To interpret a 13C NMR spectrum, first count the peaks (ignoring TMS at 0 ppm) to find the number of carbon environments, then use each chemical shift to identify the type of carbon, using the ranges on the OCR Data Sheet.Approximate ranges (always check the Data Sheet in the exam):• C−C (alkyl carbons): about 5 to 55 ppm
• carbon bonded to Cl or Br, or to N: about 30 to 70 ppm
• C−O (alcohols, ethers, esters): about 50 to 90 ppm
• C=C (alkenes) and aromatic carbons: about 100 to 160 ppm
• C=O in carboxylic acids and esters: about 160 to 185 ppm
• C=O in aldehydes and ketones: about 190 to 220 ppmThen combine the evidence with the molecular formula and any chemical tests or other spectra.Worked example: compound W, C4H8O, gives four peaks at 8, 29, 37 and 209 ppm.
• 4 peaks: 4 different carbon environments, so there is no symmetry.
• 209 ppm: a C=O in an aldehyde or ketone.
• 8, 29 and 37 ppm: three alkyl carbons.
Butanal, CH3CH2CH2CHO, would also give 4 peaks, but its C=O carbon is at about 200 ppm, so the shifts alone cannot always separate them. A Tollens' test would decide: W gives no silver mirror, so W is butanone, CH3COCH2CH3.Check your answer by predicting the spectrum of your proposed structure and comparing it with the data.
• carbon bonded to Cl or Br, or to N: about 30 to 70 ppm
• C−O (alcohols, ethers, esters): about 50 to 90 ppm
• C=C (alkenes) and aromatic carbons: about 100 to 160 ppm
• C=O in carboxylic acids and esters: about 160 to 185 ppm
• C=O in aldehydes and ketones: about 190 to 220 ppmThen combine the evidence with the molecular formula and any chemical tests or other spectra.Worked example: compound W, C4H8O, gives four peaks at 8, 29, 37 and 209 ppm.
• 4 peaks: 4 different carbon environments, so there is no symmetry.
• 209 ppm: a C=O in an aldehyde or ketone.
• 8, 29 and 37 ppm: three alkyl carbons.
Butanal, CH3CH2CH2CHO, would also give 4 peaks, but its C=O carbon is at about 200 ppm, so the shifts alone cannot always separate them. A Tollens' test would decide: W gives no silver mirror, so W is butanone, CH3COCH2CH3.Check your answer by predicting the spectrum of your proposed structure and comparing it with the data.