Module 6: Chromatography and spectroscopy

Question 1

How to carry out TLC?

Answer 1

1. Take a TLC plate with silica gel. Draw a pencil line 1cm from the end of the plate.
2. Using a capillary tube, put a small spot of sample onto the base line.
3. Pour some solvent into a beaker to a depth of 0.5cm and put the TLC plate in with a watch glass on top.
4. Allow the solvent to rise up the TLC plate until solvent front. Remove the plate from the beaker and mark solvent front with pencil.

Question 2

Stationary and mobile phase in TLC?

Answer 2

Stationary phase - thin metal sheet coated in silica. The solute molecules ADSORB onto the surface. The more they interact with the stationary phase, the more they will ‘stick’ to it.

Mobile phase - a polar or non-polar liquid (solvent) that carries components of the compound being investigated.
Polar solvents - water or alcohol.
Non-polar solvents - alkanes.

Question 3

How can we locate the spots in TLC?

Answer 3

UV light, ninhydrin, iodine vapour.

Question 4

Gas chromatography is used for analysing …

Answer 4

Gases, volatile liquids, solids in their vapour form.

Question 5

Stationary and mobile phase for gas chromatography?

Answer 5

Stationary phase - high boiling liquid adsorbed onto inert solid support.

Mobile - inert carrier gas like helium.

Question 6

Gas chromatogram.

Answer 6

Once sample molecules reach the detector, their retention times are recorded. This is the time taken for a component to travel through the column. It depends upon the attraction between the solute and the stationary and mobile phases as well as the volatility and nature of the solute.

The retention times are recorded on a chromatogram where each peak represents a volatile compound in the analysed sample. The relative sizes (i.e. areas) of the peaks are related to how much of each compound is present in the mixture.

The larger the retention time, the greater the interaction of that component with the stationary phase.

Question 7

Test for alkene?

Answer 7

Bromine water. Orange to colourless.

Question 8

Test for carbonyl?

Answer 8

2,4-DNP. Orange precipitate.

Question 9

Test for aldehyde?

Answer 9

Tollens reagent and warm. Silver mirror.

Question 10

Test for carboxylic acid?

Answer 10

Aqueous sodium carbonate. Effervescence of CO2.

Question 11

Test for haloalkane?

Answer 11

Silver nitrate in ethanol and warm. White, cream or yellow precipitate.

Question 12

Test for phenol?

Answer 12

Bromine water. Bromine decolourises and white precipitate forms. This is an electrophilic substitution reaction.