propanal low high resolution 1H proton nmr spectrum diagram n+1 rule splitting analysis interpretation of chemical shifts ppm spin spin line splitting H1 propionaldehyde 1-H nmr doc brown's advanced organic chemistry revision notes

Advanced pre-university Organic Chemistry: The 1H NMR spectrum of propanal CH₃CH₂CHO

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Interpreting and explaining the H-1 (proton) NMR spectrum of propanal (propionaldehyde)

[Author © Dr Phil Brown GRIC, PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses, IB chemistry & US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectroscopy analysis of propanal [spectra page updated RE-EDIT]

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H-1 proton NMR spectroscopy - spectra index

See also The isomers of molecular formula C₃H₆O (with some selected spectra data)

Introductory note on the 1H NMR spectra of propanal

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

The chemical shift δ splitting pattern effects for propanal 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 propanal 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 propanal molecule.

The most common solvent used for investigating the 1H NMR spectrum of compounds like propanal, 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 propanal analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 1-H nmr for propionaldehyde 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 - propanal here.

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

propanal (propionaldehyde), C₃H₆O, , , ,

An aldehyde The molecular structure and naming of aldehydes and ketones

Interpreting the H-1 NMR spectrum of propanal (propionaldehyde)

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

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

CH₃CH₂CHO

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

Although there are 6 hydrogen atoms in the molecule, there only three possible chemical environments for the hydrogen atoms in propanal molecule, so only three ¹H chemical shifts observed.

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

The high resolution spectrum of propanal

All low and high resolution spectra of propanal show 3 groups of protons and in the ratio expected from the formula of propanal.

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

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

(a) Chemical shift 1.12 ppm CH₃ protons (3 protons): CH₃CH₂CHO

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

Evidence for the presence of a CH₂ group in the molecule of propanal

(b) Chemical shift 2.47 ppm CH₂ protons (2 protons): CH₃CH₂CHO

You might think the CH₂ proton resonance is split from both sides by the CH₃ and CH protons to give a 1:4:6:4:1 quintet (4 protons, n+1 = 5), but this is not so.

This CH₂ proton resonance is split by the CH₃ protons into a quartet (n+1 = 4), which is then split into doublets by the CH (aldehyde proton, n+1 = 2).

This produces a 1:3:3:1 series of 4 doublets, so more complicated than you expect.

(This is more university level NMR spectroscopy, so don't worry, concentrate on the basic proton ratio of 3:2:1 to match the structure of propanal).

(c) Chemical shift 9.80 ppm CH proton of CHO group (1 proton): CH₃CH₂CHO

The lone CH proton resonance is split into a 1:2:1 triplet by the CH₂ protons.

Evidence for the presence of a CH₂ group in the molecule of propanal.

Adding D₂O to the sample under investigation, dissolved in CDCl₃, makes no difference to the 1H NMR spectrum of propanal because there are no labile protons to exchange with deuterium ⁽²H) e.g. as in alcohols (O-H) or amines (N-H).

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) and applied to the ¹H NMR spectrum of propanal.

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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See also The isomers of molecular formula C₃H₆O (with some selected spectra data)

The chemistry of ALDEHYDES and KETONES revision notes INDEX

The infrared spectrum of Propanal (propionaldehyde)

The mass spectrum of Propanal (propionaldehyde)

The C-13 NMR spectrum of Propanal (propionaldehyde)

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

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Index of advanced (pre-university) organic chemistry revision notes

The chemistry of alkanes and the petrochemical industry

The chemistry of alkenes
The chemistry of haloalkanes
The chemistry of alcohols
The chemistry of aldehydes and ketones
The chemistry of carboxylic acids and derivatives
The chemistry of organo-nitrogen compounds

The chemistry of aromatic compounds

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