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Advanced Organic Chemistry: H-1 NMR spectrum of propan-2-ol

The H-1 hydrogen-1 (proton) NMR spectrum of propan-2-ol  (2-propanol)

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 propan-2-ol

low and high resolution H-1 proton nmr spectrum of propan-2-ol analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 1-H nmr for 2-propanol 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 - propan-2-ol here.

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

Propan-2-ol C3H8O, alcohols and ether structure and naming (c) doc b , alcohols and ether structure and naming (c) doc balcohols and ether structure and naming (c) doc b , alcohols and ether structure and naming (c) doc b

Interpreting the H-1 NMR spectrum of propan-2-ol  (2-propanol, isopropyl alcohol)

For relatively simple molecules, the low resolution H-1 NMR spectrum of propan-2-ol is a good starting point.

The hydrogen atoms (protons) of propan-2-ol occupy 3 different chemical environments so that the low resolution NMR spectra should show 3 peaks of different H-1 NMR chemical shifts (diagram above for propan-2-ol).

(CH3)2CHOH  or  CH3CH(OH)CH3

Note the ratio 6:1:1 of the 3 colours of the protons in the 3 chemically different environments

Although there are 8 hydrogen atoms in the molecule, there only 3 possible chemical environments for the hydrogen atoms in propan-2-ol molecule.

The proton ratio 6:1:1 observed, corresponds with the structural formula of propan-2-ol.

The high resolution spectrum of propan-2-ol

All low and high resolution spectra of propan-2-ol show 3 groups of protons and in the ratio expected from the formula of propan-2-ol.

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

So, using the chemical shifts and applying the n+1 rule to propan-2-ol

Chemical shift 1.20 ppm the two CH3 groups of protons (6 protons)

The single CH proton splits the CH3 resonance into a 1:1 doublet (n+1 = 2).

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

Chemical shift 2.16 ppm the lone OH proton (1 proton)

Generally speaking, unless ultra dry and very high resolution, the hydroxyl proton resonance is not split and neither does it cause a splitting effect itself (see note at end).

Chemical shift 4.01 ppm the lone CH proton (1 proton)

The single CH proton is split by the 6 methyl protons into a 1:6:15:20:15:6:1 septet (n+1 = 7), in terms of the relative intensities of the H-1 NMR resonance lines.

Evidence for the presence two methyl groups in the molecule of propan-2-ol

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


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: 2-propanol isopropyl alcohol Interpreting the proton H-1 NMR spectra of propan-2-ol, low resolution & high resolution proton nmr spectra of propan-2-ol, H-1 nmr spectrum of propan-2-ol, understanding the hydrogen-1 nmr spectrum of propan-2-ol, explaining the line splitting patterns in the high resolution H-1 nmr spectra of propan-2-ol, revising the H-1 nmr spectrum of propan-2-ol, proton nmr of propan-2-ol, ppm chemical shifts of the H-1 nmr spectrum of propan-2-ol, 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 propan-2-ol, how to work out the number of chemically different protons in the structure of the propan-2-ol organic molecule, how to analyse the chemical shifts in the hydrogen-1 H-1 proton NMR spectrum of propan-2-ol using the n+1 rule to explain the spin - spin coupling ine splitting in the proton nmr spectrum of propan-2-ol deducing the nature of the protons from the chemical shifts ppm in the H-1 nmr spectrum of propan-2-ol examining the 1H nmr spectrum of  propan-2-ol analysing the 1-H nmr spectrum of propan-2-ol how do you sketch and interpret the H-1 NMR spectrum of propan-2-ol 2-propanol isopropyl alcohol


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