Are ethyl butanoate (1) and propyl propanoate (2) metamers or functional group isomers? My book claims they are metamers, but here one group is $\ce{-CO-OR}$ and another is $\ce{-CO-OR'}$ where $\ce{R}$ is $2$ carbon and $\ce{R'}$ is $3$ carbon. Though both of them are esters, aren't the functional groups different?
Metamerism occurs when isomers have different alkyl chains on the sides of the functional groups. In the case of an ester, the relevant functional group is the ester (..O-C=O..) group. The length of R and R' has no bearing on the functional group in this case. An example of a functional isomer would be something like ethyl butanoate and caproic acid.
