So this is carbon number one, carbon number two, and carbon number three. And the ultimate goal is to take this entire group that is shown in blue and to move it from carbon number two onto this carbon that is part of this methyl group. Now, how exactly is this reaction actually catalyzed? Well, that's where vitamin B twelve actually comes into play. So part of that vitamin B twelve molecule is shown in this diagram and it contains a radical. So we have a single electron on this carbon.