while imine is getting hydrolysed, why does not nitrile group hydrolyse to give carboxylic acid?
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5$\begingroup$ Imines are much easier to hydrolyse. Nitrile hydrolysis takes much more forcing conditions (usually heat) $\endgroup$– orthocresolCommented Oct 12, 2017 at 9:34
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3$\begingroup$ A word on your mechanism: You cannot have $\ce{tBuO-}$ and $\ce{H3O+}$ in the same reaction vessel. And likewise, as soon as you have a few molecules of $\ce{NH4+}$, you will no longer see any $\ce{H3O+}$. $\endgroup$– JanCommented Oct 12, 2017 at 13:00
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1$\begingroup$ Is the water coming in at the work up stage? It wouldn't normally occur that you have water in with butoxide from the beginning $\endgroup$– BeerhunterCommented Oct 12, 2017 at 17:49
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$\begingroup$ I couldn't upload the mechanism from the book. Posted it from wikipedia. $\endgroup$– user481311Commented Oct 13, 2017 at 16:02
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1 Answer
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It has to do with hybridization. The nitrogen in a nitrile is sp hybridized; imine sp2 and ammonia sp3. The more s-character in hybrid orbitals, the less basic the electron pair (protonated nitrile pKa ~ -10; pKb ~24). Ammonia has the lowest percent s-character (25%, NH4+ pKa 9.25). Imines lie in between although most examples you will find are measured in DMSO and are loaded with biasing phenyl groups. Compare the carbon series: acetylene (sp, pKa ~25), ethylene (sp2, ~44), methane (sp3, pKa ~50). The closer the electron density is to carbon, the more acidic the C-H bond. If electrons are held closer to carbon, then it is more difficult for homolytic cleavage of a C-H bond. That is, acetylene is willing to give up a proton but not a hydrogen atom. Thus, Bond Dissociation Energy runs in the opposite order as acidity.
The harder it is to protonate, the harder it is to hydrolyze.
There were no conditions provided by user 481311 for the hydrolysis. Imines, such as the acetaldehyde methylimine (CH3CH=NCH3), will hydrolyze in water. As the User illustrates, imines hydrolyze readily in acid medium. These hydrolyses may be conducted at ambient temperature or at mild temperatures. Nitriles, owing to their low basicity, require more vigorous conditions like refluxing aqueous acid. Its all about conditions. I hope this helps.
