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

H-1 hydrogen-1 (proton) NMR spectrum of propanal (propionaldehyde)

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 propanal

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, aldehydes and ketones nomenclature (c) doc b , aldehydes and ketones nomenclature (c) doc b , aldehydes and ketones nomenclature (c) doc b, aldehydes and ketones nomenclature (c) doc b

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 H-1 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 3 possible chemical environments for the hydrogen atoms in propanal molecule.

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.

(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


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

Key words & phrases: propionaldehyde Interpreting the proton H-1 NMR spectra of propanal, low resolution & high resolution proton nmr spectra of propanal, H-1 nmr spectrum of propanal, understanding the hydrogen-1 nmr spectrum of propanal, explaining the line splitting patterns in the high resolution H-1 nmr spectra of propanal, revising the H-1 nmr spectrum of propanal, proton nmr of propanal, ppm chemical shifts of the H-1 nmr spectrum of propanal, explaining and analyzing spin spin line splitting in the H-1 nmr spectrum, how to construct the diagram of the H-1 nmr spectrum of propanal, how to work out the number of chemically different protons in the structure of the propanal organic molecule, how to analyse the chemical shifts in the hydrogen-1 H-1 proton NMR spectrum of propanal using the n+1 rule to explain the spin - spin coupling ine splitting in the proton nmr spectrum of propanal deducing the nature of the protons from the chemical shifts ppm in the H-1 nmr spectrum of propanal examining the 1H nmr spectrum of  propanal analysing the 1-H nmr spectrum of propanal how do you sketch and interpret the H-1 NMR spectrum of propanal

Molecular structure diagram of the proton NMR diagram for the 1H NMR spectrum of propanal. The proton ratio in the 1H NMR spectrum of propanal. Deducing the number of different chemical environments of the protons in the propanal molecule from the 1H chemical shifts in the hydrogen-1 NMR spectrum of propanal. Analysing the high resolution 1H NMR spectrum of propanal. Analysing the low resolution 1H NMR spectrum of propanal. You may need to know the relative molecular mass of propanal to deduce the molecular formula from the proton ratio of the 1H NMR spectrum of propanal. Revision notes on the proton NMR spectrum of propanal. Matching and deducing the structure of the propanal molecule from its hydrogen-1 NMR spectrum.


Links associated with propanal

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 8 points at the top of the 1H NMR index page)

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