Description: Primary amides can be converted to
nitriles with a dehydrating reagent such as P2O5 .
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.
Why do we use phosporus pentoxide (p2o5) when dehydrating amide? could we dehydrating amide without reagent phosporus pentoxide? and in what condition dehytrating of amides can be happen faster?
BalasHapusthanks for your question vebria,i think the answer is
BalasHapusApplications of phosphorus pentoxide Phosphorus pentoxide is a potent dehydrating agent as indicated by the exothermic nature of its hydrolysis:
P4O10 + 6 H2O → 4 H3PO4 (–177 kJ)
However, its utility for drying is limited somewhat by its tendency to form a protective viscous coating that inhibits further dehydration by unspent material. A granular form of P4O10 is used in desiccators. Consistent with its strong desiccating power, P4O10 is used in organic synthesis for dehydration. The most important application is for the conversion of amides into nitriles
P4O10 + RC(O)NH2 → P4O9(OH)2 + RCN