Bisphenol A

Bisphenol A, or BPA, is two phenol rings joined at a single carbon atom that also carries two methyl groups. The A stands for acetone, which supplies that central carbon. It's a white crystalline solid, first made in 1891.

It's made by mixing phenol with acetone, using plenty of extra phenol, over an acid catalyst, often a solid acid resin. The acetone's central carbon, the one bonded to oxygen, joins to the ring of one phenol and then another, and the oxygen leaves as water. Two phenols and one acetone make one BPA. The product is crystallized out of the leftover phenol, which is recycled.

Its shape matters. Two rings joined at one point make a stiff, bulky core, and each ring ends in an -OH group that can react further. In epoxy resin, those ends are capped with epoxide groups from epichlorohydrin, and the rigid core gives cured epoxy its strength and heat resistance. Much of the rest goes into polycarbonate, the clear, tough plastic in safety glasses.

BPA can mimic the hormone estrogen, so its use in baby bottles and food packaging has been restricted in many countries.