6.3.1 Chromatography and Qualitative Analysis: Module 6: Organic Chemistry & Analysis Chemistry OCR A A Level

What are the basic principles of all kinds of chromatography?

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

What is the mobile phase?

Carries the soluble components of the mixture.

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

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

What does the stationary phase do?

Holds back components of the mixture that are attracted to it.

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

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

How are substances separated by chromatography?

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 the mixture. Thus, they move at different rates and are separated over time.

Why will different substances show different Rf values?

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

What does TLC stand for?

Thin Layer Chromatography.

What are the advantages of TLC over paper chromatography?

Runs faster

Smaller amounts of a mixture can be separated

TLC plates are more robust that paper

How can you observe colourless spots?

Shine UV light on them. Or spray with a developing agent (e.g. ninhydrin turns amino acid spots from colourless to purple, so they can be seen) (heating needed with ninhydrin).

How do you calculate the Rf value?

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 R f using: Rf = distance moved by spot ➗ distance moved by solvent front.

What does Rf value stand for?

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

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

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.

What is the stationary phase in gas-liquid chromatography?

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.

What is the mobile phase in gas-liquid chromatography?

Carrier gas, inert e.g. N2 or He.

What do you measure in gas-liquid chromatography?

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

What are the advantages of GLC?

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

What are GLC’s uses?

Test athletes’ and horses’ blood and urine for drugs.

How can you use GC or GCMS to identify substances?

Match Gas Chromatograph 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.

How does GCMS work?

Gas Chromatography is run, retention time is recorded, then mixture is run through a Mass Spectrometer. Fragmentation pattern/molecular ion peak confirms identity.

How do you test for alkenes? What is the result?

Shake with bromine water, result is bromine water is decolourised (orange to colourless).

How do you test for haloalkanes? What is the result?

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

Result: precipitate of AgX (for Cl=white, for Br=cream, for I=yellow).

How do you test for aldehydes? What is the result? (2 ways)

1. Warm with Fehling’s solution, result: brick red ppt forms (from blue solution)

2. Warm with Tollens’ reagent, result: “silver mirror” (Ag(s) ppt) forms

How do you test for carboxylic acids? What is the result?

Add Na 2 CO3(aq), result: CO2 (g) given off -effervescence.

How do you test for carbonyl compounds?

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