C29 Chromatography And Spectroscopy

Question 1

What are the basic principles of all kinds of chromatography

Answer 1

A family of separation techniques that depend on the principle that a mixture is separated if it’s dissolved in a solvent and this mobile phase is passed over a solid (the stationary phase)

Question 2

What’s the mobile phase

Answer 2

Carries the soluble components of the mixture

(Does move, normally liquid or gas)

Question 3

What relationship between a sample and the mobile phase makes the move faster

Answer 3

More soluble components/compounds with more affinity to the solvent move faster

Question 4

What does stationary phase do

Answer 4

Holds back components of the mixture that are attracted to it

(doesn’t move, normally is a solid or a liquid supported on a solid)

Question 5

What relationship between a sample and the stationary phase that makes the sample move slower? What kind of bonding does this often involve

Answer 5

More affinity for the stationary phase means that a component moves slower; often attracted by hydrogen bonding

Question 6

How are substances separated by chromatography

Answer 6

If suitable stationary/mobile phases are chosen, the balance between affinity for the mobile phase and affinity for the stationary phase is different for each component of mixture. Thus, they move at different rates and are separated over time

Question 7

Why will different substances show different Rf values

Answer 7

They’re bonded differently and have different polarities - more polar bonds mean longer retention time or smaller Rf value, since hydrogen bonding/dipoles are attracted more strongly to the stationary phase

Question 8

What does TLC stand for

Answer 8

Thin layer chromatography

Question 9

What’s the stationary phase in TLC

Answer 9

Plastic

Glass

Metal sheet or ‘plate’ coated in silica (SiO2) or alumina (Al2O3)

Question 10

What are the advantages of TLC over paper chromatography

Answer 10

Runs faster

Smaller amounts of a mixture can be separated

TLC plates are more robust that paper

Question 11

How can you observe colourless spots

Answer 11

Shine UV light on them

Or

Spray with developing agent (e.g. ninhydrin turns amino acid spots from colourless to purple, so they can be seen)

(Heating needed with ninhydrin)

Question 12

How do you calculate Rf value.

Answer 12

Measure the distance from the initial line (that the mixture was spotted onto) to the solvent front, and the distance from the initial line to the spot.

Calculate Rf using:
distance moved by spot / distance moved by solvent front

Question 13

What does Rf value stand for?

Answer 13

Retention factor

A measure of the rate of movement of a component through the chromatography apparatus; a ratio between the rate of movement of the solvent and that component

Question 14

How could you confirm the identity of a substance from its Rf value

Answer 14

Compare your Rf value to accepted values Rf for that substance run in the same solvent and set-up; if they match, then identity is confirmed.

Question 15

What’s the stationary phase in gas-liquid chromatography

Answer 15

Powder, coated with oil. Packed into a long, thin, capillary tube (100m long, 0.5mm diameter). Coiled and placed in an oven, the temperature of which can be varied.

Question 16

What is the mobile phase in gas-liquid chromatography

Answer 16

Carrier gas, inert e.g. N2 or He

Question 17

What do you measure in gas-liquid chromatography

Answer 17

Retention time; different components of the mixture take different amounts of time to move through

Question 18

What are the advantages of GLC

Answer 18

Very sensitive; GC can detect minute traces of substances in foodstuffs, and link pollution on beaches to the specific tanker the oil came from.

Question 19

What are the GLC’s uses

Answer 19

Test athletes’ and horses’ blood and urine for drugs

Question 20

How can you use GC or GCMS to identify substances

Answer 20

Match gas chromatography to that of a known substance under the same conditions; retention time should exactly match. Substance’s identity can be confirmed by mass spectrometry, NMR or infrared spectroscopy.

Question 21

How does GCMS work

Answer 21

Gas chromatography is run, retention time is recorded, then mixture is run through a mass spectrometer. Fragmentation pattern/molecular ion peak confirms identity.

