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What are the products produced in the decomposition reaction between p-nitroaniline and sulfuric acid at temperatures above 392 degrees Fahrenheit?
Answer 1:

The decomposition products of p-nitroaniline (also known as 4-nitroaniline) and sulfuric acid are not known exactly, but the reaction has been studied by NASA for its ability to form a pyrostat, which is a fire quenching substance. The decomposition reaction forms a solid brown- black foam. A substance that forms foam when heated is known as an aphrogen. Therefore p- nitroaniline is an aphrogenic pyrostat. The structure of p-nitroaniline is shown here

According to the article from NASA, “nitroaniline-sulfuric acid decomposition consists of three stages:”

1) “The first, or pre-expansion, stage is the interval up to about 230°C. Water is the major gaseous constituent, with some nitroaniline subliming at higher mole ratios; sulfur dioxide is absent (iodine test). Dehydration(i) and sulfonation(ii) are the main chemical processes; some deamination (iii) also takes place.

2) “The second, or aphrogenic (intumescent), stage begins at about 230°C for 4- nitroaniline … and is over within a 50° interval when the heating rate is 6°/min (iv). Sulfur dioxide and water are major gaseous products (v).

3) “The third stage, in air, represents oxidative decomposition of the residue (the primary product of the exothermal reaction) (vi) and dissociation of variable amounts of ammonium salts present in the residue (vii).”

(i) A dehydration reaction involves the loss of water from a molecule.

(ii) Sulfonation is a reaction in which an aromatic ring reacts with sulfuric acid to replace a hydrogen with a sulfonic acid group (SO3H).

(iii) Deamination is the loss of an amine group (NH3) from a compound.

(iv) This is the part where the foam expands rapidly.

(v) Carbon dioxide is also likely produced at this stage.

(vi) Essentially this means that the foam oxidizes in air after the expansion.

(vii) The deamination leads to ammonium salts remaining in the foam.

The relative atomic proportion of elements in p- nitroaniline and in the foam is:

Element p-nitroaniline foam
C 6 6.0
H 6 ~3.0
N 2 ~1.8
S 0 ~0.15
O 2 ~1.3

The foam is most likely a cross-linked network formed from whatever remains of the nitroaniline and therefore the reaction is sometimes referred to as an explosive polymerization.

You can watch a video of the reaction on youtube here:

watch reaction

If you are interested, here are some other interesting excerpts from the article:

“The mole ratio of nitroaniline to sulfuric acid, however, is not critical; voluminous foams have been obtained from mixtures over a mole ratio range of 0.3 to 2.0 if a proper heating schedule is maintained. Sulfur dioxide and water are the principal gaseous products evolved during these expansions.”

“Nitroaniline contains two nitrogen atoms for every six carbons. With the exception of 3- nitroaniline at mole ratios of 1 and 2, the atomic proportion of residual nitrogen is significantly less than 2. This result, along with the presence of ammonium ions, demonstrates extensive deamination; the extent of deamination is greatest when the amount of sulfuric acid is high.”

“The foams were good thermal insulators inert to chemicals, e.g., concentrated mineral acids, alkalies, organic and inorganic solvents, and oxidants.”

“If the foam is exposed to a flame, it glows without burning and oxidizes or erodes very slowly; the radiant glow extinguishes immediately when the foam is taken away from the flame.”

“The exact structural nature of the insoluble residue constituting the foam is unknown. All the sulfur in the residue is present as sulfonic acid groups attached to the benzene nuclei. Secondary amino or azinium groups resulting from deammoniative coupling must be present in order to account for ammonium ions and Nitrogen content less than 2 per benzene nucleus. A high nitrogen-to-oxygen ratio indicates that few if any nitro groups remain. Thus, both nitro arid sulfonic acid groups are involved in oxidative processes.”

Poshkus, A. C. and Parker, J. A. (1970), Studies on nitroaniline–sulfuric acid compositions: Aphrogenic pyrostats. J. Appl. Polym. Sci., 14: 2049–2064. doi: 10.1002/app.1970.070140813

aromatic sulfonation
dehudration reaction
d eamination

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