The Carbon-13 NMR spectrum of bromoethane

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 bromoethane

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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 - bromoethane here.

Interpreting the C-13 NMR spectrum of bromoethane

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

CH₃CH₂Br

(Note the 2 different colours indicating the 2 different chemical environments of the carbon atoms in bromoethane).

Chemical shifts (a) to (b) on the C-13 NMR spectrum diagram for bromoethane.

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

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