- -CH2- group (bound by two single bonds) present e.g. in dichloromethane (alternative name is methylene chloride);
- =CH2 group (bound by one double bond), present e.g. in formaldehyde (can be named methylene oxide, but it is not used/recommended). The better example is methylenecyclopropene.
- :CH2 is a carbene known as methylene, carbenes are highly reactive organic molecules with a divalent carbon atom with only six valence electrons.
Increasing the number of methylene groups in a chain or ring increases the size and the lipid nature (lipophilicity) of the compound.