butanal low high resolution H-1 proton nmr spectrum of butyraldehyde analysis interpretation of chemical shifts ppm spin spin line splitting H1 1-H nmr doc brown's advanced organic chemistry revision notes

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Advanced Organic Chemistry: H-1 NMR spectrum of butanal

The H-1 hydrogen-1 (proton) NMR spectrum of butanal (butyraldehyde)

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 H-1 NMR spectra of butanal

H-1 nmr spectrum of butanal analysis of chemical shifts ppm interpretation of proton chemical shifts ppm of butyraldehyde 1-H proton nmr 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 - butanal here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of butanal 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 butanal molecule.

butanal (butyraldehyde), C₄H₈O, , ,

Interpreting the H-1 NMR spectrum of butanal

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

The hydrogen atoms (protons) of butanal occupy 4 different chemical environments so that the low resolution NMR spectra should show 4 peaks of different H-1 NMR chemical shifts (diagram above for butanal).

CH₃CH₂CH₂CHO

Note the ratio 3:2:2:1 of the four colours of the protons in the four chemically different environments

Although there are 8 hydrogen atoms in the molecule, there only three possible chemical environments for the hydrogen atoms in butanal molecule.

The proton ratio 3:2:2:1 observed, corresponds with the structural formula of butanal.

The very high resolution spectrum of butanal

CH₃CH₂CH₂CHO

low and high resolution H-1 proton nmr spectrum of butanal analysis interpretation of chemical shifts ppm spin spin line splitting diagram butyraldehyde H1 1-H nmr doc brown's advanced organic chemistry revision notes

All low and high resolution spectra of butanal show four groups of protons and in the ratio expected from the formula of butanal. The ppm quoted represent the peak of resonance intensity for a particular proton group in the molecule of butanal.

The 0.97 ppm chemical shift

The adjacent CH₂ protons split the CH₃ proton resonance into a 1:2:1 triplet.

The 9.76 ppm chemical shift

The adjacent CH₂ protons split the CH proton (of the aldehyde group) resonance into a 1:1 triplet.

CH₃CH₂CH₂CHO

The 1.64 ppm chemical shift:

From the n+1 rule, the 'left-hand' CH₂ protons (H₂) are split by CH₃ protons (H₃) and by the middle CH₂ protons (H₂), (5 protons in total), into a 1:5:10:10:5:1 sextet of resonance lines.

This is pattern of resonances is a good indication of a propyl group (CH₃CH₂CH₂) and is clearly seen on the very high resolution spectrum of butanal.

The 2.37 ppm chemical shift:

The middle right-hand CH₂ protons (H₂) are split on the left sides by CH₂ protons (H₂) and on the right by the CH proton (H), (3 protons in total), so you would a 1:3:3:1 quartet of resonance lines.

However, the high resolution spectrum gives a 1:2:1 triplet and 1:1:1:1 quartet, which I'm afraid I cannot explain at the moment.

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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