Interpreting the ¹³C NMR spectrum of 2-chloro-2-methylpropane

 The chemistry of organic halogen compounds

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See also Comparing infrared, mass, 1H NMR & 13C NMR spectra of the 4 structural isomers of C₄H₉Cl

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-chloro-2-methylpropane here.

Interpreting the C-13 NMR spectrum of 2-chloro-2-methylpropane

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

(CH₃)₃CCl

(Note the 2 different colours indicating the 2 different chemical environments of the carbon atoms in 2-chloro-2-methylpropane).

¹³C chemical shifts (a) and (b) on the C-13 NMR spectrum diagram for 2-chloro-2-methylpropane.

The three methyl group carbon atoms are all chemically equivalent in the 2-chloro-2-methylpropane molecule.

The carbon atom of the C-Cl bond does have a unique ¹³C NMR chemical shift.

The carbon-13 NMR spectra provides direct evidence of 2 different carbon atom environments for the 4 carbon atoms in the 2-chloro-2-methylpropane molecule, deduced from the presence of 2 different ¹³C chemical shifts (ppm).

The C-13 NMR spectrum of 2-chloro-2-methylpropane (tert-butyl chloride) is elegantly simple due to its high symmetry and limited carbon environments. Here's a structured breakdown:

Key Carbon Signals in the C-13 NMR spectrum of 2-chloro-2-methylpropane

(CH₃)₃CCl

Note: Exact shifts may vary slightly depending on solvent and instrument, but the pattern remains consistent.

Common Misconceptions about the C-13 NMR spectrum of 2-chloro-2-methylpropane (see also below)

• Expecting more signals: Students may assume four carbon signals due to four atoms, forgetting that symmetry reduces this to two distinct environments.
• Misassigning the quaternary carbon: Some learners confuse the downfield signal (~50–55 ppm) with a carbonyl or aromatic carbon. Clarify it's due to the electron-withdrawing effect of chlorine.
• Overlooking integration: Unlike ¹H NMR, ¹³C NMR signals are not proportional to the number of atoms unless DEPT or quantitative techniques are used.

Exam Tips for questions involving the C-13 NMR spectrum of 2-chloro-2-methylpropane (see also above)

• Count unique environments: Use symmetry to predict the number of signals—tert-butyl chloride gives two.
• Use chemical shift logic: The more deshielded quaternary carbon (C–Cl) appears downfield due to chlorine’s electronegativity.
• Compare with t-butanol: t-butanol also shows two signals, but the quaternary carbon appears slightly upfield (~45 ppm) due to the less electronegative OH group.
• Watch for distractors: Spectra with three or more signals likely indicate less symmetrical isomers—use this to eliminate options.

Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 4 halogenoalkane isomers of C4H9Cl NOTE: The images are linked to their original detailed spectral analysis pages AND can be doubled in size with touch screens to increase the definition to the original 1-chlorobutane, 2-chlorobutane, 1-chloro-2-methylpropane and 2-chloro-2-methylpropane image sizes.  These four molecules are structural isomers of molecular formula C4H9Cl and exemplify the infrared, mass, 1H NMR and 13C NMR spectra of lower aliphatic halogenoalkanes (haloalkanes, alkyl halides, chloroalkanes, alkyl chlorides).
INFRARED SPECTRA (above): Apart from the significant differences in the fingerprint region at wavenumbers 1500 to 400 cm-1, there are no other great striking differences, but each could be identified from its infrared spectrum. The infrared spectrum of 2-chloro-2-methylpropane is noticeably simpler in the fingerprint region, perhaps due to the greater symmetry of the molecule.
MASS SPECTRA (above): Theoretically, all four can give the parent molecular ions of m/z 92 and 94, but they are all relatively tiny peaks. 2-chlorobutane and 2-chloro-2-methylpropane give a base ion peak of m/z 57. The base ion peak for 1-chlorobutane is m/z 56 and that of 1-chloro-2-methylpropane is m/z 43. Each gives different patterns of pairs of m/z values two mass units apart, in the peak height ratio of 3:1, if the positive fragment contains a chlorine atom (35Cl or 37Cl) e.g look for m/z pairs 49/51, 63/65 and 77/79 in their mass spectra.
1H NMR SPECTRA (above): The 1H NMR spectra of all four molecules give different integrated proton ratios i.e.1-chlorobutane four peaks of ratio 3:2:2:2; 2-chlorobutane four peaks of ratio 3:3:2:1, 1-chloro-2-methylpropane three peaks of ratio 6:2:1 and 2-chloro-2-methylpropane gives just one peak '1' (effectively no ratio involved), so all four molecular structures can be distinguished from each other by their 1H NMR spectra proton ratios, numbers of peaks and (n+1) rule splitting patterns.
13C NMR SPECTRA (above): The 13C NMR spectra of the four molecules show various numbers of carbon-13 chemical environments i.e 1-chlorobutane and 2-chlorobutane show four 13C NMR resonances, 1-chloro-2-methylpropane three 13C NMR resonances and 2-chloro-2-methylpropane only two 13C resonances (3 and 2 chemical environments respectively. Therefore 1-chloro-2-methylpropane and 2-chloro-2-methylpropane can be distinguished from the other three by their number of resonances in their 13C NMR spectra, but 1-chlorobutane and 2-chlorobutane cannot be distinguished from each other from their number of 13C NMR resonance lines - other data would be required.

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