13C NMR Spectroscopy

¹³C NMR detects the ¹³C isotope, which makes up only about 1.1% of carbon atoms. This is enough to give a spectrum, although it takes longer to record than a ¹H spectrum.Each peak in a ¹³C NMR spectrum corresponds to one carbon environment. Carbon atoms in the same environment (equivalent carbons, related by the symmetry of the molecule) give one peak. So:
• the number of peaks = the number of different carbon environments
• the chemical shift of each peak shows what the carbon is bonded toIn the spectra you meet at A level, each environment appears as a single line, and peak heights are not used to count carbons.Typical ¹³C shifts (use the Data Booklet): C-C alkyl 5 to 55; C-O in alcohols, ethers and esters 50 to 90; C=C in alkenes and arenes 100 to 160; C=O in acids and esters 160 to 185; C=O in aldehydes and ketones 190 to 220.Worked example: distinguishing propan-1-ol and propan-2-ol.
Propan-1-ol, CH₃CH₂CH₂OH, has three different carbon environments, so it gives 3 peaks (the carbon bonded to O at about 64).
Propan-2-ol, CH₃CH(OH)CH₃, is symmetrical: the two CH₃ carbons are equivalent, so it gives only 2 peaks.Similarly, propanone, CH₃COCH₃, gives 2 peaks, one at about 205 for the C=O carbon. Benzene, with six equivalent carbons, gives just one peak.