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1) I am unable to understand how benzenesulfonyl chloride acts as a dehydrating agent in dehydration of amides? What is the driving force for losing chloride and taking the oxygen from the amide?

2) Why I can't use other dehydrating agents like $\ce{P4O10}$, $\ce{H2SO4}$, or $\ce{CaCl2}$?

3) How can I estimate what is suitable dehydrating agent for a particular reactant?

4) Is it necessary to use only those dehydrating agents that undergo chemical change? Or can I use dehydrating agents that adsorb water like silica gel?

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One key detail is that this reaction is performed in the presence of base. Typically pyridine is the solvent. Amides are nucleophilic at oxygen, explained by the resonance structure shown. Displacement of chloride by the amide oxygen gives the O-sulfonylated intermediate. After neutralization, the O-sulfonylimidate is set up for an E2 elimination across the C-N bond to give the nitrile.

Dehydration mechanism

In addition to sulfonyl chlorides, $\ce{P4O10}$ and $\ce{POCl3}$ are common reagents for this reaction. In general, the reagent should be electrophilic and be able to act as a leaving group.

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The sulphur of the sulphonyl group is pretty electrophilic due to the two oxygens and one chlorine atom attached to it. So the nitrogen oxygen from the amide attacks on this sulphur atom, displacing the chlorine atom on it. The $\ce{Cl-}$ ion takes a proton from the amide with it and forms $\ce{HCl}$. The nitrogen then pushes its lone pair towards the former carbonyl carbon, causing the benzenesulphonyl group along with the former carbonyl carbon to take off, leaving a triple bond between the nitrogen and the carbon. The benzenesulphonate anion then picks up the proton left on the nitrogen, giving the products $\ce{R-C#N}$ and $\ce{Ph-SO3H}$.

Generally $\ce{P4O10}$ is a very strong dehydrating agent, so must be used where dehydration is difficult, or more dehydration than done by normal dehydrating agents is required. $\ce{H2SO4}$ is a good dehydrating agent, but probably isn't good enough for this particular reaction. It is difficult to predict which dehydrating agent must be used for any arbitrary reactant (atleast to my knowledge).

$\ce{CaCl2}$ or silica gel are physical dehydrating agents (more accurately drying agents) and cannot be used for chemical dehydration reaction like the above reaction.

Edit: My original mechanism was shown to be wrong by @jerepierre and @orthocresol, and hence i have corrected it above.

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    $\begingroup$ Do you have a reference for the proposed dehydration? I doubt that mech is correct. $\endgroup$ – jerepierre Apr 16 '16 at 14:38
  • $\begingroup$ I don't have a reference :/ But your mechanism does seem to make more sense...but I am not able to find concrete reference for the oxygen acting as a nucleophile, and not nitrogen. That is the part where I went off on a wrong track, otherwise your mechanism easily follows. $\endgroup$ – FreezingFire Apr 16 '16 at 18:20
  • $\begingroup$ @jerepierre It seemed intuitive to me initially that the nitrogen would act as a nucleophile....that is the reason for me proposing the above mechanism. $\endgroup$ – FreezingFire Apr 16 '16 at 18:21
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    $\begingroup$ The most nucleophilic part of an amide (aka the HOMO) is the carbonyl oxygen. The nitrogen lone pair is tied up by delocalisation into the C=O π*. $\endgroup$ – orthocresol Apr 16 '16 at 19:29
  • $\begingroup$ @orthocresol Okay. Thank you for correcting me! I have corrected the mechanism in my answer now! $\endgroup$ – FreezingFire Apr 17 '16 at 12:33

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