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Advanced Organic Chemistry: H-1 NMR spectrum of ethanol

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The H-1 hydrogen-1 (proton) NMR spectrum of ethanol (ethyl alcohol)

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

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H-1 proton NMR spectroscopy - spectra index

low and high resolution H-1 proton nmr spectrum of ethanol analysis interpretation of chemical shifts ppm spin spin line splitting diagram for ethyl alcohol 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 shifts, called chemical shifts, depend on the individual (electronic) chemical environment of the hydrogen atoms in an organic molecule - ethanol here.

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

Ethanol C2H6O, alcohols and ether structure and naming (c) doc b , alcohols and ether structure and naming (c) doc b , alcohols and ether structure and naming (c) doc b , alcohols and ether structure and naming (c) doc b

Revision notes on the structure and naming (nomenclature) of aliphatic ALCOHOLS and ETHERS

Interpreting the H-1 NMR spectrum of ethanol

For relatively simple molecules, the low resolution H-1 NMR spectrum of ethanol is a good starting point.

The hydrogen atoms (protons) of ethanol occupy 3 different chemical environments so that the H-1 proton low resolution NMR spectra should show 3 peaks (diagram above).

CH3CH2OH

Note the ratio of the 3 colours for the 3 proton chemical environments in ethanol.

In terms of the H-1 chemical shifts for ethanol (a) to (c) and applying the n+1 rule:

(a) Centred at 1.22 ppm, the CH3 protons are split by the CH2 protons into a 1 : 2 : 1 triplet (n+1 = 3).

(b) Centred at 3.69 ppm, the CH2 protons are split by the CH3 protons into a 1 : 3 : 3 : 1 quartet (n+3 = 4).

(c) The hydroxy proton O-H gives a chemical shift of 2.61 ppm and shows no significant splitting.

Normally the O-H proton resonance is not split by adjacent protons and neither does it, in turn, split the resonance of the same adjacent carbon atom protons - see extra notes below.

Very high resolution 1H NMR spectrum of ultra-pure ethanol under the right condition!

With ultra-pure anhydrous ethanol it is possible to observe the splitting effects by, and of, the hydroxyl proton OH - but its a bit tricky in place!

(a) The CH3 protons give a triplet from the CH2 protons- as above (n+1 = 3) - no change.

(b) You might think the CH2 proton resonance might seem to be split into a quintet (n+1 = 5) by the CH3 and OH protons on either side, and not a quartet. Some diagrams I've seen look like this, but this not actually what happens.

In fact the CH2 protons are split by the CH3 protons into a 1:3:3:1 quartet, but this quartet is split into doublets by the single OH proton (n+1 = 2), so a series of 1:3:3:1 doublets, or 1:1:3:3:3:3:1:1 to be a purist, but this is going beyond pre-university level!

(c) As above, the OH resonance would be split into a triplet (n+1 = 3) by the CH2 protons.

(This is more university level NMR spectroscopy, so don't worry, concentrate on the basic proton ratio of 3:2:1 to match the structure of ethanol).

The integrated NMR proton ratio observed of 3 : 2 : 1, corresponds with the structural formula of ethanol.

See also comparing the IR, mass, 1H NMR and 13C NMR spectra of isomers of C2H6O below.

Extra note on the hydroxy (O-H) resonance in the H-1 NMR of alcohols

(a) The first important point to make is, that despite the variety of different chemical shifts for the OH proton resonance quoted in data books, textbooks and internet sources, the integration of the NMR resonances always gives the correct proton ratio in the molecule, in this case 3 : 2 : 1 for ethanol CH3CH2OH

(b) Apart from the O-H group resonance the NMR spectra of most alcohols conform to what would be expected e.g. the triplet and quartet of the spin-spin splitting effects for the alkyl part resonances of the ethanol molecule.

(c) However, the OH proton NMR resonance for ethanol is typically quoted as ~2.6 ppm (here) and ~5.3 ppm, and some in between too!

BUT, the main difference you find from various sources is not the OH proton chemical shift, but is it a singlet or a triplet?

(d) The problem arises if the alcohol is impure e.g. containing water or any source of labile protons, because water and the alcohol, ethanol, exchange protons e.g.

CH3CH2OH  +  H-O-H    CH3CH2OH  +  H-O-H

This means the CH2 protons no longer experience a 'simple' local field from one singlet proton from two possible orientations, but, over a finite period, experience the averaging effect of exchanging protons.

