2,4-DNPH test + recrystallisation method Flashcards
What does this test for?
C=O bond
How to complete the test to identify C=O?
Test tube = 2,4-DNPH + methanol : concentrated sulfuric acid
What confirms presence of C=O bond?
Bight orange precipitate is formed
Method to obtain bright orange precipitate?
Recrystallisation
What do we first redissolve the precipitate with?
A suitable solvent which dissolves ppt when hit but not when cold using minimum volume so the crude product is at high concentrations
Why do we use minimum volumes of the solvent?
To make recrystallisation of ppt once cooled more likely
During hot filtration what is left in the conical flask?
The crystals have dissolved but any insoluble impurities will remain undissolved
Hot filtration
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
What do we do after hot filtration?
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
What do we do with the cooled solution?
Cold filter but keep residue (crude product remains crystallised in Büchner funnel)
What happens to the soluble impurities?
Filtered out and separated from crude product crystals
During the cold filtration what we do?
Wash with small volume of cold solvent so the soluble impurities stay in solution and are washed through into the filtrate
How to dry recrystallised crude product?
Using drying agent eg anhydrous CaCl2
How to find mass of crude product made?
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
Melting point analysis
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)
How to carry out melting point test
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
Apparatus for melting point test
Thiele tube
Repeat of melting point test we should….
Ensure the oil is at room temperature
So it is the same temperature as the crude product before beginning
Pure products will…
Will melt at the data temperature stated and sharply at one temperature
Impure products will…
Melt over a range of different temperatures and at a lower temperature
Melting point and boiling point of an impure product
The boiling point is increased
But melting point is decreased
Thus the range of values the product is a liquid at is increased
What step does recrystallisation occur at?
Once the solution has cooled down
AFTER hot filtration
