butanone low high resolution H-1 proton nmr spectrum of butanone analysis interpretation of chemical shifts ppm spin spin line splitting for 2-butanone butan-2-one doc brown's advanced organic chemistry revision notes

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Advanced Organic Chemistry: H-1 NMR spectrum of butanone

The H-1 hydrogen-1 (proton) NMR spectrum of butanone

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses involving molecular spectroscopy analysing H-1 NMR spectra of butanone (2-butanone, butan-2-one)

low and high resolution H-1 proton nmr spectrum of butanone analysis interpretation of chemical shifts ppm spin spin line splitting diagram for 2-butanone butan-2-one doc brown's advanced organic chemistry revision notes

TMS is the acronym for tetramethylsilane, formula Si(CH₃)₄, whose protons are arbitrarily given a chemical shift of 0.0 ppm. This is the 'standard' in ¹H NMR spectroscopy and all other proton shifts, called chemical shifts, depend on the individual (electronic) chemical environment of the hydrogen atoms in an organic molecule - butanone here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of butanone represent the peaks of the intensity of the chemical shifts of (which are often groups of split lines at high resolution) AND the relative integrated areas under the peaks gives you the ratio of protons in the different chemical environments of the butanone molecule.

Butanone C₄H₈O, , , ,

Interpreting the H-1 NMR spectrum of butanone

For relatively simple molecules, the low resolution H-1 NMR spectrum of butanone is a good starting point.

The hydrogen atoms (protons) of butanone occupy 3 different chemical environments so that the H-1 proton low resolution NMR spectra should show 3 peaks (diagram above for butanone).

CH₃COCH₂CH₃

(Note the ratio 3:2:3 of the three colours of the protons in the 3 chemically different environments)

Although there are 8 hydrogen atoms in the molecule, there only 3 possible chemical environment for the hydrogen atoms.

The proton ratio observed 3:2:3, corresponds with the structural formula of butanone.

The high resolution spectrum of butanone

So, using the chemical shifts and applying the n+1 rule to CH₃COCH₂CH₃:

The chemical shift at 2.16 ppm is for the 'left' methyl group and no splitting is evident because there s no adjacent proton on a the carbonyl (C=O) carbon atom.

The CH₂ protons are split into a 1:3:3:1 quartet by the 'right-hand' CH₃ group at 2.45 ppm - evidence of a CH₃ group in the molecule.

The 'right-hand' CH₃ protons are split into a 1:2:1 triplet by the CH₂ group at 1.06 ppm - evidence of a CH₂ group in the molecule.

Note that the 'real spectra' intensity ratios e.g. 1:2:1 and 1:3:3:1 are not perfect, but do look something like the theory predicts for butanone!

Number of protons ¹H causing splitting	Splitting pattern produced from the n+1 rule and the theoretical ratio of line intensities
0 means no splitting							1
1 creates a doublet						1		1
2 creates a triplet					1		2		1
3 creates a quartet				1		3		3		1
4 creates a quintet			1		4		6		4		1
5 creates a sextet		1		5		10		10		5		1
6 creates a septet	1		6		15		20		15		6		1

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