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Advanced Organic Chemistry: 1H NMR spectrum of propanoic acid

Interpreting the H-1 proton NMR spectrum of propanoic acid (propionic 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 propanoic acid

1H proton nmr spectrum of propanoic acid low/high resolution diagrams C3H6O2 CH3CH2COOH analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for propionic 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 - propanoic acid here.

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

Propanoic acid  (propionic acid) C3H6O2   (c) doc b   (c) doc b   (c) doc b

Interpreting the H-1 NMR spectrum of propanoic acid

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

For relatively simple molecules, the low resolution H-1 NMR spectrum of propanoic acid is a good starting point (high resolution diagram above, but just imagine 3 'unsplit' peaks and with an integrated proto ratio of 1:2:3 from left to right).

The hydrogen atoms (protons) of propanoic 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 (diagram above for propanoic acid).

CH3CH2COOH

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

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

Although there are 6 hydrogen atoms in the molecule, there are only 3 possible different chemical environments for the hydrogen atoms in propanoic acid molecule.

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

The high resolution 1H NMR spectrum of propanoic acid

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

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

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

(a) 1H Chemical shift 1.16 ppm, methyl protons: CH3CH2COOH

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

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

(b) 1H Chemical shift 2.38 ppm, CH2 protons: CH3CH2COOH

This 1H resonance is split by the CH3 protons into a 1:3:3:1 quartet (n+3 = 4).

Evidence for the presence of a CH3 group in the molecule of propanoic acid

(c) 1H Chemical shift 11.7 ppm, the hydroxyl group proton: CH3CH2COOH

This 1H resonance is NOT split because there are no protons on the adjacent carbon atom.

Evidence for the presence of an 'isolated' proton e.g. in a -COOH  group with no adjacent carbon atom attached to a proton (as in the molecule of propanoic acid or any other carboxylic acid).

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

Note the particularly large chemical shift for the proton of the hydroxyl group with


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

Key words & phrases: C3H6O2 CH3CH2COOH Interpreting the proton H-1 NMR spectra of propanoic acid, low resolution & high resolution proton nmr spectra of propanoic acid, H-1 nmr spectrum of propanoic acid, understanding the hydrogen-1 nmr spectrum of propanoic acid, explaining the line splitting patterns from spin-spin coupling  in the high resolution H-1 nmr spectra of propanoic acid, revising the H-1 nmr spectrum of propanoic acid, proton nmr of propanoic acid, ppm chemical shifts of the H-1 nmr spectrum of propanoic acid, 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 propanoic acid, how to work out the number of chemically different protons in the structure of the propanoic acid organic molecule, how to analyse the chemical shifts in the hydrogen-1 H-1 proton NMR spectrum of propanoic acid using the n+1 rule to explain the spin - spin coupling ine splitting in the proton nmr spectrum of propanoic acid deducing the nature of the protons from the chemical shifts ppm in the H-1 nmr spectrum of propanoic acid examining the 1H nmr spectrum of  propanoic acid analysing the 1-H nmr spectrum of propanoic acid how do you sketch and interpret the H-1 NMR spectrum of propanoic acid interpreting interpretation of the 1H proton spin-spin coupling causing line splitting in the NMR spectrum of propanoic acid  assignment of chemical shifts in the proton 1H NMR spectrum of propanoic acid formula explaining spin-spin coupling for line splitting for propionic acid


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