Ephoton
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A question on the hydrolisis of nitriles to acids.
The normal procedure would be to heat the nitrile
with some kind of mineral acid.
My question is if one is to leave the nitrile in the mineral acid for a very extended period would this also result in its coversion to an acid, or
would it stop at the amide.
If anyone knows of other was of converting a nitrile
to an acid other than using an alkali I would love to hear it.
[Edited on 31-5-2008 by Ephoton]
e3500 console login: root
bash-2.05#
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chemrox
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depends on the compound. Is it a secret? Why not heat it?
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Nicodem
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Nitriles are hard to hydrolyze and will not easily do so at room temperature. Just do a kinetic consideration using the rule of thumb that every
increase of 10K doubles the reaction speed. So if a complete hydrolysis of your nitrile would requires 5 hours at 110°C (let's say a temperature of
some diluted H2SO4 reflux) it could require something like 5*2<sup>9</sup> hours at 20°C which is more than three months. And this is
only using a rule of thumb estimation which means little, since other factors derived from the biphasic conditions, like different solubilities at
room temperature and phase transfer rates could reduce the speed even much more (it could actually take more than a year!). Not to mention the
annoyance of having to monitor the reaction progress routinely at least with a TLC and having the stirrer occupied for months.
The general method for the hydrolysis of nitriles to amides is to use a hydroxide in with at least one equivalent of hydrogen peroxide (the
HOO<sup>-</sup> is a strong nucleophile and adds on the nitrile triple bond much more easily than OH<sup>-</sup>, thus forming
the R-C(OOH)=NH which decompose to oxygen and the corresponding amide). This way the reaction is easily stopped at the amide stage while other methods
of either acidic or basic hydrolysis generally can not give amides in good yields since they tend to hydrolyze further at the conditions used.
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chemrox
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Yes, NaOH with 6-12% H2O2 is the normal procedure. Acid catalyzed hydrolysis is a second choice and may require more energetic conditions. I'm still
curious why you would want to leave it in acid for extended lengths of time and as my friend says, tie up a stirrer 'for months...' You can stop at
the amide using MnO2 on silica if that's of any interest. Structure seems irrelevant after all.
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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kmno4
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| Quote: | Originally posted by Ephoton
My question is if one is to leave the nitrile in the mineral acid for a very extended period would this also result in its coversion to an acid, or
would it stop at the amide.
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... so let us put H2O2 aside 
This reference is taken from OS:
HYDROLYSIS OF ARYLACETONITRILES
WILHELM WENNER
J. Org. Chem.; 1950; 15(3); 548-551
It is about only one class of nitriles but shows what conditions lead to what produsts. The first page is freely available:
http://pubs.acs.org/cgi-bin/searchRedirect.cgi/joceah/1950/1...
[Edited on 1-6-2008 by kmno4]
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Nicodem
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Thanks a lot kmno4! Coincidently, that paper gave me an answer to a problem that buggered me for some time already.
As for the hydrolysis of arylacetonitriles the paper pretty much says the hydrolysis rate was fast enough to give carboxylic acids only above 50°C:
"Up to temperatures of 50°C the resulting phenylacetamide is not easily saponified further." But what is most interesting is that up to the amide
stage, the hydrolysis with HCl(aq) is detectable already at 10°C! The other surprising thing is that "the formation of phenylacetic acid is speeded
up considerably when the amide is heated with more dilute acid."
I would however not tend to generalize these results with acids other than HCl(aq) (like H2SO4 commonly used). They even report that "hydrobromic acid
will not lead to saponification under similar conditions" [to phenylacetamide at 10°C]. Also generalization to all aliphatic nitriles is a bit
dubious, let alone to benzonitriles. Yet, find the full paper attached.
Attachment: HYDROLYSIS OF ARYLACETONITRILES.pdf (305kB)
This file has been downloaded 724 times
…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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