[chemical substances]
   compounds
     Class hydrocarbons [CxHy] - compounds consisting of carbon and hydrogen only
       2.1.01.1 Class alkanes [CnH2n+2] - saturated acyclic hydrocarbons
       2.1.03.1 Class olefins
       2.1.03.2 Class acetylenes
       2.1.11.1 Class cycloalkanes - saturated monocyclic hydrocarbons (with or without side chains)
       2.1.12.4 Class arenes - monocyclic and polycyclic aromatic hydrocarbons
       2.1.21.1 Class cumulenes [R2C[=C]nR2 (n≥3)]
       2.1.21.2 Class acenes - polycyclic aromatic hydrocarbons consisting of fused benzene rings in a rectilinear arrangement
       2.2.03.1.2 Class annulenes [CnHn (n=even) or CnHn+1 (n=odd)]