Group Additivity Approach For Estimating Reactive Hazards

During process development or plant operation it is often necessary to estimate energy of reaction based on chemical formulae representation alone. This heat of decomposition represents the potential energy that can be released and is therefore a measure of explosion potential. One can therefore envision that estimation of heats reaction based on chemical similarities should be possible similar to Benson’s group additivity for estimating heats of formation.

I have compiled heats of polymerization and heats of formation from a variety of sources. You can use the functional groups from these tables to estimate heats of reaction for your system of interest.

Heats of Polymerization

Functional Group	Structure	Mean heat of reaction	Min…..Max.
		kcal/mol	kcal/mol
Olefins	-C=C-	20
Acetylenes	-C≡C-	30
	-C=O-	5
	-C=N-	1.4
	-C≡N-	7
Sulfolane	-S=O	7
	-C=S-	2

Heats of Reaction – Carbon Containing Groups

Functional Group	Structure	Mean -∆H	Min…..Max.
Epoxide	-C-C-	20	17-24
Hydro peroxide	-OOH	60	55-67
Dialkyl peroxides	R-O-O-R	50	43-55
Peroxyacids	-C(=O)OOH	63	57-69
Diacyl peroxy	-C(=O)-O-O-C(=O)	75	55-86
Hydrazine	-N-N-	18	15-21
Azo	-N=N-	50	28-185
Diazo	-N2+	40	38-43
	>C=N≡N	50	41-57
Azide	N3	53	48-58

Heats of Reaction – Nitrogen Containing Groups

Functional Group	Structure	Mean -∆H	Min…Max
In a ring	-N-N-	24	4-72
Imidazole	N-C-N	31	10-51
Tetrazole		50	33-84
Trizene	-N=N-N<	50	33-84
C-Nitro	-C-NO2	75	60-130
O-Nitro	-O-NO2	110	105-115
N-Nitro	-N-NO2	100	93-108
Nitroso	-N=O-	48	33-70
N-Oxide	-N-O-	34	21-51
Oxime	>N-OH	34	25-68
Oxazole	N-C-O	19	8-43
Isocyanate	-N=C=O	15	12-18

Examples

1. Heat of polymerization for Styrene

To predict heat of polymerization I will use –C=C-.
Mean – 20 kcal/mol
Min – 12 kcal/mol
Max – 22 kcal/mol

Thus we estimate heat of polymerization of Styrene to be 217 cal/g [Range 130-239 cal/g]. The experimental heat of reaction for styrene is 151 cal/g.

2. Heat of reaction for Propylene oxide

To predict heat of polymerization I will use value for epoxide group.
Mean – 20 kcal/mol
Min – 17 kcal/mol
Max – 24 kcal/mol

Thus we estimate heat of polymerization of propylene oxide to be 343 cal/g [Range 292-412 cal/g]. The experimental heat of reaction for styrene is 266 cal/g.

To test the values in the two tables, I predicted reaction energies for 105 compounds.

For the 105 compound test set the average absolute error was 87 cal/g. I therefore believe that you can predict heats of reactions within 100 cal/g using the proposed values.

Knowing the heat of reaction for a compound gives you an idea of its explosion potential. As a rule of thumb, if the heat of reaction  is greater than 750 cal/g, the compound is packing high energy.