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Advanced Organic Chemistry: 13C NMR spectrum of 2-methylpropanoic acid

Interpreting the C-13 NMR spectrum of 2-methylpropanoic acid (isobutyric acid)

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-methylpropanoic acid

13C nmr spectrum of 2-methylpropanoic acid C4H8O2 (CH3)2CHCOOH analysis of chemical shifts ppm interpretation of C-13 chemical shifts ppm of isobutyric acid C13 13-C nmr doc brown's advanced organic chemistry revision notes 

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-methylpropanoic acid here.

2-methylpropanoic acid (isobutyric acid), C4H8O2  (c) doc b   (c) doc b   (c) doc b

Interpreting the C-13 NMR spectrum of 2-methylpropanoic acid

As you can see from the diagram above there are 3 different chemical shift lines in the C-13 NMR spectrum of 2-methylpropanoic acid indicating 3 different chemical environments of the 4 carbon atoms of 2-methylpropanoic acid.

CH3CH(CH3)COOH  or  (CH3)2CHCOOH

(Note the 3 different colours indicating the 3 different chemical environments of the 4 carbon atoms in 2-methylpropanoic acid).

13C chemical shifts (a) to (c) on the C-13 NMR spectrum diagram for 2-methylpropanoic acid.

The two methyl group carbon atoms are equivalent to each other, due to the symmetry of the 2-methylpropanoic acid molecule, and therefore have the same 13C NMR chemical shift.

Note the decreasing effect on the 13C chemical shift as the carbon atom is further from the more electronegative oxygen atoms in 2-methylpropanoic acid.

The carbon-13 NMR spectra provides direct evidence of 3 different carbon atom environments for the 4 carbon atoms in the 2-methylpropanoic acid molecule, deduced from the presence of 3 different 13C NMR chemical shifts (ppm).


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