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Advanced Organic Chemistry: 1H NMR spectrum of 2-methylpropanal

Interpreting the H-1 NMR spectrum of 2-methylpropanal (isobutyraldehyde)

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 2-methylpropanal

low/high resolution 1H proton nmr spectrum of 2-methylpropanal C4H8O (CH3)2CHCHO analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for isobutyraldehyde explaining spin-spin coupling for line splitting 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 resonances, called chemical shifts, are measured with respect to the TMS, and depend on the individual (electronic) chemical environment of the hydrogen atoms in an organic molecule - 2-methylpropanal here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of 2-methylpropanal 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 2-methylpropanal molecule.

2-methylpropanal   C4H8O   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 2-methylpropanal

In terms of spin-spin coupling from the possible proton magnetic orientations, for 2-methylpropanal I have only considered the interactions of non-equivalent protons on adjacent carbon atoms e.g. -CH-CH3, -CH-CH- protons etc.

For relatively simple molecules, the low resolution H-1 NMR spectrum of 2-methylpropanal is a good starting point and would show protons in the ratio 6:1:1.

The hydrogen atoms (protons) of 2-methylpropanal occupy 3 different chemical environments so that the low resolution NMR spectra should show 3 principal peaks of different H-1 NMR chemical shifts (I've only constructed the diagram above for 2-methylpropanal).

(CH3)2CHCHO

Note the proton ratio 6:1:1 of the 3 colours of the protons in the 3 chemically different environments and the integrated signal proton ratio observed in the high resolution H-1 NMR spectrum, corresponds with the structural formula of 2-methylpropanal.

Chemical shifts (a) to (c) on the H-1 NMR spectrum diagram for 2-methylpropanal.

Although there are 8 hydrogen atoms in the molecule, there are only 3 possible different chemical environments for the hydrogen atoms in 2-methylpropanal molecule.

The high resolution 1H NMR spectrum of 2-methylpropanal

All low and high resolution spectra of 2-methylpropanal show 3 groups of proton resonances and in the ? ratio expected from the formula of 2-methylpropanal.

The ppm quoted on the diagram represent the peak of resonance intensity for a particular proton group in the molecule of 2-methylpropanal - 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 2-methylpropanal below.

So, using the chemical shifts and applying the n+1 rule to 2-methylpropanal and make some predictions using some colour coding! (In problem solving you work the other way round!)

(a) 1H Chemical shift 1.06 ppm, methyl protons: (CH3)2CHCHO

This 1H NMR resonance is split into a 1:1 doublet by the adjacent CH proton (n+1 = 2).

Evidence for the presence of a CH group in the molecule of 2-methylpropanal

(b) 1H Chemical shift 2.39 ppm, CH proton of the alkyl group: (CH3)2CHCHO

This 1H NMR resonance is split into an octet the adjacent CH3 (x 2) and CH protons (n+1 = 8)

Evidence for the presence of a (CH3)2C-CH grouping in the molecule of 2-methylpropanal

(c) 1H Chemical shift 9.57 ppm, CH proton of the aldehyde group: (CH3)2CHCHO

This 1H NMR resonance is split into a 1:1 doublet by the adjacent CH proton (n+1 = 2).

Note the large chemical shift exhibited by the aldehyde group proton (much more than the other CH proton).

Evidence for the presence of a 2nd CH group in the molecule of 2-methylpropanal

Note the decreasing effect on the 1H chemical shift as the proton is further from the more electronegative oxygen atom in 2-methylpropanal.


Number of directly adjacent protons 1H causing splitting Splitting pattern produced from the n+1 rule on spin-spin coupling 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

7 protons create a resonance split of a 1:7:21:35:35:21:7:1 octet.


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