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I was told that urea on reaction with nitrous acid gives nitrogen gas, carbon dioxide and water. While on the other hand, thiourea on reaction with nitrous acid gives $\ce{H+}$, thiocyanate ion and water.

I tried looking for the mechanism and thought that this would be related:

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I carried out the above procedure on both the nitrogens of urea and was able to obtain the products of the first reaction enlisted above. Now, my questions are:

• Isn't the nucleophilicity of the lone pairs of urea's nitrogens negligible, and any reaction should proceed via attack by oxygen's lone pairs? (I thought that this was similar to amidines and amides)

• Why are the products of the reactions with urea and thiourea different, as the carbonyl oxygen, or sulphur doesn't seem involved in the reaction according to me

• More importantly, is this even the correct mechanism?

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This question has an open bounty worth +50 reputation from YUSUF HASAN ending in 3 days.

This question has not received enough attention.

Please provide a clear mechanism scheme(arrow-pushing) if possible and also explain the discrepancy between urea and thiourea's reactivity

  • $\begingroup$ It is similar as in amidines and amides and it doesn't matter. Any attacks on O are unproductive, so effect of conjugation is rather kinetical. $\endgroup$ – Mithoron Jan 12 at 19:17
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What I've so far is that Thiourea is a stronger nucleophile (Sulfur is less electronegative than oxygen, thus more nucleophilic). Thiourea reacts at the site of the sulfur whereas urea reacts the the amine sites.

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  • $\begingroup$ Thanks for the idea, but it would be great help if you could provide a mechanism scheme $\endgroup$ – YUSUF HASAN Jan 10 at 13:06

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