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Advanced Organic Chemistry: H-1 NMR spectrum of ethanol

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

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.

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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:

Centred at 1.22 ppm, the CH3 protons are split by the CH2 protons into a 1 : 2 : 1 triplet.

Centred at 3.69 ppm, the CH2 protons are split by the CH3 protons into a 1 : 3 : 3 : 1 quartet.

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.

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

 

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 gives 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 used in the NMR machine, often CDCl3 - deuterated trichloromethane.


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

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 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


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