This removes the spin - spin coupling effect and the OH proton resonance just shows up as a singlet if the ethanol contains even a trace of water (or acid).

This sort of exchange cannot happen with the alkyl protons, but is common with molecules containing a hydroxylic (OH) hydrogen atom like alcohols and carboxylic acids.

Not only that, you also get proton transfer between the alcohol molecules i.e.

CH3CH2OH  +  H-O-CH2CH3    CH3CH2OH  +  H-O-CH2CH3

which can give the same effect as traces of water of acid.

Normally the result is the O-H proton resonance is not split by adjacent protons and neither does it, in turn, split the resonance of the same adjacent carbon atom protons - see extra notes below.

(e) So, in ethanol, all you usually see in the H-1 NMR spectrum is the mutual splitting of the CH2 and CH3 proton resonances plus a singlet line for the OH proton resonance.

Note the actual chemical shift for the OH proton resonance of alcohols can depend on the solvent and concentration used in the NMR machine, often CDCl3 - deuterated trichloromethane solvent.

See also comparing the IR, mass, 1H NMR and 13C NMR spectra of isomers of C2H6O below.


Number of protons 1H causing splitting Splitting pattern produced from the n+1 rule 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

Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 2 isomers of C2H6O

NOTE: The images are linked to their original detailed spectral analysis pages AND can be doubled in size with touch screens to increase the definition to the original ethanol (ethyl alcohol) and methoxymethane (dimethyl ether) image sizes.

INFRARED SPECTRA: Apart from the significant differences in the fingerprint region at wavenumbers 1500 to 400 cm-1, the most striking difference is the broad O-H stretching band ~3400 cm-1, found in the infrared spectrum of alcohols, but absent in the infrared spectrum of ethers.

MASS SPECTRA: Both ethanol and methoxymethane show some similarities in their mass spectra, but their base ion peaks are quite different - for ethanol it is m/z 31 and for methoxymethane it is m/z 45.

1H NMR SPECTRA: The 1H NMR spectra of ethanol and methoxymethane are quite significantly different. Ethanol gives 3 peaks in the proton ratio 3:2:1 (3 different chemical environments), whereas methoxymethane only gives one 1H chemical shift peak (all 6 protons in the same chemical environment). Ethanol does not have the symmetry of methoxymethane and has the hydroxyl group, thus producing 3 different chemical environments

13C NMR SPECTRA: The 13C NMR spectra of ethanol and methoxymethane are different. Ethanol gives two 13C resonances, but methoxymethane only one (2 different 13C chemical environments and a 13C single chemical environment).

Key words & phrases: Interpreting the proton H-1 NMR spectra of ethanol, low resolution & high resolution proton nmr spectra of ethanol, H-1 nmr spectrum of ethanol, understanding the hydrogen-1 nmr spectrum of ethanol, explaining the line splitting patterns in the high resolution H-1 nmr spectra of ethanol, revising the H-1 nmr spectrum of ethanol, proton nmr of ethanol, ppm chemical shifts of the H-1 nmr spectrum of ethanol, explaining and analyzing spin line splitting in the H-1 nmr spectrum, how to construct the diagram of the H-1 nmr spectrum of ethanol, how to work out the number of chemically different protons in the structure of the ethanol organic molecule, how to analyse the chemical shifts in the hydrogen-1 H-1 proton NMR spectrum of ethanol Molecular structure diagram of the proton NMR diagram for the 1H NMR spectrum of ethanol. The proton ratio in the 1H NMR spectrum of ethanol. Deducing the number of different chemical environments of the protons in the ethanol molecule from the 1H chemical shifts in the hydrogen-1 NMR spectrum of ethanol. Analysing the very high high resolution 1H NMR spectrum of ultra-pure ethanol. Analysing the low resolution 1H NMR spectrum of ethanol. You may need to know the relative molecular mass of ethanol to deduce the molecular formula from the proton ratio of the 1H NMR spectrum of ethanol. Revision notes on the proton NMR spectrum of ethanol. Matching and deducing the structure of the ethanol molecule from its hydrogen-1 NMR spectrum explaining the chemical shift splitting pattern in the 1H proton nmr of ethanol and complications in the spectra due to hydrogen bonding


Associated links

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

The infrared spectrum of Ethanol (ethyl alcohol)

The mass spectrum of Ethanol (ethyl alcohol)

The C-13 NMR spectrum Ethanol (ethyl alcohol)

The chemistry of ALCOHOLS revision notes INDEX

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