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Diphenyl Ether From Phenol?
I was wondering if it is possible to make diphenyl ether from phenol via an acid-ether synthesis (using H2SO4 catalyst) instead of using a Williamson
ether synthesis. The Williamson ether synthesis would not be an issue, I would just prefer to do the first method as I wouldn't need to make
bromobenzene (I'm pretty lazy, so the less work, the better  )
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Nicodem
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Thread Moved
4-8-2015 at 16:24 |
Nicodem
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No, it is not possible to condense phenol to diphenyl ether with sulfuric acid. See the mechanism of acid catalyzed ether formation and you will see
why it does not apply to phenol.
Williamson ether synthesis with phenol and bromobenzene is impossible, as bromobenzene does not undergo SN2 substitutions for obvious
reasons. However, these two reactants give diphenyl ether in the Ullmann condensation reaction. Arylations of phenols work well also with palladium
based catalysts (via oxidative addition of iodo- or bromobenzenes followed by halide/phenolate ligand exchange and finally reductive elimination).
With electron poor halobenzenes (for example with 4-fluoronitrobenzene, etc.) direct arylation or phenols is possible via aromatic nucleophilic
substitution (but this does not work with PhBr).
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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The Chemistry Shack
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Quote: Originally posted by Nicodem  | No, it is not possible to condense phenol to diphenyl ether with sulfuric acid. See the mechanism of acid catalyzed ether formation and you will see
why it does not apply to phenol.
Williamson ether synthesis with phenol and bromobenzene is impossible, as bromobenzene does not undergo SN2 substitutions for obvious
reasons. However, these two reactants give diphenyl ether in the Ullmann condensation reaction. Arylations of phenols work well also with palladium
based catalysts (via oxidative addition of iodo- or bromobenzenes followed by halide/phenolate ligand exchange and finally reductive elimination).
With electron poor halobenzenes (for example with 4-fluoronitrobenzene, etc.) direct arylation or phenols is possible via aromatic nucleophilic
substitution (but this does not work with PhBr). |
Forgive my confusion, I am relatively new to O-chem and haven't studied any more than basic mechanisms like Nucleophilic substitution, condensation,
addition, elimination, esterification, etc..
Does the acid-catalyzed synthesis not work with the phenol because the phenol is acidic, making it harder to protonate? Or is it something to do with
the alkene groups?
Also, I misspoke about the Williamson synthesis. It is actually a modification of that mechansim using bromobenzene and Sodium phenoxide, according to
the wiki article: https://en.wikipedia.org/wiki/Diphenyl_ether
"It is synthesized by a modification of the Williamson ether synthesis, here the reaction of phenol and bromobenzene in the presence of base and a
catalytic amount of copper:
PhONa + PhBr → PhOPh + NaBr"
[Edited on 5-8-2015 by The Chemistry Shack]
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Nicodem
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| Quote: Originally posted by The Chemistry Shack | | Does the acid-catalyzed synthesis not work with the phenol because the phenol is acidic, making it harder to protonate? Or is it something to do with
the alkene groups? |
The mechanism of the acid catalysed ether formation involves an SN2 in the ether bond formation step (in the case of tert-alcohols
and certain others, it involves a SN1). Phenols obviously cannot undergo SN2 substitutions at the ipso position as that
carbon is sp2 hybridized. SN1 is also impossible for phenols as phenyl carbocations are among the most instable carbocations of
them all.
For the mechanism of this and the Willliamson reactions, see: http://www.mhhe.com/physsci/chemistry/carey/student/olc/ch15... (note the step 2 of the acid catalyzed reaction)
http://www.masterorganicchemistry.com/2014/11/14/ether-synth...
(and many other educational sites that you can find yourself)
Try applying that mechanism to phenol and you will see it simply cannot go.
Don't take what Wikipedia authors write for granted. Either check the references or read further. That reference that Wikipedia cites uses the Ullmann
condensation reaction, which is obviously something quite different from the Williamson reaction. Wikipedia acknowledges this by mentioning that
copper catalysis is used, but oversimplifies by calling it a "modified Williamson ether synthesis". Whenever you see the word "modified" applied in
organic synthesis, you need to be skeptical whether it is truly still the same reaction with the change in technique, conditions or reagents standing
for the "modified", or is it actually mechanistically different. Such confusion sometimes appears even in the scientific articles. It is a fuzzy
area, especially for reactions that can proceed through various pathways under overlapping conditions to give the same product (and there are many
such).
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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The Chemistry Shack
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| Quote: Originally posted by Nicodem |
The mechanism of the acid catalysed ether formation involves an SN2 in the ether bond formation step (in the case of tert-alcohols
and certain others, it involves a SN1). Phenols obviously cannot undergo SN2 substitutions at the ipso position as that
carbon is sp2 hybridized. SN1 is also impossible for phenols as phenyl carbocations are among the most instable carbocations of
them all. |
Ok, thank you for the help. After looking at the mechanisms, it does make sense that SN2 won't work (it appears that the pi electrons get
in the way of the oxygen's lone electron pair, making substitution impossible, although I might be misunderstanding this).
I have tried to apply the second step of the mechanism to phenol, and as I said before, from drawing everything out it seems as if the pi electrons on
the phenol get in the way and prevent the nucleophilic oxygen from attaching to the ring, although I couldn't find any justification for this online,
other than it seems to make sense to me.
And I will definitely note what you said about using wikipedia as a source for organic reactions!
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