2,4-DNPH test + recrystallisation method

Question 1

What does this test for?

Answer 1

C=O bond

Question 2

How to complete the test to identify C=O?

Answer 2

Test tube = 2,4-DNPH + methanol : concentrated sulfuric acid

Question 3

What confirms presence of C=O bond?

Answer 3

Bight orange precipitate is formed

Question 4

Method to obtain bright orange precipitate?

Answer 4

Recrystallisation

Question 5

What do we first redissolve the precipitate with?

Answer 5

A suitable solvent which dissolves ppt when hit but not when cold using minimum volume so the crude product is at high concentrations

Question 6

Why do we use minimum volumes of the solvent?

Answer 6

To make recrystallisation of ppt once cooled more likely

Question 7

During hot filtration what is left in the conical flask?

Answer 7

The crystals have dissolved but any insoluble impurities will remain undissolved

Question 8

Hot filtration

Answer 8

Pour hot solution through Büchner funnel into conical flask so the insoluble impurities will remain in the filter paper and the pure redissolved crude product (+ soluble impurities) in solution in conical flask

Question 9

What do we do after hot filtration?

Answer 9

Keep hot filtrate then cool solution so the crude product will recrystallise out of solution (solvent only dissolves product when hot)
AND soluble impurities remain dissolved in solution

Question 10

What do we do with the cooled solution?

Answer 10

Cold filter but keep residue (crude product remains crystallised in Büchner funnel)

Question 11

What happens to the soluble impurities?

Answer 11

Filtered out and separated from crude product crystals

Question 12

During the cold filtration what we do?

Answer 12

Wash with small volume of cold solvent so the soluble impurities stay in solution and are washed through into the filtrate

Question 13

How to dry recrystallised crude product?

Answer 13

Using drying agent eg anhydrous CaCl2

Question 14

How to find mass of crude product made?

Answer 14

Weighing by difference: weigh watch glass + filter paper + crystals
Scrape off crystals
Reweigh empty watch glass and filter paper so difference is mass of crude product

Question 15

Melting point analysis

Answer 15

Once we obtained pure crude product we can find the temp it melts at to determine what the original carbonyl is (based on mp of hydrazine derivative)

Question 16

How to carry out melting point test

Answer 16

Assemble apparatus for mp test
Heat arm of tube with Bunsen burner to create convection currents so the oil and sample will warm steadily
Record temp at which product melts using thermometer

Question 17

Apparatus for melting point test

Answer 17

Thiele tube

Question 18

Repeat of melting point test we should….

Answer 18

Ensure the oil is at room temperature
So it is the same temperature as the crude product before beginning

Question 19

Pure products will…

Answer 19

Will melt at the data temperature stated and sharply at one temperature

Question 20

Impure products will…

Answer 20

Melt over a range of different temperatures and at a lower temperature

Question 21

Melting point and boiling point of an impure product

Answer 21

The boiling point is increased
But melting point is decreased
Thus the range of values the product is a liquid at is increased

Question 22

What step does recrystallisation occur at?

Answer 22

Once the solution has cooled down
AFTER hot filtration