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In a reaction network built from nearly 13 million recorded reactions originating from limonene, molecules were separated by an average of about six or fewer synthesis steps. That is a network-distance observation—not proof that every compound can be made from every other one in six practical, verified steps.
What “six degrees” means in chemistry
The phrase is an analogy to the idea that people in a social network may be connected through short chains of acquaintances. In the chemical analysis, the links are recorded reactions: molecules are represented as vertices in a graph, and reactions connect them. The reported pattern concerns the average number of reaction steps separating molecules in that particular graph.
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A short average does not mean every pair of molecules is connected, or that any route between a chosen pair is known. It describes distances across the analyzed network, not a universal rule about chemistry.
What the researchers analyzed
Philipp-Maximilian Jacob and Alexei Lapkin examined nearly 13 million reactions in Reaxys that originated from limonene, a bio-based feedstock. The work was motivated by a practical question from industry: “which molecules can you give us?” The challenge is to connect sustainably sourced starting materials with potentially higher-value products.
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The researchers converted reaction records into a network using Python and assessed graph properties including average reaction connectivity, clustering among highly connected molecules, and the average number of reactions separating pairs of molecules. The 2018 report describes the resulting network as showing the six-degrees pattern. It does not provide a further numerical network statistic suitable for quoting here.
How network analysis could help chemists
Finding routes worth investigating
Mapping reactions as a network may help researchers navigate a large body of recorded chemistry and notice shorter pathways that might otherwise be overlooked. A graph can suggest connections for chemists to examine; it does not itself establish that a proposed route works in the laboratory.
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Thinking about how reaction records grow
The report also describes network metrics as potentially useful for estimating how the total number of recorded reactions might change as unexpected reactions are discovered. This is a possible application of the analysis, not a demonstrated forecast for all chemical databases.
Other reaction networks
Reaction-pathway researcher Judit Zador suggested that these ideas may be relevant to networks in combustion or atmospheric chemistry. Those are prospective applications she identified, not results tested by the limonene study.
Why six steps is not a synthesis guarantee
A database path is only as informative as the records and graph representation behind it. The reported network came from one database and a subset of reactions originating from limonene. A path in that graph does not, by itself, show that a route is selective, safe, economical, scalable, available under a chemist’s conditions, or experimentally validated.
So the finding is best read as a way to describe connectivity in recorded reaction data. It is not a claim that any chosen product can be reached from any chosen starting material in six feasible steps.
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The study and its context
The 2018 report identifies the underlying paper as P.-M. Jacob and A. Lapkin, Reaction Chemistry & Engineering, volume 3, page 102, DOI 10.1039/c7re00129k. The network-analysis result is a snapshot of how molecules connect in the limonene-origin records analyzed, rather than an exhaustive map of all chemical transformations.
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