Thermal decomposition of calcium propionate

What are the decomposition products?

For example:

Calcium formate decomposes into formaldehyde and calcium carbonate.

$$\ce{Ca(HCOO)2 -> CaCO3 + HCHO}$$

Calcium acetate decomposes into acetone and calcium carbonate.

$$\ce{Ca(CH3COO)2 -> CaCO3 + (CH3)2CO}$$

How does calcium propionate decompose?

There is a paper (1) where TG-GLC-MS with subsequent XRD analysis of solid products has been utilized for the analysis of evolved gases during the decomposition of calcium propionate at various heating rates. Detected products were $\ce{H2O}$, $\ce{CO2}$, $\ce{C2H5CHO}$, $\ce{C2H5COCH3}$, $\ce{C2H5COC2H5}$, $\ce{CaCO3}$.

The Discussion section of the paper suggests the following mechanism of formation:

Following the free radical mechanism of Hites and Biemann (2), proposed for the decomposition of the calcium salts of monocarboxylic acids we suggest that the initial stage in the decomposition of calcium propanoate can be represented as:

$$\ce{Ca(C2H5COO)2 -> CaCO3 + C2H^.5 + C2H5C^.=O,}$$

Combination of the two free radicals results in the formation of the major product, 3-pentanone:

$$\ce{C2H5C=O + C2H^.5 -> C2H5 - CO - C2H5}$$

The other trace products observed, 2-butanone and propanal, can be accounted for by disproportionation reactions involving the ethyl radical:

$$\ce{C2H^.5 + C2H^.5 -> C3H^.7 + CH^.3}$$

and

$$\ce{C2H^.5 -> C2H5 + H^.}$$

The methyl and hydrogen radicals so produced may react with the large excess of the original propanoyl radicals to give 2-butanone and propanal respectively.

$$\ce{C2H^.5=O + CH^.3 -> C2H5COCH3}$$

and

$$\ce{C2H5C=O + H -> C2H5CHO}$$

The $\ce{C3H7}$ may combine with any of the other radicals or may disproportionate. In any event there was no evidence for compounds larger than 3-propanone. However on close examination of the GC curves there were indications of a very small peak before that due to $\ce{CO2}$ which could be attributed to low molecular weight alkanes and alkenes.

(1) Barnes, P. A.; Stephenson, G.; Warrington, S. B. Journal of Thermal Analysis 1982, 25 (2), 299–311. DOI: 10.1007/BF01912955

(2) Hites, R. A.; Biemann, K. J. Am. Chem. Soc. 1972, 94 (16), 5772–5777. DOI: 10.1021/ja00771a039

• The low molecular weight alkanes and alkenes correspond to disproportionation products, right? – Eashaan Godbole Jul 15 '17 at 16:09
• @EashaanGodbole Could be, authors hasn't investigated that phenomena due to it's low impact on overall picture. – andselisk Jul 15 '17 at 16:13

Abbrev.

1. MEK - Methyl ethyl ketone(butanone)
2. CP - Calcium propanoate or propionate
3. CC - Calcium carbonate

It can be answered logically. The formula for calcium propanoate is $\ce{Ca(C2H5COO)2}$ or specifically $\ce{Ca(CH3CH2COO)2}$. See, for every time, there is a common decomposition product i.e calcium carbonate ($\ce{CaCO3}$). So, remove $\ce{CaCO3}$ from $\ce{Ca(CH3CH2COO)2}$ and you get is $\ce{C2H5COCH3}$ known as butanone(MEK). In each case, there is an increase in methyl group. So, according to it, the decomposition products would be calcium carbonate and MEK.

For further reading about the reaction mechanism and condition, give a quick read at the following papers:

The thermal decomposition of CP began when the carboxylic radical was evaporated from 565 K to 759 K for which corresponding mass loss percentage was 47.79%. The residual was subsequently decomposed to release carbon dioxide$\ce{^{*}}$ between 843 K and 1012 K. The latter phase of the process occurred more readily than with CC because of the loose structure of CP resulting from evaporation of the carboxylic radical in the low temperature zone, which could be seen directly by scanning electron microscope. The maximum mass loss rates of this phase occurred at temperatures of 972 K and 1012 K for CP and CC, respectively. The Ozawa-Flynn-Wall method was used to calculate the activation energy during the thermal decomposition process at heating rates of 5, 7.5, 10 and 15 K/min. The result further confirmed the multistage characteristic of CP thermal decomposition, which could be seen in differential thermogravimetry curves. The reaction orders of CP in the conversion range 20%–80%, calculated using the Avrami theory were from 0.061 to 0.608, smaller than those of CC, which were 1.647 to 2.084.

There is a thing to note. Applying further heat (going beyond the temperature in which CP completely decompose to MEK and CC), calcium carbonate decompose to calcium oxide($\ce{CaO}$) and MEK decomposes to emit irritating and harmful gases as noted by pubchem, $\ce{^{*}}$most probably carbon dioxide and carbon monoxide .

When heated to decomposition it emits acrid smoke and irritating fumes.

Lewis, R.J. Sax's Dangerous Properties of Industrial Materials. 9th ed. Volumes 1-3. New York, NY: Van Nostrand Reinhold, 1996., p. 637