We have only begun to explore the intricate world of identifi cation of structure by spectros copy. It is important that you recognize that structures are assigned, not because of some theoretical reason or because a reaction ‘ought’ to give a certain product, but because of sound evidence from spectra. You have seen four powerful methods—mass spectra, 13C and 1H NMR, and IR spectroscopy—in this chapter. In Chapter 13 we delve more deeply into the most important of all (1H NMR) and, finally, in Chapter 18 we shall take each of these methods a little further and show how the structures of more complex unknown compounds are really deduced. The last problem we have discussed here is not really solvable without 1H NMR and in reality, no-one would tackle any structure problem without this most powerful of all tech niques. From now on spectroscopic evidence will appear in virtually every chapter. Even if we do not say so explicitly every time a new compound appears, the structure of this compound will in fact have been determined spectroscopically. Chemists make new compounds, and every time they do they characterize the compound with a full set of spectra. No scientific c journal will accept that a new compound has been made unless a full description of all of these spectra are submitted with the report. Spectroscopy lets the science of organic chemistry advance.