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

Predicting Carbon-13 NMR Spectra

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