In the chemical formula
<chem>\left( \frac{a}{2} \frac{b}{3} \right) O</chem>
I get an error TeX parse error: Missing or unrecognized delimiter for \left
A live example is at https://en.wikipedia.org/w/index.php?title=User:Salix_alba/sandbox&oldid=809956844
The error stems from trying to convert the first formula in https://en.wikipedia.org/wiki/Chemical_oxygen_demand to use the chem environment.