The standard IUPAC system for chemical nomenclature assigns a name to essentially any possible compound. But even among hydrocarbons with fairly few atoms not all have ever been considered interesting enough to list in standard chemical databases. Thus for example the following compares the total number of conceivable alkanes (paraffins) to the number actually listed in the 2001 standard Beilstein database:
Any tree with up to 4 connections at each node can in principle correspond to an alkane with chemical formula CnH2n + 2. The total number of such trees—studied since 1875—increases roughly like 2.79n n-5/2. If every node has say 4 connections, then eventually one gets dendrimers that cannot realistically be constructed in 3D. But long before this happens one runs into many alkanes that presumably exist, but apparently have never explicitly been studied. The small unbranched ones (methane, ethane, propane, butane, pentane, etc.) are all well known, but ones with more complicated branching are decreasingly known. In coal and petroleum a continuous range of alkanes occur. Branched octanes are used to reduce knocking in car engines. Biological systems contain many specific alkanes—often quite large—that happen to be produced through chemical pathways in biological cells. (The n = 11 and n = 13 unbranched alkanes are for example known to serve as ant pheromones.)
In general the main way large molecules have traditionally ended up being considered chemically interesting is if they occur in biological systems—or mimic ones that do. Since the 1980s, however, molecules such as the fullerenes that instead have specific regular geometrical shapes have also begun to be considered interesting.