Taking Away: Elimination

Abstract

Abstract The hot spots of molecules that I have identified as double bonds (two shared pairs of electrons lying between the same two carbon atoms) and their triple bond cousins are often desirable entities. They are desirable either in their own right or because they can be used in the course of the construction of an elaborate molecule. For instance, a double bond can make the molecule stiffer and resistant to twisting. In Reaction 28 you will see that one particular natural product, quinine, must have a double bond in a particular position for it to be able to function—Nature is very particular about the shape of a molecule that she uses—and the drug’s synthesizers had to find a way to introduce it. How, though, can a double bond be introduced into a molecule that begins life with only single bonds? One approach is ‘elimination’, the expulsion of groups of atoms on neighbouring carbon atoms, leaving those two atoms free to form a second or even third bond to each other. One approach is to pull an H atom (as a proton) or some other group of atoms off one C atom, and then hope that the ensuing convulsions of the electron cloud will result in its accumulation to form a double bond between that C atom and its neighbour. There are two common approaches, one involving an acid and the other a base. Let’s watch what happens when sulfuric acid, 1, is added to 2. The acid, a proton donor, generates H3O+ ions in the usual way by transferring a proton to a neighbouring water molecule and leaving behind an HSO4– ion, and we see one of these ions sidle up to the target molecule. A proton hops across onto the O atom from the H3O+ ion, so forming a positively charged –OH2+ group. There is an immediate convulsion of the electron cloud, and that group escapes as an H2O molecule, leaving behind a positively charged ion with the positive charge mostly localised on the C atom. This ion is unstable but survives briefly.