The Carbon-13 NMR spectrum of 2-bromopropane

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses involving molecular spectroscopy analysing C-13 NMR spectra of 2-bromopropane

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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-bromopropane here.

Interpreting the C-13 NMR spectrum of 2-bromopropane

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

CH₃CHBrCH₃

(Note the 2 different colours indicating the 2 different chemical environments of the 3 carbon atoms in 2-bromopropane).

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

The end carbon atoms are equivalent to each other.

Note the decreasing effect on the chemical shift as the carbon atom is further from the more electronegative bromine atom of 2-bromopropane.

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