with a small amount of TMS. Placed inside NMR spectrometer where it is spun and the sample is given a pulse of radiation, any absorptions of energy are detected and displayed on a computer screen

Question 22

What are deuterated solvents and why are they used?

Answer 22

Solvent where 1H atoms have been replaced by 2H atoms (deuterium), which produces no NMR signal in frequency ranges

Question 23

What does the number of peaks in carbon NMR tell you?

Answer 23

The number of different carbon environments within the molecule (each peak = different environment)

Question 24

What does the shift of peaks in carbon NMR tell you?

Answer 24

The types of carbon environments present within the molecule

Question 25

When do carbon atoms have the same environment in C NMR and why?

Answer 25

If they are positioned symmetrically within a molecule they have the same chemical environment. They will then absorb radiation at same chemical shift and contribute to same peak

Question 26

What does the number of peaks in proton NMR tell you?

Answer 26

The number of different proton (H) environments present

Question 27

What does the chemical shift of peaks in proton NMR tell you?

Answer 27

The types of different proton (H) environments present

Question 28

What does the relative peak areas/integration trace in proton NMR tell you?

Answer 28

The relative numbers of each type of proton (H)

Question 29

What does the spin splitting pattern in proton NMR tell you?

Answer 29

The number of non equivalent protons adjacent to a given proton

Question 30

What is the n+1 rule?

Answer 30

For a proton (H) with n protons attached to an adjacent carbon atom, the number of sub peaks in a splitting pattern = n +1

Question 31

What are the common splitting patterns/what they show?

Answer 31

No H on adjacent atoms = singlet 1 H on adjacent C atom = doublet (CH) 2 H on adjacent C atom = triplet (CH2) 3 H on adjacent C atom = quartet (methyl) 6 H on adjacent C atom = heptet (dimethyl)

Question 32

What do multiplets in proton NMR often suggest?

Answer 32

Aromatic protons

Question 33

What type of peaks do N-H and O-H groups create in proton NMR?

Answer 33

Broad peaks

Question 34

How does proton exchange identify -OH and -NH protons?

Answer 34

1. normal proton NMR spectrum run 2. small volume of D2O added, mixture shaken and a second spectrum run Deuterium exchanges and replaces OH and NH protons with deuterium atoms, which are not absorbed on chemical shift frequency, so peaks disappear

Question 35

What is a typical sequence for identification?

Answer 35

Elemental analysis (% comp by mass) Mass spectra Infrared spectra NMR spectra