A substance that is an isomer of acetic acid.

First of all, of course, you need to know what isomers are. And these are substances that have the same molecular formula and atomic composition, but differ in the arrangement of atoms in the molecule of a given substance. So the chemical formula doesn’t change. And this is what it looks like.

HCO2CH3.

But a change in spatial arrangement leads to the formation of an isomer and this is methiformate or methylmethanate. And the trivial name for this isomer is formic acid methyl ester.

But acetic acid has no other isomers. But the acetic acid isomer is a different chemical compound and exhibits different chemical properties.