Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Juaqui on June 12, 2019, 04:37:24 PM
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Hello!
I'm not understanding a mechanism. I have this reaction:
https://ibb.co/dbBKQW0
I think the hydroxylamine hydrochloride is for forming the oxyme and the formic acid is the solvent, but which is the role of the sodium formate?
Thank you!
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If i'm not wrong, this should be reductive amination and then the oxime gets converted to nitrile via beckman rearrangement.
full mechanism: https://www.masterorganicchemistry.com/reaction-guide/beckmann-rearrangement/
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@OP, When a weak acid and its conjugate base are present in comparable amounts, what do you have?
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@OP, When a weak acid and its conjugate base are present in comparable amounts, what do you have?
You have a buffer, but how is the formation of the nitrile then?
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You have a double-bond and want a triple-bond, what reaction can do that?
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@OP, When a weak acid and its conjugate base are present in comparable amounts, what do you have?
You have a buffer, but how is the formation of the nitrile then?
My memory may be off, but I seem to recall that the rate of similar reactions is maximal at slightly acidic pH. I will try to double-check my memory on this as soon as I can.
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Sodium formate removes HCl and increases the boiling temperature of the reaction mixture. Removing of HCl prevents the Beckman rearrangement and allows the thermal decomposition of oxime formate with good yield.
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Oxime eliminates water to give triple bond. Acids (HCl, Formic acid) catalyze the protonation of the OH, while the base (sodium formate) takes the H+.
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Thank you, people!
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And why not acetic acid and sodium acetate? ???
Could it be because of the more negative density of the oxygen in the formic acid, necessarily for substrating the H bonded to the N at the end?
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Such a method is also used but with the addition of acetic anhydride.
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In other words, is kind of Wohl degradation, isn't it?
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Not exactly. Alfred Wohl used this reaction in 1893 for the conversion of glucose oxime to nitrile. He isolated oxime and tested its geometry. He knew that only syn-oximes react with acetic anhydride with the formation of nitrile. Next step reaction with sodium ethoxide was a real degradation with carbon chain shortening.
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Whenever I see descriptions of synthesis in the late 1800s I am so impressed at their ability to deduce structure.
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Not exactly. Alfred Wohl used this reaction in 1893 for the conversion of glucose oxime to nitrile. He isolated oxime and tested its geometry. He knew that only syn-oximes react with acetic anhydride with the formation of nitrile. Next step reaction with sodium ethoxide was a real degradation with carbon chain shortening.
And what is the way in which formiate removes HCl to increase the boilling point of the mixture?
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HCOONa + HCl = HCOOH + NaCl
Sodium formate is very soluble in formic acid. Formic acid boils at about 101 ° C and forms azeotrope with water boiling at about 107 ° C. Addition of approximately 3 moles of sodium formate to 1 kilogram of formic acid, after deducting its consumption to neutralize hydroxylamine hydrochloride, increase the boiling point of the reaction mixture (see ebullioscopic constant of formic acid) above the boiling point of the azeotrope. In this way, we can remove water from the reaction environment by shifting the equilibrium towards ester formation - oxime formate, which at this temperature decomposes thermally to the nitrile.
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It could be that formic acid formylates the oxime before the elimination-step, if you use acetic anhydride you get acetylation. The acyl group is a better leaving group then the protonated oxim hydroxyl group
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On page 467 of Catalysis in Chemistry and Enzymology (WP Jencks) there is a discussion of the pH dependence of the reaction of aldehydes and ketones with hydroxylamine. The rate for acetone and hydroxyl amine is maximal between pH 4 and pH 5 (Figure 1 on page 469). This issue may be tangential to the present discussion.
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Thank you very much, guys!!!!