Advanced Organic Chemistry: 1H NMR spectrum of butan-2-ol (2-butanol, sec-butyl alcohol)

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Interpreting the H-1 (proton) NMR spectrum of butan-2-ol

CH3CH(OH)CH2CH3 (2-butanol, sec-butyl 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 of butan-2-ol

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Links associated with butan-2-ol

H-1 proton NMR spectroscopy - spectra index

C4H10O CH3CH(OH)CH2CH3 low and high resolution 1H proton nmr spectrum of butan-2-ol analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for 2-butanol sec-butyl alcohol 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 - butan-2-ol here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of butan-2-ol 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 butan-2-ol molecule.

Butan-2-ol    C4H10O    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 

Interpreting the H-1 NMR spectrum of butan-2-ol

In terms of spin-spin coupling from the possible proton magnetic orientations, for butan-2-ol 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 butan-2-ol is a good starting point (low resolution inset diagram above).

The hydrogen atoms (protons) of butan-2-ol occupy 5 different chemical environments so that the low resolution NMR spectra should show 5 principal peaks of 5 different H-1 NMR chemical shifts (diagram above for butan-2-ol).

CH3CH(OH)CH2CH3

Note the proton ratio 3:1:1:2:3 of the 5 colours of the protons in the 5 chemically different environments

Chemical shifts (a) to (e) on the H-1 NMR spectrum diagram for butan-2-ol.

Although there are 10 hydrogen atoms in the molecule, there are only 5 possible different chemical environments for the hydrogen atoms in butan-2-ol molecule.

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

The high resolution 1H NMR spectrum of butan-2-ol

All low and high resolution spectra of butan-2-ol show 5 groups of proton resonances and in the 3:1:1:2:3 ratio expected from the formula of butan-2-ol.

The ppm quoted on the diagram represent the peak of resonance intensity for a particular proton group in the molecule of butan-2-ol - 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 butan-2-ol below.

So, using the chemical shifts and applying the n+1 rule to butan-2-ol and make some predictions using some colour coding! (In problem solving you work the other way round!)

BUT, 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 don't 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.

(a) 1H Chemical shift 1.17 ppm for methyl protons: CH3CH(OH)CH2CH3

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

Evidence for the presence of a CH group in the molecule of butan-2-ol

(b) 1H Chemical shift 3.71 ppm for CH proton: CH3CH(OH)CH2CH3

This 1H resonance is split into a 1:5:10:10:5:1 sextet by the adjacent CH3 and CH2 protons on either side (n+1 = 6).

Evidence for the presence of a CH3-CHx-CH2 grouping in the molecule of butan-2-ol

(c) 1H Chemical shift 1.46 ppm for CH2 protons: CH3CH(OH)CH2CH3

This 1H resonance is split into a 1:4:6:4:1 quintet by the adjacent CH3 and CH protons on either side (n+1 = 5).

Evidence for the presence of a CH-CHx-CH3 group in the molecule of butan-2-ol

(d) 1H Chemical shift 0.93 ppm for methyl protons: CH3CH(OH)CH2CH3

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

Evidence for the presence of a ? group in the molecule of butan-2-ol

(e) 1H Chemical shift 2.37 ppm for the hydroxyl proton: CH3CH(OH)CH2CH3

This 1H resonance is observed as a singlet - assuming no splitting effect from the CH proton.

Evidence for the presence of an O-H or an 'isolated' C-H group (no adjacent C-Hx) in the molecule of butan-2-ol, but there is no isolated CH proton in the molecule.

Note the decreasing effect on the 1H chemical shift as the proton is further from the more electronegative oxygen atom in butan-2-ol.

Extra note on the OH proton resonance

If the alcohol is impure, containing water or any source of labile protons, because water and the alcohol exchange protons e.g.

R-O-H  +  H-O-H    R-O-H  +  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 butan-1-ol 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.

R-O-H  +  H-O-R    R-O-H  +  H-O-R

which gives the same effect as traces of water of acid.

So, in butan-2-ol, all you usually see in the H-1 NMR spectrum is the mutual splitting of the CH, CH2 and CH3 proton resonances plus a singlet line for the OH proton resonance.

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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Links associated with butan-2-ol

The infrared spectrum of butan-2-ol (sec-butyl alcohol)

The mass spectrum of butan-2-ol (sec-butyl alcohol)

The C-13 NMR spectrum of butan-2-ol (sec-butyl alcohol)

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