PAG 06: Synthesis of an organic solid- Preparation of methyl 3-nitrobenzene Flashcards
What two reactants are used to synthesise methyl 3- nitrobenzoate?
- Methyl benzoate
- Nitric acid
What is purpose of concentrated sulfuric acid in synthesis of methyl 3- nitrobenzoate?
Catalyst (speeds up rate of reaction)
In preparation stage of methyl 3-nitrobenzoate, what should temperature be kept below? Why?
Below 10 degrees, to maximise yield
Describe process of filtration under reduced pressure
- Place a piece of filter paper in a Buchner funnel
- Place funnel in a Buchner flask. Connect the side arm of the flask to a water pump
- Connect the water pump to a tap and turn the tap on to create the vacuum. Pour solution into the funnel.
- Once solutions has drained through, rinse the solid on the filter paper with a small volume of solvent.
Describe process of recrystallisation
- Dissolve solid in minimum volume of hot solvent
- Filter to remove insoluble impurities
- Cool the filtrate
- Filter to remove soluble impurities (the product remains on filter paper)
Why is ethanol used for recrystallisation of methyl 3-nitrobenzoate?
- Methyl 3-nitrobenzoate is soluble in ethanol but not water
- Ethanol is a cheap and relatively safe solvent
During recrystallisation, why is the product dissolved in a minimum amount of solvent?
If there is a large volume of solvent when the mixture is cooled ( and solubility decreased), some of the population may remain in solution
How does recrystallisation improve the purity of a product?
The desired product is dissolved in an appropriate solvent that the impurities are insoluble in. Subsequent filtration leaves the insoluble impurities on the filter paper while solution flows through
Why is the solution cooled in ice immediately after recrystallisation?
To decrease the solubility of the product so that it crystallises. Lower temperatures increase the size of crystals formed.
Why must the crystals be completely dry before measuring the mass to calculate the percentage yield?
Excess solvent on the crystals could falsely increase the yield.
Give 3 reasons why the yield may be lower than 100%
- Product may be lost when filtering/ transferring between apparatus
- Some product may stay in solution after recrystallisation
- Other side reactions may occur
What technique could be used to investigate the purity of the product before and after recrystallisation?
Thin layer chromatography- comparison of Rf values from product before and after recrystallisation to a pure lab sample
Which piece of apparatus is used to transfer a small amount of each liquid onto chromatography plate?
Capillary tube
During chromatography, why should the beaker containing the solvent and TLC plate be covered with a clear watch glass?
- To prevent evaporation of solvent
- The watch glass should be clear to allow the plate to be monitored
In chromatography, what is solvent front?
The furthest distance travelled by solvent up the TLC plate
Why should baseline be drawn in pencil on a TLC plate?
Pencil is insoluble in solvent so it won’t travel up the plate and obscure the results
When using chromatography to asses purity of methyl 3-nitrobenzoate, what is used to make spots visible?
Iodine crystals
How do you calculate Rf values?
distance travelled by substance/ distance travelled by solvent
Without using chromatography, how can purity of a sample be assessed?
Use melting point apparatus
Pure substances have a specific melting point whereas impure substances will melt over a larger range of temperatures. The smaller the range, the purer the sample.
