Advanced Organic Chemistry: 1H NMR spectrum of butanoic acid

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Interpreting the H-1 hydrogen-1 NMR spectrum of butanoic acid (butyric 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 butanoic acid

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H-1 proton NMR spectroscopy - spectra index

1H proton nmr spectrum of butanoic acid low/high resolution diagrams C4H8O2 CH3CH2CH2COOH analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for butyric 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 - butanoic acid here.

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

Butanoic acid (butyric acid)  (c) doc b  (c) doc b  (c) doc b a carboxylic acid

Interpreting the H-1 NMR spectrum of butanoic acid

In terms of spin-spin coupling from the possible proton magnetic orientations, for butanoic acid I have only considered the interactions of non-equivalent protons on adjacent carbon atoms e.g. -CH2-CH3, -CH2-CH2-.

For relatively simple molecules, the low resolution H-1 NMR spectrum of butanoic acid is a good starting point (high resolution diagram above, but just 'blur' the groups of lines into integrated proton ratio of 3:2:2:1).

The hydrogen atoms (protons) of butanoic acid occupy 4 different chemical environments so that the low resolution NMR spectra should show 4 principal peaks of different H-1 NMR chemical shifts (diagram above for butanoic acid).

CH3CH2CH2COOH

Note the proton ratio 3:2:2:1 of the 4 colours of the protons in the 4 chemically different proton environments

Chemical shifts (a) to (d) on the H-1 NMR spectrum diagram for butanoic acid.

The integrated signal proton ratio 3:2:2:1 observed in the high resolution H-1 NMR spectrum, corresponds with the structural formula of butanoic acid.

The high resolution 1H NMR spectrum of butanoic acid

All low and high resolution spectra of butanoic acid show 4 groups of proton resonances and in the 3:2:2:1 ratio expected from the structural formula of butanoic acid.

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

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

(a) 1H Chemical shift 0.98 ppm, methyl protons: CH3CH2CH2COOH

This 1H resonance is split by the adjacent CH2 protons into a 1:2:1 triplet (n+1 = 3)

Evidence for the presence of a CH2 group in the molecule of butanoic acid

(b) 1H Chemical shift 1.68ppm, CH2 protons: CH3CH2CH2COOH

This 1H resonance is split by the adjacent CH2 and CH3 protons into a 1:5:10:10:5:1 sextet (n+5 = 6)

Evidence for the presence of a CH3-CHx-CH3 group in the molecule of butanoic acid

(c) 1H Chemical shift 2.33 ppm, CH2 protons: CH3CH2CH2COOH

This 1H resonance is split by the adjacent CH2 protons into a 1:2:1 triplet (n+1 = 3)

Evidence for the presence of a 2nd CH2 group in the molecule of butanoic acid

(d) 1H Chemical shift 11.5 ppm, hydroxyl proton: CH3CH2CH2COOH

This resonance is NOT split, there is no proton on an adjacent atom.

Evidence for the presence of an isolated proton in the molecule of butanoic acid

Note the decreasing effect on the 1H chemical shift as the proton is further from the more electronegative oxygen atoms in butanoic 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 butanoic 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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