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I found the Wikipedia article for melamine to be incorrect in "synthesis and reaction" section; so I decided to edit it. But on the other hand I'm not sure my assumption to be correct that the entry is incorrect. So I decided to ask here to seek assist.

Today most industrial manufacturers use urea in the following reaction to produce melamine: $$\ce{6(NH2)2CO -> C3H6N6 + 6NH3 + 3CO2}$$

In the first step, urea decomposes into cyanic acid and ammonia: $$\ce{(NH2)2CO -> HNCO + NH3}$$

Cyanic acid polymerizes to cyanuric acid, which condenses with the liberated ammonia forming melamine. The released water reacts with cyanic acid, which helps to drive the reaction:

$$\ce{6HNCO + 3NH3 -> C3H6N6 + 3CO2 + 3NH3}$$

Questions:

  1. So are there two methods of obtaining melamine; one by decomposing $6$ moles of urea and another by mixing $6$ moles of cyanic acid with $3$ moles of ammonia?

  2. Why is $\ce{3NH3}$ both in reactors and products parts of the last reaction?

  3. So where are the cyanuric acid and water mentioned in the last reaction? They are in the text but not in the reaction equation.

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  • $\begingroup$ I combined the latter two reactions and I got this: $$\ce{(NH2)2CO + 5HNCO -> C3H6N6 + 3CO2 + NH3}$$, which differs from the first reaction. $\endgroup$ Commented Aug 17, 2023 at 13:52
  • $\begingroup$ Think it like this way: Urea decomposing to cyanic acid which then polymerizes to melamine. $\endgroup$ Commented Aug 17, 2023 at 17:01
  • $\begingroup$ And a species can be at both sides of the reaction. It is the net moles that matter. See for example: chemistry.stackexchange.com/questions/34138/… $\endgroup$ Commented Aug 17, 2023 at 17:03

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In the first step, urea decomposes into cyanic acid and ammonia: $$\ce{(NH2)2CO -> HNCO + NH3}$$

Cyanic acid polymerizes to cyanuric acid,

$$\ce{3 HCNO -> (HO)3C3N3}$$

which condenses with the liberated ammonia forming melamine.

$$\ce{(HO)3C3N3 + 3 NH3 -> (NH2)3C3N3 + 3 H2O}$$

The released water reacts with cyanic acid,

$$\ce{HCNO + H2O -> NH3 + CO2}$$

which helps to drive the reaction: $$\ce{6HNCO + 3NH3 -> C3H6N6 + 3CO2 + 3NH3}$$

It gives the summary reaction:

$$\ce{6(NH2)2CO -> C3H6N6 + 6NH3 + 3CO2}$$

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  1. So are there two methods of obtaining melamine; one by decomposing 6 moles of urea and another by mixing 6 moles of cyanic acid with 3 moles of ammonia?

That's not what the article is saying.

The first equation it presents describes the stoichiometry of an overall process for converting urea into melamine, ammonia, and carbon dioxide.

The second two equations each describe contributing reactions: first urea decomposes into cyanic acid and ammonia; then, some of those intermediate products react further to produce the final products.

In the industrial processes described, both of these reactions happen in the same reaction vessel, and they are going on at the same time. And in the real world, probably neither runs all the way to completion, and probably there are some side reactions, too, diverting some of the input material to different products.

  1. Why is 3NH3 both in reactors and products parts of the last reaction?

Because ammonia is both consumed and produced. The reaction does not happen without ammonia: "cyanuric acid [...] condenses with [...] ammonia", but overall, the ammonia that is consumed is also replaced. Putting it on both sides of the reaction conveys that.

  1. So where are the cyanuric acid and water mentioned in the last reaction? They are in the text but not in the reaction equation.

Yes, the last equation presented is also a summary scheme. It does not show the cyanuric acid and water being both produced and consumed (as opposed to consumed and (re)produced), possibly because they are short-lived intermediates, possibly because the author considered those details less important, possibly just for brevity. The prose description sketches in those details. But if you preferred, you could say $$\ce{3HNCO -> C3N3(OH)3}$$ $$\ce{C3N3(OH)3 + 3NH3 -> C3N3(NH2)3 + 3H2O}$$ $$\ce{3HNCO + 3H2O -> 3NH3 + 3CO2}$$ The last reaction drives this sub-process forward by consuming the water produced during the condensation and replenishing the ammonia consumed in that step. But note well that this can't happen without the condensation happening first, and the condensation requires ammonia.

As for

I found the Wikipedia article for melamine to be incorrect in "synthesis and reaction" section

, I don't see any basis for that conclusion.

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  • $\begingroup$ I don't see any basis for that conclusion. Yes, and I myself told everyone that I'm not sure about my assumption. $\endgroup$ Commented Aug 18, 2023 at 2:52

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