Advanced Organic Chemistry: 1H NMR spectrum of 2-methylpropanoic acid

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Interpreting the 1H NMR spectrum of 2-methylpropanoic acid (isobutyric acid)

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-methylpropanoic acid

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H-1 proton NMR spectroscopy - spectra index

1H proton nmr spectrum of 2-methylpropanoic acid low/high resolution diagrams C4H8O2 (CH3)2CHCOOH analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for isobutyric acid 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-methylpropanoic acid here.

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

2-methylpropanoic acid (isobutyric acid), C4H8O2  (c) doc b   (c) doc b   (c) doc b

Interpreting the H-1 NMR spectrum of 2-methylpropanoic acid

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

For relatively simple molecules, the low resolution H-1 NMR spectrum of 2-methylpropanoic acid is a good starting point (low resolution diagram above).

The hydrogen atoms (protons) of 2-methylpropanoic acid occupy 3 different chemical environments so that the low resolution NMR spectra should show 3 principal peaks of different H-1 NMR chemical shifts (JUST 'blur' the high resolution diagram above for 2-methylpropanoic acid).

CH3CH(CH3)COOH  or  (CH3)2CHCOOH

Note the proton ratio 6:1:1 of the 3 colours of the protons in the 3 chemically different proton environments

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

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

The integrated signal proton ratio 6:1:1 observed in the high resolution H-1 NMR spectrum, corresponds with the structural formula of 2-methylpropanoic acid.

The high resolution 1H NMR spectrum of 2-methylpropanoic acid

All low and high resolution spectra of 2-methylpropanoic acid show 3 groups of proton resonances and in the 6:1:1 ratio expected from the structural formula of 2-methylpropanoic acid.

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

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

(a) 1H Chemical shift 1.20 ppm, six methyl protons: (CH3)2CHCOOH

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

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

(b) 1H Chemical shift 2.58 ppm, CH alkyl proton: (CH3)2CHCOOH

This resonance is split into a 1:6:15:20:15:6:1 septet by the two groups of methyl protons (n+6 = 7).

Evidence for the presence of a CH3-CH-CH3 group in the molecule of 2-methylpropanoic acid

(c) 1H Chemical shift 11.88 ppm, hydroxyl proton: (CH3)2CHCOOH

This resonance is observed as a singlet because there is no proton on an adjacent carbon atom (n+0 = 1).

Evidence for the presence of an 'isolated' proton in the molecule of 2-methylpropanoic acid.

The chemical shift indicates a hydroxyl proton.

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

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

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Links associated with 2-methylpropanoic acid

The chemistry of CARBOXYLIC ACIDS and DERIVATIVES revision notes INDEX

H-1 proton NMR spectroscopy index  (Please read 8 points at the top of the 1H NMR index page)

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

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