PAG 06 - Synthesis Of An Organic Solid Flashcards
(20 cards)
What 2 reactants are used to synthesise
methyl 3-nitrobenzene
Nitric acid
Methyl benzoate
What’s the purpose of conc H2SO4 in synthesis of 3-nitrobenzene
Catalyst (speeds up reaction rate)
In preparation stage of methyl 3-nitrobenzoate, what should the temp be kept below? Why?
Below 10*C
To maximise yield
Describe process of filtration under reduced pressure
- Place a piece of filter paper in a Büchner funnel.
- Place the funnel in a Büchner 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 the solution into the funnel.
- Once the solution has drained through, rinse the solid on the filter paper with a small volume of solvent.
Describe process of recrystallisation
- Dissolve the solid in the minimum volume of hot solvent
- Filter to remove insoluble impurities
- Cool the filtrate
- Filter to remove soluble impurities (the product remains on the filter paper)
Why is ethanol used for recrystallisation of
methyl 3-nitrobenzene
Methyl 3-nitrobenzene is soluble in ethanol but not water
Ethanol is a cheap and relatively safe solvent
During recrystallisation, why is product dissolved in a minimum amount of solvent
If there’s a large enough vol of solvent when mixture is cooled (and solubility decreased), some of the product may remain in solution
How does recrystallisation improve purity of a product
Desired product is dissolved in an appropriate solvent that the impurities are insoluble in.
Subsequent filtration leaves insoluble impurities on filter paper while solution flows through
Why is solution cooled in ice immediately after recrystallisation
To decrease solubility of product so that it crystallises
Lower temp increase size of crystals formed
Why must crystals be completely dry before measuring their mass to calculate % yeild
Excess solvent on crystals could falsely increase yeild
Give 3 reasons why 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 purity of product before and recrystallisation
Thin layer chromatography
Comparison of Rf values from product before and after recrystallisation to a pure lab sample
What piece of apparatus is used to transfer a small amount of each liquid onto chromatography plate
Capillary tube
During chromatography, why should beaker containing solvent and TLC plate be covered with a clear watch glass
To prevent evaporation of solvent
Watch glass should be clear to allow plate to be monitored
In chromatography what is the solvent front
Furthest distance travelled by solvent up TLC plate
Why should baseline be drawn in pencil on TLC plate
Pencil is insoluble in solvent so it won’t travel up plate and obscure results
When using chromatography to assess purity of a sample of methyl 3-nitrobenzoate, what is used to make the spots visible
Iodine crystals
How do you calculate Rf value
Distance travelled by substance
———————————————
Distance travelled by solvent
Without using chromatography, how can purity of sample be assessed
Use MP apparatus
Pure substances have specific mp whereas impure substances will melt over a large range of temps.
The smaller the range, the purer the sample
What safety precautions should be taken during this practical
- Methyl 3-nitrobenzoate is an irritant so wear safety goggles and avoid contact with skin
- Iodine crystals are harmful if in contact with skin or when inhaled so avoid contact with skin and keep away from your face
- Methyl benzoate is an irritant so wear safety goggles and avoid
contact with skin - Ethanol is highly flammable so keep away from naked flames
