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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Luscious on January 28, 2012, 07:03:11 PM

Title: Williamson Ether Synthesis Experiment
Post by: Luscious on January 28, 2012, 07:03:11 PM

We did a Williamson Ether Synthesis in lab last week and I am confused about one of the steps.

We reacted an o-cresol with KOH... the results being the conjugate base of the cresol to become soluble in H20, K+, H20, and an excess of KOH... chloroacetic acid was added, and an acid base reaction happens between the KOH and chloroacetic acid.... Sn2 reaction happens between conjugate cresol base and deprotonated chloroacetic acid... making the o-methylphenoxyacetic acid.. a final addition of HCl neutralizes the species for separation..

What would be the consequence of added only 1 mole equivalent of KOH instead of 4?

Is the excess KOH only needed to deprotonate the chloroacetic acid?
Why does the chloroacetic acid even need to be deprotonated?


Thanks for all your *delete me*:) 

Title: Re: Williamson Ether Synthesis Experiment
Post by: Doc Oc on January 28, 2012, 08:50:21 PM

Take a look at things stepwise.

1) You add the KOH to deprotonate cresol and get a phenoxide anion.  This is your nucleophile for the next reaction, but remember that nucleophiles can also be bases.

2) Chloroacetic acid is the electrophile, but what part of it might be a problem if you just add a nucelophile/base?  How would you solve that problem?

Title: Re: Williamson Ether Synthesis Experiment
Post by: Altered State on February 20, 2013, 03:19:30 PM

If you use just one equivalent of base (KOH), when you add monochloroacetic acid, it will "give" his H+ to the deprotonated o-cresol, becouse chloro acetic pKa is lower (about 5) than o-cresol's pKa (about 6-7), and then, Sn2 reaction won't be able to take place.

Title: Re: Williamson Ether Synthesis Experiment
Post by: Dan on February 20, 2013, 04:37:01 PM

Quote from: Altered State on February 20, 2013, 03:19:30 PM

If you use just one equivalent of base (KOH), when you add monochloroacetic acid, it will "give" his H+ to the deprotonated o-cresol, becouse chloro acetic pKa is lower (about 5) than o-cresol's pKa (about 6-7), and then, Sn2 reaction won't be able to take place.

pKa of chlorocaetic acid is ~3
pKa of o-cresol is ~10

Title: Re: Williamson Ether Synthesis Experiment
Post by: Altered State on February 20, 2013, 04:52:14 PM

Quote from: Dan on February 20, 2013, 04:37:01 PM

Quote from: Altered State on February 20, 2013, 03:19:30 PM

If you use just one equivalent of base (KOH), when you add monochloroacetic acid, it will "give" his H+ to the deprotonated o-cresol, becouse chloro acetic pKa is lower (about 5) than o-cresol's pKa (about 6-7), and then, Sn2 reaction won't be able to take place.

pKa of chlorocaetic acid is ~3
pKa of o-cresol is ~10

Thanks for the correction.
It was a "fast estimation" based of my 2nd chemistry year knowledge.

Title: Re: Williamson Ether Synthesis Experiment
Post by: bcoots on February 22, 2013, 11:15:05 AM

It would be better to use the sodium salt of the chloroacetic acid to begin with.
1) it is much less toxic than the acid
2) you don't need the extra KOH to neutralize it