Predicting Carbon-13 NMR Spectra (CIE A Level Chemistry)

Predicting ¹³C molecular environments

• The key to carbon-13 NMR spectroscopy is identifying different carbon environments 
  • It can help to look for symmetry in the molecules
• For example, propanol
  • There are 2 isomers of propanol: propan-1-ol and propan-2-ol
  • Since both isomers contain 3 carbons, the maximum possible number of ¹³C NMR peaks is 3
  • Propan-1-ol
    • There is no symmetry or equivalent carbons in a molecule of propan-1-ol
    • Therefore, there will be 3 peaks in the ¹³C NMR spectrum of propan-1-ol
  • Propan-2-ol
    • There is a line of symmetry through the second carbon with the hydroxyl / OH group attached
    • This means that the CH₃ groups on either side are equivalent 
    • Therefore, there will be 2 peaks in the ¹³C NMR spectrum of propan-2-ol

Identifying molecular environments in propanol

Propan-1-ol has no symmetry / equivalent carbons in its structure, while propan-2-ol has symmetry and, therefore, 2 equivalent carbons

• For example, pentane-1,4-diol
  • Since pentane-1,4-diol contains 5 carbons, the maximum possible number of ¹³C NMR peaks is 5
  • There are 2 carbons with hydroxyl / OH groups attached but these are not equivalent 
    • Carbon-1 is CH₂OH  with a neighbouring CH₂ 
    • Carbon-4 is CHOH with a neighbouring CH₂ and a neighbouring CH₃ 
    • This means that there is no symmetry within the molecule and, therefore, no equivalent carbons
    • So, there will be 5 peaks in the ¹³C NMR spectrum of pentane-1,4-diol

Identifying molecular environments in pentane-1,4-diol

Pentane-1,4-diol has no symmetry / equivalent carbons in its structure

• The same principle of carbon environments also applies to aromatic compounds
• For example, benzene
  • Since benzene contains 6 carbons, the maximum possible number of ¹³C NMR peaks is 6
  • The benzene molecule has several lines of symmetry leading to the fact that all of the carbons are equivalent 
  • So, there will be 1 peak in the ¹³C NMR spectrum of benzene

Identifying molecular environments in benzene

Benzene has symmetry / equivalent carbons in its structure