Dehydration of amides to give nitriles

by James

Description: Primary amides can be converted to nitriles with a dehydrating reagent such as P2O5 .
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Notes: Note that the net effect of this reaction is to remove two H atoms and one O from the amide. For this reason this is called a “dehydration”.

Only primary amides work for this reaction. Other reagents can be used for this, however, such as thionyl chloride (SOCl2)

Examples:

Notes:

Mechanism:

The reaction begins with the oxygen of the amide attacking phosphorus (through a resonance form) forming an O–P bond (Step 1, arrows A, B, and C). After a proton transfer (Step 2, arrows D and E) a lone pair from nitrogen forms a new C–N bond, expelling oxygen (Step 3, arrows F and G). Finally the nitrogen is deprotonated (Step 4, arrows H and I) to give the neutral nitrile.

Notes:

There are certainly other reasonable ways to draw proton transfer (Step 2) as well as other bases to use for deprotonation (Step 4) besides phosphate. This is just one reasonable possibility.

It’s also reasonable to show fragmentation of the P–O–P bond in step 3, although for simplicity’s sake this was not drawn.

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{ 2 comments… read them below or add one }

Chan

Hi James,
My question may sound silly,but i seriously have no idea.Why ‘P’ in P2O5 is attracted by the pi bonds and not the lone pairs of Amine group?

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james

Lone pair of amide is strongly donating into carbonyl carbon – there is pseudo double bond character between carbon and nitrogen. Oxygen is very nucleophilic.

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