Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: fran008 on January 19, 2006, 11:36:55 AM
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A by-product of the coal-tar industry is a liquid containing phenol mixed with hydrocarbons and organic bases. How would you separate the phenol?
Could anyone explain in principle how one would check the purity of the sample of phenol obtained by this method?
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Distillation? Bp phenol bp 181.7c, most of the hydrocarbons will be rather long chain and have bp's above phenol. As for the organic bases, should there be any in the distillate at this temperature you could seperate by pH alterations and solvent extract the phenol.
-Josh
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Try dissolving the solutions in ether and adding a strong base. I believe this should bring the phenol out of solution. (Take this suggestion with a grain of salt though. I'm no expert with organic analysis :P)
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Try dissolving the solutions in ether and adding a strong base. I believe this should bring the phenol out of solution. (Take this suggestion with a grain of salt though. I'm no expert with organic analysis :P)
That ought to work. The pKa of phenol is about 10.
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S0 WHat happens is that when you add an ether, 2 layers form. One layer contains phenol and hydrocarbons and the other organic bases in aquoeous solution. The two mixtures are seperated using a seperating funnel. Then a base such as NaOH will precipitate sodium phenoxide?
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When aqueous base is added to the solution, the base will deprotonate the phenols, creating charged phenolate ions which are soluble in the aqueous phase. The hydrocarbons and organic bases are not acidic, so they will not react with the base to form charged salts. Therefore, the phenolate will be in the aqueous layer while the hydrocarbons and organic bases will remain in the organic (ether) phase. Isolation of the organic phase, then neutralizing with aqueous acid will precipitate the phenol from the aqueous phase.
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Distillation? Bp phenol bp 181.7c, most of the hydrocarbons will be rather long chain and have bp's above phenol. As for the organic bases, should there be any in the distillate at this temperature you could seperate by pH alterations and solvent extract the phenol.
-Josh
I thought BP of phenol will be higher than that of hydrocarbons due to the presence of hydrogen bonding.
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Comparing hydrogen bonding is really only valid for molecules of comparable size. When there is a big difference in size of the molecules then other effects become more important.
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And about checking the purity of the smple of phenol obtained, is checking boiling point a good idea?
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Phenol is soluble in water so why should make fenolate? i think water alone can extract phenol from the ether phase though may be in basic media that works better as Ph-ONa insoluble in ether ;)..
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Phenol is soluble in water so why should make fenolate? i think water alone can extract phenol from the ether phase though may be in basic media that works better as Ph-ONa insoluble in ether ;)..
Exactly. You want to maximize your partition coefficient.
BP for checking purity of your phenol should be okay. Melting point is usually more accurate though.
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Why is melting point more accurate? and how can u measure that?
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Melting point is usually more accurate because it isn't so dependent on pressure. Also, compounds tend to have a sharper melting point range than boiling point range. You don't get problems with azeotropes either, although you will observe melting point depression if the sample is impure.
To measure a MP, you usually use a special aparatus that is made for checking MPs, but you can get away with a regular oil bath which you heat slowly until you sample, usually placed in a capillary tube, melts.