Since I posted this, I’ve been getting a lot of questions about how Geosmin is formed so let’s talk about this.
Petrichor (the smell of earth after rain) doesn’t come from rain itself, it comes from the soil below it. The molecule that produces it is called Geosmin, which also makes soil smell the way it does. It is synthesized by microbes in the ground and when the rain disturbs the soil, this molecule gets released into the air. But, how is Geosmin produced? It all begins with a simple condensation reaction with ethyl vinyl ketone, and 2-methyl-cyclohexanone in presence of sodium ethoxide (NaOEt) creating the second ring. But now there is an extra ketone on that second ring, so a second step of a special reduction method is required. This involves the addition of (CH2SH)2 with BF3 to create a thioketal derivative, and then desulfurization with Raney nickel.