Benzene Is Less Reactive Than Ethene:
Benzene is a very stable molecule. This stability is due to the delocalization of π electrons whereas no such delocalization of π electrons is possible in ethene. In ethene, the π electronic cloud is present above and below the plane of atoms and is more exposed to electrophile attack.
Therefore, benzene undergoes electrophilic substitution reactions and is less reactive towards electrophilic addition reactions.
Benzene Is More Reactive Than Ethane:
Although, benzene is less reactive than ethene but more reactive than ethane. This is because benzene contains π electrons and gives electrophilic substitution reactions. Whereas ethane contains no π electrons and all the bonds in ethane are sigmas ( σ) covalent bonds. This makes ethane less reactive than benzene.
Generally, ethane undergoes free radical substitution reactions rather than electrophilic substitution/addition reactions.