We report integral, differential, and momentum transfer cross sections for elastic scattering of low-energy electrons by cyclobutane (c-C4H8), which is an isomer of the C4H8 molecule and has a closed chain. Our calculations were performed with the Schwinger multichannel method with pseudopotentials at the static-exchange level of approximation, for energies up to 50 eV. We compare the cross sections of cyclobutane with the cross sections of other three isomers of C4H8, namely, isobutene, cis-2-butene, and skew-1-butene. These isomers have open chain and were the subject of a previous study by our group [Lopes et al. J. Phys. B 37, 997 (2004)]. We also show previous calculated integral and momentum transfer cross sections for isomers of C3H4, C3H6 and C4H6, which have open and closed chains. For each isomeric group we discuss the isomer effect focusing on the closed chain isomer. The isomer effect is related to differences in the isomers cross sections due to differences in their geometries.