Interpreting the ¹³C NMR spectrum of 2-methylpropan-2-ol (tert-butyl alcohol)

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TMS is the acronym for tetramethylsilane, formula Si(CH₃)₄, whose ¹³C atoms are arbitrarily given a chemical shift of 0.0 ppm. This is the 'standard' in ¹³C NMR spectroscopy and all other ¹³C resonances, called chemical shifts, are measured with respect to the TMS, and depend on the individual (electronic) chemical environment of the ¹³C atoms in an organic molecule - 2-methylpropan-2-ol here.

The molecular structure and naming of aliphatic alcohols and ethers

Interpreting the C-13 NMR spectrum of 2-methylpropan-2-ol

As you can see from the diagram above there are 2 different chemical shift lines in the C-13 NMR spectrum of 2-methylpropan-2-ol indicating 2 different chemical environments of the 4 carbon atoms of 2-methylpropan-2-ol.

(CH₃)₃COH

(Note the 2 different colours indicating the 2 different chemical environments of the 4 carbon atoms in 2-methylpropan-2-ol).

¹³C chemical shifts (a) to (b) on the C-13 NMR spectrum diagram for 2-methylpropan-2-ol.

Note, because of the symmetry of the molecule, the three carbon atoms of the methyl groups are all chemically equivalent to each other, they give the same C-13 chemical shift (31.2 ppm) because all three carbon atoms share the same chemical environment.

The 4th carbon atom of the C-O-H grouping of the alcohol functional group has a different C-13 chemical shift (69.1 ppm) because of its different chemical environment.

Note that the electronegative oxygen atom produces a much greater C-13 NMR shift on the C or the C-OH grouping, typical of molecules like alcohols.

The carbon-13 NMR spectra provides direct evidence of 2 different carbon atom environments for the 2 carbon atoms in the 2-methylpropan-2-ol molecule, deduced from the presence of 2 different 13C NMR chemical shifts (ppm).

The ¹³C NMR spectrum of 2-methylpropan-2-ol shows two distinct signals: one for the three equivalent methyl carbons (~31 ppm) and one for the quaternary carbon bonded to OH (~69 ppm).

Key ¹³C NMR Features of 2-Methylpropan-2-ol

2-Methylpropan-2-ol (tert-butanol) has the formula (CH₃)₃COH and contains four carbon atoms in two unique environments. Its ¹³C NMR spectrum reflects this symmetry:

Sources: Vaia Chemistry Solutions

Common Misconceptions in Exams

• Expecting four peaks for four carbons: Only two signals appear due to symmetry—three methyl carbons are equivalent.
• Misassigning the quaternary carbon: Students may incorrectly assign the downfield peak (~69 ppm) to a carbonyl or aromatic carbon.
• Assuming splitting patterns: ¹³C NMR spectra are typically proton-decoupled—no splitting is observed.

Exam Revision Tips

• Count unique carbon environments: Use symmetry to predict the number of peaks—this is often tested.
• Use chemical shift ranges: Learn typical δ values for methyl (~10–40 ppm), quaternary (~40–80 ppm), and carbonyl (>160 ppm) carbons.
• Compare with similar alcohols: Primary and secondary alcohols show more distinct carbon environments and peaks.
• Annotate spectra: Label each peak with its carbon type and environment to reinforce understanding.

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