Natura - nature actualia - actual entities Mundus Plinius - physical world naturalia - natural bodies "crystal-kingdom" chemical substances chemical compounds Class hydrocarbons [CxHy] - compounds consisting of carbon and hydrogen only 1.1.1 Class alkanes [CnH2n+2] - saturated acyclic hydrocarbons 1.1.2 Class cycloalkanes - saturated monocyclic hydrocarbons (with or without side chains) 1.2 Class olefins 1.3 Class acetylenes 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 2.2.3.1.2 Class annulenes [CnHn (n=even) or CnHn+1 (n=odd)]