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Advanced Organic Chemistry: 1H NMR spectrum of 3-hydroxybutanone

Interpreting the H-1 hydrogen-1 (proton) NMR spectrum of 3-hydroxybutanone  (acetoin)

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 3-hydroxybutanone

1H proton nmr spectrum of 3-hydroxybutanone low/high resolution diagrams C4H8O2 CH3COCH(OH)CH3 analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for acetoin 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 - 3-hydroxybutanone here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of 3-hydroxybutanone 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 3-hydroxybutanone molecule.

C4H8O2, 3-hydroxybutanone 3-hydroxybutan-2-one acetoin CH3COCH(OH)CH3 C4H8O2, 3-hydroxybutanone, 3-hydroxybutan-2-one, 'acetoin', CH3COCH(OH)CH3

Interpreting the H-1 NMR spectrum of 3-hydroxybutanone

In terms of spin-spin coupling from the possible proton magnetic orientations, for 3-hydroxybutanone I have only considered the interactions of non-equivalent protons on adjacent carbon atoms e.g. -CH-CH3, protons, but note that the hydroxyl proton (O-H) resonance is not split (singlet) and neither does this proton cause splitting in the CH or CH3 protons.

For relatively simple molecules, the low resolution H-1 NMR spectrum of 3-hydroxybutanone is a good starting point (low resolution diagram above).

The hydrogen atoms (protons) of 3-hydroxybutanone occupy 4 different chemical environments so that the low or low resolution NMR spectra should show 4 principal peaks of different H-1 NMR chemical shifts in the integrated proton ratio of 3:1:1:3 (diagram above for 3-hydroxybutanone).

CH3COCH(OH)CH3

Note the proton ratio 3:1:1:3 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 3-hydroxybutanone.

Although there are 8 hydrogen atoms in the molecule, there are only 4 possible different chemical environments for the hydrogen atoms in 3-hydroxybutanone molecule.

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

The high resolution 1H NMR spectrum of 3-hydroxybutanone

The high resolution spectra of 3-hydroxybutanone shows 4 groups of proton resonances and in the 3:1:1:3 ratio expected from the structural formula of 3-hydroxybutanone.

The ppm quoted on the diagram represent the peak of resonance intensity for a particular proton group in the molecule of 3-hydroxybutanone - 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 3-hydroxybutanone below.

So, using the chemical shifts and applying the n+1 rule to 3-hydroxybutanone and make some predictions using some colour coding! (In problem solving you work the other way round!)

(a) 1H Chemical shift 2.21 ppm, methyl protons: CH3COCH(OH)CH3

This 1H NMR resonance appears as a singlet because there are no protons on the adjacent carbon atom.

From the proton ratio, evidence for the presence of an 'isolated' methyl group in the molecule of 3-hydroxybutanone

(b) 1H Chemical shift 4.26 ppm, CH proton: CH3COCH(OH)CH3

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

Evidence for the presence of a CH3 group in the molecule of 3-hydroxybutanone

(c) 1H Chemical shift 3.65 ppm: CH3COCH(OH)CH3

This 1H NMR resonance appears as a singlet because there is no interaction observed with the CH proton.

Evidence for the presence of an 'isolated' proton in the molecule of 3-hydroxybutanone

(d) 1H Chemical shift 1.39 ppm: CH3COCH(OH)CH3

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

Evidence for the presence of a CH3 group in the molecule of 3-hydroxybutanone

From the proton ratio, evidence for the presence of a 2nd CH3 group in the molecule of 3-hydroxybutanone

An important note about the hydroxyl group on butan-2-ol (for pre-university students):

Unless the alcohol is completely free of water (difficult), the hydrogen on the -O-H hydroxyl group and any hydrogens on the adjacent carbon atoms do not interact to produce any spin-spin splitting. Therefore the -OH peak shows up as a singlet and you don't usually have to consider its effect on any hydrogen atoms, if present on the adjacent carbon atom (C-OH), and, neither do you have to consider the splitting effect of adjacent C-H protons on the hydrogen of the OH group.
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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