1. Consider each of the following solvent pairs and determine if the pair would
ID: 1000806 • Letter: 1
Question
1. Consider each of the following solvent pairs and determine if the pair would be a good crystallization solvent mixture. Explain why or why not. (a) water-hexane (b) water-methanol (c) pentane-diethyl ether (d) toluene-hexane
2. Dimethylsufoxde (DMSO, bp 189 °C) is an excellent organic solvent, but is not routinely employed in crystallization. Explain.
3. If you perform a recrystallization on 435 mg of impure benzoic acid and recover 360 mg of crystals, what is the percent recovery? thank you very much i will rate fast
Explanation / Answer
1)
i) Solvent pair used for recrystallization behaves like a single solvent for recrystallization.
ii) A very important criterion for choosing solvents to form solvent pair is that they must be miscible with each other.
iii) “Like dissolves like” is the principle. Polar solvent are miscible with polar solvents and non-polar with non-polar one.
Based on this we can say that
a) Water – Hexane will not be good recrystallization solvent pair (RSP)as water is highly polar solvent and Hexane will be non-polar and they will be immiscible with each other and will not behave as single solvent for recrystallization.
b) Water – Methanol will be a good RSP. Both water and methanol are polar and are miscible through Hydrogen bonding. Hence will behave as single solvent.
c) Pentane – diethyl ether will be good RSP. Polarities are comparable and hence can be miscible at high temperature and hence can be used for recrystallization of non-polar compounds.
d) Toluene – Hexane will be very good RSP for non-polar organic compounds. Toluene and Hexane both are non-polar and hence miscible with each other hence can act as single solvent.
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3)
i) Dimethyl sulfoxide (DMSO) is very good aprotic polar solvent for various polar organic compounds.
ii) It’s enough polar that it dissolves some of the organic compound even at normal temperature.
iii) This property only makes DMSO a bad recrystallization solvent. Because this property of dissolving solvent at normal temperature is against the principle of recrystallization.
iv) For the solvent to be good recrystallization solvent it must dissolve compound to be recrystallized at high temperature and at low temperature it should not dissolve compound into it so that on cooling compound should come out and hence the recrystallization is achieved.
v) But as said DMSO dissolves some compounds (impirities also) even at low temperature and hence recrystallization for purification will not be achieved.
Hence DMSO is not good recrystallization solvent.
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3)
Amount of benzoic acid before recrystallization = 435 mg
Amount of benzoic acid after recrystallization = 360 mg
% recovery = Amount after recrystallization/Amount before recrystallization x 100
% recovery = 360/453 x 100 = 79.47 %
% recovery of benzoic acid is 79.47%
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