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Advanced Organic Chemistry: 1H NMR spectrum of 2-chlorobutane

The H-1 hydrogen-1 (proton) NMR spectrum of 2-chlorobutane

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 2-chlorobutane

C4H9Cl CH3CHClCH2CH3 low and high resolution 1H proton nmr spectrum of 2-chlorobutane analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for sec-butyl chloride 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 - 2-chlorobutane here.

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

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Interpreting the H-1 NMR spectrum of 2-chlorobutane

In terms of spin-spin coupling from the possible proton magnetic orientations, for 2-chlorobutane I have only considered the interactions of non-equivalent protons on adjacent carbon atoms e.g. -CH-CH3.

For relatively simple molecules, the low resolution H-1 NMR spectrum of 2-chlorobutane is a good starting point (low resolution diagram above).

The hydrogen atoms (protons) of 2-chlorobutane occupy 4 different chemical environments so that the low/high resolution NMR spectra should show 4 principal peaks of different H-1 NMR chemical shifts (diagram above for 2-chlorobutane).

CH3CHClCH2CH3

Note the proton ratio 3:1:2:3 of the 4 colours of the protons in the 4 chemically different environments

Chemical shifts (a) to (d) on the H-1 NMR spectrum diagram for 2-chlorobutane.

Although there are 9 hydrogen atoms in the molecule, there are only 4 possible different chemical environments for the hydrogen atoms in 2-chlorobutane molecule.

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

The high resolution 1H NMR spectrum of 2-chlorobutane

All low and high resolution spectra of 2-chlorobutane show 4 groups of proton resonances and in the ratio expected from the formula of 2-chlorobutane.

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

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

(a) 1H Chemical shift 1.50 ppm, CH3 protons: CH3CHClCH2CH3

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

Evidence for the presence of a CH3 group in the molecule of 2-chlorobutane

(b) 1H Chemical shift 3.97 ppm, CH proton :  CH3CHClCH2CH3

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

Evidence for the presence of a CH3-C-CH2 grouping in the molecule of 2-chlorobutane

(c) 1H Chemical shift 1.71 ppm, CH2 protons : CH3CHClCH2CH3

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

Evidence for the presence of a CH3-C-CH group in the molecule of 2-chlorobutane

(d) 1H Chemical shift 1.02 ppm, CH3 protons : CH3CHClCH2CH3

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

Evidence for the presence of a CH2 group in the molecule of 2-chlorobutane

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


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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