Preparative chromatography

Question 1

What type of technique is TLC?

Answer 1

Analytical technique

Question 2

What type of technique is column chromatography?

Answer 2

Preparative technique

Question 3

Which range of values do you want the Rf values to be within?

Answer 3

0.2-0.4

Question 4

What is the first step for running column chromatography?

Answer 4

• Add a slurry of silica gel in the eluent to a glass column

Question 5

What is the second step for running the column for column chromatography?

Answer 5

Open the tap to allow solvent to drain

Question 6

What is the third step for running the column for column chromatography?

Answer 6

Add a thin layer of sand to the top of the silica to protect the surface

Question 7

What is the 4th step for running the column in column chromatography?

Answer 7

Add a concentrated solution of the mixture

Question 8

What is the 5th step for running the column in column chromatography?

Answer 8

Add the eluent and do not allow the column to run dry

Question 9

What are the components to the column chromatography?

Answer 9

Glass column 
Sand layer 
Stationary phase 
Glass Frit
Collection vessel 
Eluent

Question 10

What is gradient elution?

Answer 10

You change the solvent during elution

to move polar compounds quicker increase solvent polarity

Question 11

What are the two most common options for stationary phase?

Answer 11

Silica

Alumina

Question 12

What is the pH like of silica?

Answer 12

Slightly acidic

Question 13

What is the pH like of alumina?

Answer 13

Slightly basic

Question 14

What does silica retain more strongly?

Answer 14

Bases

Question 15

What does alumina retain more strongly?

Answer 15

acids

Question 16

What are characteristics of silica?

Answer 16

Always wet

Uses compound hydrolysis

Question 17

What happens when you collect colourless compounds from the column?

Answer 17

Collect regular samples in tubes

Retain a sample of the original mixture for comparison

Question 18

How do you analyse colourless compounds?

Answer 18

Spot original alongside all of the fractions collected in the right order
Run TLC using the same solvent as the column

Question 19

How do you detect the colourless compounds on the TLC plate? What are the two methods?

Answer 19

Under UV light

By chemical means

Question 20

How does detection work by placing under a UV light?

Answer 20

The TLC plates are coated with fluorescent material

Place under a UV light and the compounds will be able to seen

Question 21

How does detection work by chemical means?

Answer 21

Spray plate or dip into a reagent

Question 22

Which general reagents are used for detection by chemical means?

Answer 22

Iodine vapour
Potassium permanganate
Vanillin
Phosphomolybdic acid

Question 23

Which reagents can be used to identify aldehydes and ketones?

Answer 23

2,4 - dinitrophenylhydrazine

Question 24

Which reagent is used to identify amino acids and amines?

Answer 24

Ninhydrin

Question 25

What happens once you have identified the spots with the same Rf values?

Answer 25

You combine those fractions and then evaporate off the solent

Question 26

What do you need to do to the flask before you add the fractions?

Answer 26

Pre-weigh it so you can work out the dry product to work out the yield for the reaction

Question 27

Why must you add the solvent when the eluent is below the sand?

Answer 27

The product will dissolve in the mobile phase which will cause poor separation and low yield for the pure sample

Question 28

What are two problems that can occur in column chromatography?

Answer 28

Changes in the solvent composition can change the Rf values | There may be overlapping of fractions that can occur

Question 29

What does the length of the column determine?

Answer 29

The separation of the compounds

Question 30

What happens if the column is too short?

Answer 30

The difference in Rf will be too small

Question 31

What does a longer column mean?

Answer 31

More time to separate effectively

Question 32

What does the width of the column determine?

Answer 32

How thin the band is

Question 33

What happens if the column is too wide?

Answer 33

There will be too much compound added

Question 34

What does a thinner band mean?

Answer 34

Better separation of compounds