Open-chain compound

[2][3] Many of the simple molecules of organic chemistry, such as the alkanes and alkenes, have both linear and ring isomers, that is, both acyclic and cyclic.

For those with 4 or more carbons, the linear forms can have straight-chain or branched-chain isomers.

Cycloalkanes are isomers of alkenes, not of alkanes, because the ring's closure involves a C-C bond.

For example, in living organisms, the open-chain isomer of glucose usually exists only transiently, in small amounts; D-glucose is the usual isomer; and L-glucose is rare.

For example, the straight-chain alkanes are wavy or "puckered", as the models below show.