In chemistry|lang=en terms the difference between isomeric and propiolactone

is that isomeric is (chemistry) exhibiting isomerism while propiolactone is (chemistry) either of two isomeric chemical compounds of the lactone family, α-propiolactone and and β-propiolactone.

As an adjective isomeric

is (chemistry|physics) being an isomer.

As a noun propiolactone is

(chemistry) either of two isomeric chemical compounds of the lactone family, α-propiolactone and and β-propiolactone.