A diene $\mathrm{C}{}_5\mathrm{H}{}_8$ on reductive ozonolysis gives 2 mol of methanal and 1 mol of propanedial, $\mathrm{OHC}-\mathrm{CH}{}_2-\mathrm{CHO}$, per mole of diene. The diene is

Two $\mathrm{HCHO}$ come from two terminal $=\mathrm{CH}{}_2$ groups; the three-carbon remainder is $=\mathrm{CH}-\mathrm{CH}{}_2-\mathrm{CH}=$, which gives $\mathrm{OHC}-\mathrm{CH}{}_2-\mathrm{CHO}$. The diene is $\mathrm{CH}{}_2=\mathrm{CH}-\mathrm{CH}{}_2-\mathrm{CH}=\mathrm{CH}{}_2$. Penta-1,3-diene gives methanal, ethanedial and ethanal; 2-methylbuta-1,3-diene gives 2 methanal and $\mathrm{CH}{}_3\mathrm{COCHO}$; cyclopentene gives pentanedial.