Advanced Organic Chemistry: 1H NMR spectrum of 1-iodopropane (propyl iodide)

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Interpreting the H-1 hydrogen-1 (proton) NMR spectrum of 1-iodopropane

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 1-iodopropane

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H-1 proton NMR spectroscopy - spectra index

1H proton nmr spectrum of 1-iodopropane low/high resolution diagrams C3H7I CH3CH2CH2I analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for propyl iodide 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 - 1-iodopropane here.

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

1-iodopropane, C3H7I, CH3CH2CH2I, CH3-CH2-CH2-I

Interpreting the H-1 NMR spectrum of 1-iodopropane

In terms of spin-spin coupling from the possible proton magnetic orientations, for 1-iodopropane I have only considered the interactions of non-equivalent protons on adjacent carbon atoms e.g. -CH2-CH3, -CH2-CH2-, protons etc.

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

The 7 hydrogen atoms (protons) of 1-iodopropane occupy 3 different chemical environments so that the low resolution NMR spectra should show 3 principal peaks of different H-1 NMR chemical shifts (diagram above for 1-iodopropane).

CH3-CH2-CH2-I

Note the proton ratio 3:2:2 of the 3 colours of the 7 protons of 1-iodopropane in the 3 chemically different proton environments

Chemical shifts (a) to (c) on the H-1 NMR spectrum diagram for 1-iodopropane.

Although there are 7 hydrogen atoms in the molecule, the proton NMR spectrum shows there are only 3 possible different chemical environments for the hydrogen atoms in 1-iodopropane molecule.

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

The high resolution 1H NMR spectrum of 1-iodopropane

The high resolution spectra of 1-iodopropane shows 3 groups of proton resonances and in the 3:2:2 ratio expected from the structural formula of 1-iodopropane, but we can now consider the splitting of resonance lines from the spin-spin coupling in the molecule of 1-iodopropane.

The ppm quoted on the diagram represent the peak of resonance intensity for a particular proton group in the molecule of 1-iodopropane - 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 1-iodopropane below.

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

1H NMR resonance (a) 1H Chemical shift 0.82 ppm: CH3-CH2-CH2-I

This resonance line will be split into a 1:2:1 triplet by the adjacent CH2 group protons (n+1 = 3).

Evidence for the presence of a CH2 group in the molecule of 1-iodopropane

1H NMR resonance (b) 1H Chemical shift 1.66 ppm: CH3-CH2-CH2-I

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

Evidence for the presence of a CH3-CHx-CH2 group in the molecule of 1-iodopropane (x could be 1 or 2 as in this case).

1H NMR resonance (c) 1H Chemical shift 2.99 ppm: CH3-CH2-CH2-I

This resonance line will be split into a 1:2:1 triplet by the adjacent CH2 group protons (n+1 = 3).

Evidence for the presence of a 2nd CH2 group in the molecule of 1-iodopropane with a significantly different chemical shift to the other CH2 group in the 1-iodopropane molecule - compare shifts (a) and (c).

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 1-iodopropane

The chemistry of HALOGENOALKANES (haloalkanes) revision notes INDEX

The infrared spectrum of 1-iodopropane (n-propyl iodide)

The mass spectrum of 1-iodopropane (n-propyl iodide)

The C-13 NMR spectrum of 1-iodopropane (n-propyl iodide)

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

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

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