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

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Advanced Organic Chemistry: H-1 NMR spectrum of ethanal

The H-1 hydrogen-1 (proton) NMR spectrum of ethanal (acetaldehyde)

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 ethanal

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, , , ,
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 H-1 NMR chemical shifts (diagram above for ethanal).

CH₃CHO

Note the ratio 2 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.

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.

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 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 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

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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