Question 22

How do you test for alkenes

Answer 22

Shake with bromine water

Result is bromine water is decolourised (orange to colourless)

Question 23

How do you test for haloalkanes

Answer 23

Add NaOH(aq) and warm, acidify with HNO3, add AgNO3(aq)

Result = precipitate of AgX
Cl - white
Br - cream
I - yellow

Question 24

How do you test for alcohols

Answer 24

Add acidified K2Cr2O7 (potassium dichromate (vi)) and heat

Result =
Colour change from orange to green for 1* and 2*

No change for 3* alcohols

Question 25

How do you test for aldehydes

Answer 25

Warm with Fehling’s solution Result = Brick red ppt forms from blue solution Warm with tollens reagent Result = silver mirror (Ag solid ppt forms)

Question 26

How do you test for carboxylic acids

Answer 26

Add Na2CO3(aq) CO2(g) given off - effervescence

Question 27

How do you test for phenols

Answer 27

By weak acidity - there is a neutralisation reaction reacted with NaOH but no reaction with CO3^2-

Question 28

How do you test for carbonyl compounds

Answer 28

React with 2,4- DNP and an orange precipitate should form

Question 29

What does NMR stand for

Answer 29

Nuclear magnetic resonance

Question 30

What are the basic principles of NMR

Answer 30

You can find the structures of complex molecules by placing them in a magnetic field and applying EM waves of radio frequency to them. If radio waves of the right frequency are absorbed, the nuclei flips from parallel to applied magnetic to field to anti-parallel. This energy change can be monitored and recorded. Uses the resonance of nuclei with spin.

Question 31

Give one use of NMR

Answer 31

MRI scans

Question 32

What kind of nuclei does NMR work with (and examples)

Answer 32

Those with an uneven number of nucleons, meaning they will spin e.g. 1H, 13C

Question 33

Summarise what number of signals mean for 13C NMR

Answer 33

One signal for each C environment (each set of in equivalent 13C atoms)

Question 34

Summarise what chemical shift means for 13C NMR

Answer 34

Greater from atoms closer to electronegative atoms or C=C

Question 35

Summarise what area under peak means for 13C NMR

Answer 35

No meaning

Question 36

Summarise what splitting means for 13C NMR

Answer 36

There’s no splitting for 13C NMR

Question 37

Why does the peak from O-H bonds disappear if D2O is used as a solvent?

Answer 37

The O-D bond is formed in preference to O-H due to labile protons that move/swap from one molecule to another.

Question 38

Why is it easier to get a spectrum of 1H NMR than 13C NMR

Answer 38

Most H atoms are 1H- its much more abundant than 13C This means almost all H atoms have spin so show up

Question 39

On a low resolution spectrum, what peaks would you expect to see for H NMR

Answer 39

One peak for each set of inequivalent H atoms (each chemical environment shows 1 peak)

Question 40

What does the area under the peak represent (for H NMR)

Answer 40

The area under the peak is proportional to the number of 1H atoms represented by the peak

Question 41

What is the integration trace

Answer 41

A stepped line that makes it easier to measure the area under the curve (height of line = area under that peak)

Question 42

What is TMS

Answer 42

Tetramethylsilane

Question 43

Why is TMS used

Answer 43

Can be added to sample to calibrate the NMR equipment. It provides a peak at exactly 0ppm. Is is the reference point against which all ppm are measured.

Question 44

What are other advantages of using TMS

Answer 44

Inert, non-toxic, easy to remove from the sample (as relatively volatile)

Question 45

When does splitting/spin-spin coupling occur

Answer 45

Neighbouring hydrogen atoms (3 or fewer bonds away, or on the adjacent carbon) affect the magnetic field of 1H atoms and causes their peaks to split

Question 46

What is the n+1 rule

Answer 46

If there are n inequivalent 1H atoms on the neighbouring carbon then the peak will split into (n+1) smaller peaks

Question 47

Why must solvents used for 1H NMR not contain any hydrogen atoms

Answer 47

Signals from the solvent would swamp signals from the sample, as there is much more solvent than sample.

Question 48

Which solvents are used

Answer 48

Deuterated solvents: CDCl3, D2O, C6D6 CCl4 - tetrachloromethane

Question 49

Summarise what number of signals mean for 1H NMR

Answer 49

One main signal for each set of inequivalent 1H atoms (for each Hydrogen environment)

Question 50

Summarise what chemical shift means for 1H NMR

Answer 50

Larger. For 1H atoms closer to electronegative atoms or C=C

Question 51

Summarise what splitting means for 1H NMR

Answer 51

Number of smaller peaks = 1 + number of inequivalent hydrogen atoms 3 bonds away

Question 52

Summarise what area under peak means for 1H NMR

Answer 52

Proportional to the number of atoms represented by that peak