[GeLe5000]

Good evening.

It's easy to balance a combustion equation for molecules made of C, H and O. But how can I know the fate of a molecule containing a N atom ?
For example : Paracetamol, C8H9NO2 + ? O2  ------> ? H2O + ? CO2 + ? N2.

[Borek]

N₂ is in most cases a reasonable guess.

[Enthalpy]

If the combustion gave CO₂ instead of a mix with much CO, then the oxidizer was not pure oxygen, but something like air, which already brings much nitrogen. So what should be different if the fuel too contains nitrogen?

[GeLe5000]

I was considering combustion in a calorimeter, in pure oxygen. And the production of N2 is (indeed) only a guess.

[Enthalpy]

While the combustion gases are hot, they contain much CO, some NO_x and many more species. But once they have cooled enough to leave only CO₂ without CO, N₂ is the expected product.

Out of curiosity, I've asked a software the composition of the flame, at 1atm, with just enough pure oxygen for CO₂ once cold. Here molar fractions:

CO                   2.8109e-001
CO2                  1.8832e-001
COOH                 7.2469e-007
H                    4.1505e-002
HCO                  7.9666e-007
HNO                  6.5391e-007
HNO2                 7.9769e-008
HO2                  4.4565e-005
H2                   3.3417e-002
HCOOH                1.8394e-008
H2O                  1.7123e-001
H2O2                 1.2297e-006
N                    4.7924e-006
NH                   3.5205e-007
NH2                  3.4576e-008
NO                   7.4103e-003
NO2                  2.9766e-006
N2                   2.5629e-002
N2O                  7.6986e-008
O                    5.7822e-002
OH                   7.7243e-002
O2                   1.1627e-001
O3                   7.1936e-008

[GeLe5000]

Thank you for the data.

It's far from the simplicity of : Glucose C6H12O6 + 6 O2 ---------> 6 CO2 + 6 H2O.

[AWK]

For example : Paracetamol, C8H9NO2 + ? O2  ------> ? H2O + ? CO2 + ? N2

Nobody use paracetamol as fuel in  the car engine. Even then catalyst convert CO and NO and other dangerous compounds that may be formed during combustion to N₂ and CO₂.

Just end balancing this above reaction and all be OK

[GeLe5000]

C₈H₉NO₂ + 37/2 O₂  -----> 8 CO₂ + 9/2 H₂O + 1/2 N₂

[AWK]

Whole numbers and ... correct balancing.

[Borek]

Out of curiosity, I've asked a software the composition of the flame, at 1atm, with just enough pure oxygen for CO₂ once cold. Here molar fractions:

The OP asked about burning in an excess of pure oxygen, so while these numbers are interesting they are irrelevant to the case.

[GeLe5000]

It must be this one :

4 C₈H₉NO₂ + 37 O₂  -----> 32 CO₂ + 18 H₂O + 2 N₂

I was asking about that because of the metabolic fate of Paracetamol. For 1 molecule of Paracetamol, there is 37/4 = 9,25 molecules of Oxygen consumed, thus there must be an Oxygen radical somewhere. A toxic species.

The same with Ibuprofen (C₁₃H₁₈O₂ + 16,5 O₂ ---> 13 CO₂ + 9 H₂O). I hope there's no mistake in this one.

But I don't know if it could be an explanation of the toxicity of these drugs.

[AWK]

C13H18O2 + 16,5 O2 ---> 13 CO2 + 9 H2O

Coefficients should be whole numbers.

[Arkcon]

It must be this one :

4 C₈H₉NO₂ + 37 O₂  -----> 32 CO₂ + 18 H₂O + 2 N₂

I was asking about that because of the metabolic fate of Paracetamol. For 1 molecule of Paracetamol, there is 37/4 = 9,25 molecules of Oxygen consumed, thus there must be an Oxygen radical somewhere. A toxic species.

The same with Ibuprofen (C₁₃H₁₈O₂ + 16,5 O₂ ---> 13 CO₂ + 9 H₂O). I hope there's no mistake in this one.

But I don't know if it could be an explanation of the toxicity of these drugs.

Its not rare, in biology, to balance a metabolic equation with fractions.  But before you said:

I was considering combustion in a calorimeter, in pure oxygen. And the production of N2 is (indeed) only a guess.

You're going to have to pick one or the other.  Nothing like a calorimeter happens in any living thing.

[GeLe5000]

"Nothing like a calorimeter happens in any living thing"


I agree, but the total energy balance for the reaction "Glucose C6H12O6 + 6 O2 ---------> 6 CO2 + 6 H2O + Energy (ATP)"  is given buy a measurement in a calorimeter, thus I thought that it would be the same for any compound (assuming that it can be catabolised on the same way as Glucose. Aspirin, for example can be a source of energy in our body : C9H8O4 + 9 O2   ----> 9 CO2 + 4 H2O + Energy (ATP)).
Since the enzymes handle their substrates one by one, a biochemist must sometimes use fractions. And an odd number of Oxygen atoms could mean that an Oxygen radical leaks away.

[AWK]

Aspirin, for example can be a source of energy in our body : C9H8O4 + 9 O2   ----> 9 CO2 + 4 H2O + Energy (ATP)).

Aspirin is not a source of energy for our body, at least not important one.
About 70 % of aspirine is excreted in urine as salicylic acid, its acyl and phenolic glucuronides, salicyluric acid, its phenolic glucuronide and gentisic acid. Conjugation needs some energy.

https://www.ncbi.nlm.nih.gov/pubmed/3705620