How to be a Successful Organic Chemist

30 peaks expected for the functional group. For example, if we use the presence of an O-H peak to conclude “alcohol”, we expect to find (and should look for) the presence of a C-O peak to confirm the conclusion. Likewise, the presence of a C-H sp 2 peak, must mean that the spectrum contains a C=C peak. Whenever possible, base our conclusion on all of the relevant data in the spectrum. Do not “cherry pick” your results. In general, it is a good idea to obtain an IR -spectrum of the starting mate- rial as well as an IR- spectrum of the product. This allows for a direct com- parison of the two spectra and makes it easier to establish that the starting material is no longer present, and to find new peaks that might correspond to the product. The spectra shown below come from an experiment in which a student tried to oxidize a secondary alcohol to a ketone. The top spectrum belongs to the starting material (alcohol), while the middle spectrum came from an authentic sample of the product. The bottom spectrum was obtained from a sample of the student’s product. The student’s spectrum (bottom) contains a C=O peak at 1747 cm -1 , but a comparison of this spectrum with the starting material (top) reveals an alternative interpretation. The starting material’s spectrum has a clear and dis- tinct OH-peak (and the corresponding Figure 5.2. IR spectrum of stock sample of product ketone. Figure 5.1. IR spectrum of alcohol starting material. Table 1. Simplified IR spectroscopy correlation table Bond Absorption [cm -1 ] Frequency Comments O-H 3600-3400 The peak is usually broader than most, and its intensity and frequency vary with concentration. If present, also look for C-O peak. C-H (sp 2 ) 3080-3010 Alkene or benzene. If present, also look for C=C peak. C-H (sp 3 ) 2960-2820 If the sample is a DCM-solution, you will find a very strong solvent peak here that obscures any C-H (sp 3 ) peaks that might be generated by the target compound. C-H (aldehyde) 2720 and/or 2820 Very characteristic. If present, also look for C=O peak. C=O (carbonyl) 1750-1680 Very characteristic.A large number of functional groups contain a carbonyl group,so, if present,also look for other peaks that might identify the functional group C=C 1680-1600 Alkene or benzene. C-O (alcohol, ester, ether) 1300-1100 Complementary bond in alcohol, esters and ethers (but not only those bonds). If an O-H peak is present, this might indicate an alcohol or carboxylic acid group, while the presence of a carbonyl peak and absence of O-H peak can indicate an ester group.The absence of both O-H and carbonyl peaks would indicate an ether group.