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Advanced Organic Chemistry: 1H NMR spectrum of propyl methanoate

Interpreting the H-1 hydrogen-1 (proton) NMR spectrum of propyl methanoate

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 propyl methanoate

1H proton nmr spectrum of propyl methanoate low/high resolution diagrams C4H8O2 HCOOCH2CH2CH3 analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for propyl formate 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 - propyl methanoate here.

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

Propyl methanoate (propyl formate)   (c) doc b    (c) doc b    (c) doc b 

Interpreting the H-1 NMR spectrum of propyl methanoate

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

For relatively simple molecules, the low resolution H-1 NMR spectrum of propyl methanoate is a good starting point (high resolution diagram above, 'blur' it to get a proton ratio of 1:2:2:3).

The hydrogen atoms (protons) of propyl methanoate 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 propyl methanoate).

HCOOCH2CH2CH3

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

Chemical shifts (a) to (4) on the H-1 NMR spectrum diagram for propyl methanoate.

Although there are 8 hydrogen atoms in the molecule, there are only 4 possible different chemical environments for the hydrogen atoms in propyl methanoate molecule.

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

The high resolution 1H NMR spectrum of propyl methanoate

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

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

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

(a) 1H Chemical shift 8.04 ppm: HCOOCH2CH2CH3

This resonance appears as a singlet, no line splitting because there is no proton on an adjacent atom.

Evidence for the presence of an isolated proton in the molecule of propyl methanoate

(b) 1H Chemical shift 4.16 ppm CH2 protons: HCOOCH2CH2CH3

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

Evidence for the presence of a CH2 group in the molecule of propyl methanoate

(c) 1H Chemical shift 1.61 ppm, CH2 protons: HCOOCH2CH2CH3

This resonance is split 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 2nd CH2CH2CH3 grouping in the molecule of propyl methanoate.

Note the two sets of CH2 protons are not equivalent and will split each other.

(d) 1H Chemical shift 0.96 ppm, methyl protons: HCOOCH2CH2CH3

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

Evidence for the presence of a 2nd CH2 group in the molecule of propyl methanoate

Evidence for the presence of a CH2 group in the molecule of propyl methanoate

Note the decreasing effect on the 1H chemical shift as the proton is further from the more electronegative oxygen atoms in the propyl group of propyl methanoate.


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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