The Student Room Group

Nitration Of Methyl Benzoate

1) why were crystals formed washed with water before recrystallisation?

2) what happens during recrystillisation to:
a) impurities
b) the main product

3) how is the loss of the product kept to a minium during recrystallisation?

4) what are the usual stages in the purification of an organic solid?


thanks alot !!!!!:smile::smile:
Reply 1
anyone?
Reply 2
1) This dissolves inorganic material (salts) but not your organic product so some impurities are washed away. These inorganic impurities would not dissolve in your organic recrystallistion solvent and end up in your crystallised material.

2a) The impurities stay dissolved in the solvent so that when you filter they separate from the product which you collect as a solid.

2b) The product precipitates out as a solid and you collect it by filtration.

3) You should use the minimum amount of hot solvent required to compeltely dissolve all of your crude product so that as much as pure product as possible precipitates when the solvent cools down.

4) This really depends on the product being formed but usually people do an aqueous work-up first where the product is dissolved in an organic solvent which is washed with an aqueous solvent. For methyl benzoate, if you formed it from benzoic acid you'd probably wash with aqueous sodium hydroxide to turn benzoic acid into sodium benzoate and pull it into the aqueous layer.

Then you'd dry your organic solvent with a drying agent such as magnesium sulfate which you later remove by filtration.

If the solid is still not pure (you'd see if it is by NMR) then you'd either do a recrystallisation or flash chromatography.
Reply 3
some extra information and background

Nitration of methyl benzoate


Start with: methyl benzoate Ph(benzene ring C6H5 )COOCH3 + 50:50 mixture of (HNO3 + H2SO4) forms NO2+( reagent) + H2O + HSO-4

final product: methyl -3- nitrobenzoate
Reply 4
Under nitrating conditions, you'll have sulfuric and nitric acid left at the end so washing the crystals with water will ensure all traces of acid are washed away. If you're recrystallising from ethanol later then the presence of acid could cause some transesterification which you don't want.

In this case, your recrystallisation is probably removing minor by-products where the nitro group goes on a different position on the aromatic ring other than meta. Also maybe some leftover starting material.
What impurities could still have remained in methyl 3-nitrobenzoate even after the purification stage- I want to mention how my melting point for methyl 3-nitrobenzoate is lower than literature value but I need to relate this to impurities etc. can someone help me?
Original post by haleemah-k123
What impurities could still have remained in methyl 3-nitrobenzoate even after the purification stage- I want to mention how my melting point for methyl 3-nitrobenzoate is lower than literature value but I need to relate this to impurities etc. can someone help me?

A mixture of two substances has a lower MP than either of the pure components in the mixture.

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