Question

# When malonic acid is dehydrated using ${P_4}{O_{10}}$ at ${150^o}C$, species formed has total number of atoms ________

Hint: An acid anhydride of malonic acid is formed on dehydration of malonic acid with ${{\text{P}}_{\text{4}}}{{\text{O}}_{{\text{10}}}}$, which is used as a source of carbon. It is also a stable member of linear oxocarbons $O{\text{ }} = {\text{ }}{C_n} = {\text{ }}O$.
$2C{H_2}{(COOH)_2}{\text{ }} + {\text{ }}{P_4}{O_{10}}{\text{ }} \to {\text{ }}2{H_4}{P_2}{O_7}{\text{ }} + {\text{ }}2{C_3}{O_2}$
$O{\text{ }} = {\text{ }}C{\text{ }} = {\text{ }}C{\text{ }} = {\text{ }}C = {\text{ }}O$
It is an acid anhydride of malonic acid i.e. produces acid when reacted slowly with water. It decomposes by the process of photolysis to form ketene, ${C_2}O$, which is a very reactive molecule.
Other than malonic acid, carbon suboxide is also formed during heme oxidation by heme oxygenase-1 enzyme. It can undergo polymerization into macrocyclic polycaron structures with ${({C_3}{O_2})_n}$ the common formula.