Berkeley Review Ochem question regarding recrystallization solvent choice

[erskine]

heyas all,

I'm having trouble with example 8.9 of the Ochem BR review, in the lab techniques section.


If a solid is too soluble in water at room temperature, it would be best to add which of the following solvents?
A. Hexane
B. Ethanol
C. Diethyl Ether
D. Tetrahydrofuran

The correct answer is B.

Could someone explain how this changes the overall polarity of the solvent- is it just because the ethanol reacts with the water and reduces its affinity for the solid?

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[BerkReviewTeach]

heyas all,

I'm having trouble with example 8.9 of the Ochem BR review, in the lab techniques section.


If a solid is too soluble in water at room temperature, it would be best to add which of the following solvents?
A. Hexane
B. Ethanol
C. Diethyl Ether
D. Tetrahydrofuran

The correct answer is B.

Could someone explain how this changes the overall polarity of the solvent- is it just because the ethanol reacts with the water and reduces its affinity for the solid?

Because of the R-group on the alcohol, ethanol is less polar than water and exhibits less H-bonding. This is why despite etOH being heavier than water, it still has a lower boiling point than water. Upon mixing ethanol into water, the polarity becomes an average of the two solvents and is therefore overall less polar than pure water.

Also, ethanol is the only one of the four solvent choices that is soluble in water.

 

[schlieren]

And to make it bio-relevant, ethanol extraction of DNA molecules from water is a standard bio lab tech.

 

[manohman]

sorry to bring this back. But why couldnt you add any of the other solvents to the solution? It would just make the solution less polar wouldnt it?

 

[chemtopper]

heyas all,

I'm having trouble with example 8.9 of the Ochem BR review, in the lab techniques section.


If a solid is too soluble in water at room temperature, it would be best to add which of the following solvents?
A. Hexane
B. Ethanol
C. Diethyl Ether
D. Tetrahydrofuran

The correct answer is B.

Could someone explain how this changes the overall polarity of the solvent- is it just because the ethanol reacts with the water and reduces its affinity for the solid?

Water is more polar than ethanol and in ethanol -C2H5 group is non polar part and O-H is the polar part.Solid is highly polar in nature and that is the reason it is highly soluble in water.So recrystallization with water as solvent will be difficult as solid would tend to remain in soluble form.If we use ethanol as solvent then solubility of the solid will be reduced and on recrystallization more solid would come out of the solvent.

 

[manohman]

Water is more polar than ethanol and in ethanol -C2H5 group is non polar part and O-H is the polar part.Solid is highly polar in nature and that is the reason it is highly soluble in water.So recrystallization with water as solvent will be difficult as solid would tend to remain in soluble form.If we use ethanol as solvent then solubility of the solid will be reduced and on recrystallization more solid would come out of the solvent.

but wouldnt adding hexane for example decrease the polarity of the solvent, so the solid would be less soluble in the solution?

 

[chemtopper]

but wouldnt adding hexane for example decrease the polarity of the solvent, so the solid would be less soluble in the solution?

Hexane is a non polar solvent and solid will be completely insoluble in it.We need a solvent which can dissolve it and on cooling we can get solid recrystallize out of it .With water solubility is more so recrystallization becomes difficult or may not take place.Ethanol is the best because it can dissolve the solid and since it is less polar so it helps in recrystallization also.

 

[manohman]

Hexane is a non polar solvent and solid will be completely insoluble in it.We need a solvent which can dissolve it and on cooling we can get solid recrystallize out of it .With water solubility is more so recrystallization becomes difficult or may not take place.Ethanol is the best because it can dissolve the solid and since it is less polar so it helps in recrystallization also.

I see. So you can't lower the polarity of a solution as a whole by mixing two different solvents together.

The reason why I ask is because when doing column chromatography, you can change the ratio of your Polar to NonPolar components of your solvent (i.e. Hexane to Ethanol) to change the rate of elution, so i thought the same principle applied here.

 

[chemtopper]

I see. So you can't lower the polarity of a solution as a whole by mixing two different solvents together.

The reason why I ask is because when doing column chromatography, you can change the ratio of your Polar to NonPolar components of your solvent (i.e. Hexane to Ethanol) to change the rate of elution, so i thought the same principle applied here.

Hexane and ethanol are immiscible - two solvents - one polar and other non polar