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

Advanced Organic Chemistry: H-1 NMR spectrum of ethanal CH₃CHO

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Interpreting the H-1 (proton) NMR spectrum of ethanal (acetaldehyde)

[Author © Dr Phil Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectroscopy analysis of ethanal [spectra page updated Mar 23rd 2026 *]

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H-1 proton NMR spectroscopy - spectra index

Introductory note on the 1H NMR spectra of ethanal

Students and teachers please note my explanation of the proton NMR spectrum of ethanal is designed for advanced, but pre-university, chemistry courses.

The chemical shift δ splitting pattern effects for ethanal are confined to a proton spin-spin coupling effects analysed using the n+1 rule for adjacent non-equivalent proton fields (n is the number of neighbouring protons in a non-equivalent different chemical environment for the ethanal molecule).

It is assumed that the integrated intensities of the ¹H NMR δ chemical shifts give the ratio of the protons in the different non-equivalent chemical environments of the ethanal molecule.

The most common solvent used for investigating the 1H NMR spectrum of compounds like ethanal, is CDCl₃ and other deuterated solvents to avoid confusion with a ¹H NMR signal, ²D (²H) has a different NMR chemical shift.

low and high resolution H-1 proton nmr spectrum of ethanal analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 1-H nmr for acetaldehyde 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 - ethanal here.

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

Ethanal (acetaldehyde), C₂H₄O, , , , an aliphatic aldehyde
The molecular structure and naming of aldehydes and ketones
Interpreting the H-1 NMR spectrum of ethanal

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

The hydrogen atoms (protons) of ethanal occupy 2 different chemical environments so that the low resolution NMR spectra should show 2 peaks of different ¹H NMR chemical shifts (diagram above for ethanal).

CH₃CHO

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

Although there are 4 hydrogen atoms in the molecule, there only 2 possible chemical environments for the hydrogen atoms in ethanal molecule, so you only see two peaks in the low resolution spectrum.

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

The high resolution spectrum of ethanal

All low and high resolution spectra of ethanal show 2 groups of protons and in the ratio of 3 : 1 expected from the formula of ethanal - two different chemical environments.

The ppm quoted on the diagram represent the peak of resonance intensity for a particular proton group in the molecule of ethanal - since the peak' is at the apex of a band of H-1 NMR resonances due to spin - spin filed splitting effects - see high resolution notes on ethanal below.

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

Chemical shift 2.21 ppm for the CH₃ protons

The methyl proton resonance is split into a 1 : 1 doublet by the single proton of the aldehyde CHO group (1 proton, n+1 = 2 = doublet).

Evidence for the presence of a CH group in the molecule of ethanal

Chemical shift 9.79 ppm for the CH proton

The aldehyde group proton resonance is split into a 1 : 3 : 3 : 1 quartet by the methyl group protons (3 protons, n+1 = 4 = quartet).

Evidence for the presence of a CH₃ group in the molecule of ethanal.

The very electronegative oxygen atom tends to increase the chemical shift of protons attached to the carbon atom which is also attached to that electronegative atom.

The splitting pattern from proton spin-spin coupling effects is analysed using the n+1 rule for adjacent non-equivalent proton fields (n is the number of neighbouring protons in a non-equivalent different chemical environment).

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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Links associated with ethanal

H-1 proton NMR spectroscopy index (Please read 9 points at the top of the ¹H NMR index page)

The infrared spectrum of ethanal (acetaldehyde)

The mass spectrum of ethanal (acetaldehyde)

The C-13 NMR spectrum of ethanal (acetaldehyde)

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