Green Chemistry: Oranges to epoxide to polymer to plastic?
Lately we have been reading the green chemistry headlines about turning orange into plastic. But what does this really mean? What is the chemistry behind it? How does my orange peel become plastic? Read more for answers:
Step 1: Why oranges? Orange rinds have a ringed compound called limonene that gives citrus fruits their smell.
Above is the structure of limonene, courtesy of toxipedia.
Step 2: The limonene can be turned into an epoxide with the addition of an oxygen atom.
Above is the structure of limonene oxide, with the epoxide structure shown in red. Image courtesy of Santa Cruz Biotechnology.
The compound from step 2 is not very unstable since the epoxide structure (as seen in red) has angular strain and potential strain from sterics (having many components of a structure taking up the same space and being too close to one another). The epoxide creates bond angles that are not ideal as they are too small. The small bond angles are unstable and reactive – doing whatever they can to remove that strain.
Step 3: This is where carbon dioxide comes in. With the help of a zinc complex as a catalyst, the CO2 reacts with the epoxide structure to relieve the strain, allowing the epoxide to react with other epoxides to produce a polymer structure. This polymer structure is called polylimonene carbonate.
Step 4: Now we have a polymer (a compound with repeating units) which can be used to make plastic products through research and design. Currently there are no products on the market using this technology, but we hope to see that change in the near future!
Reaction is described as seen in JACS.