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

Interpreting the H-1 hydrogen-1 (proton) NMR spectrum of 2-iodobutane

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-iodobutane

See also comparison of the infrared, mass, 1H NMR and 13C NMR spectra of the four isomers of C4H9I

1H proton nmr spectrum of 2-iodobutane low/high resolution diagrams C4H9I CH3CHICH2CH3 analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for sec-butyl 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 - 2-iodobutane here.

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

2-iodobutane (sec-butyl iodide), C4H9I, CH3-CHI-CH2-CH3

Interpreting the H-1 NMR spectrum of 2-iodobutane

In terms of spin-spin coupling from the possible proton magnetic orientations, for 2-iodobutane 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 2-iodobutane is not necessarily a good starting point (low resolution diagram above)!

The 9 hydrogen atoms (protons) of 2-iodobutane appear to occupy 4 different chemical environments so that the low resolution NMR spectra shows 4 principal resonance peaks of different H-1 NMR chemical shifts (diagram above for 2-iodobutane).

CH3-CHI-CH2-CH3

Note the proton ratio 3:1:2:3 of the 4 colours of the 9 protons of 2-iodobutane in the 4 chemically different proton environments

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

BUT, unfortunately, this is an over simplification of the 1H NMR spectrum of 2-iodobutane.

The integrated proton signal ratio of 3:1:1:1:3 observed in the very high resolution H-1 NMR spectrum, does not appear to correspond with the structural formula of 2-iodobutane (but, see notes below for an explanation).

The high resolution 1H NMR spectrum of 2-iodobutane

The high resolution spectra of 2-iodobutane shows 5 groups of proton resonances and not in the ratio expected from the structural formula of 2-iodobutane, but we can now consider the splitting of resonance lines from the spin-spin coupling in the molecule of 2-iodobutane.

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

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

1H NMR resonance (a) 1H Chemical shift 1.92 ppm: CH3-CHI-CH2-CH3

This 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 2-iodobutane.

There is no ambiguity about this interpretation, however, the same cannot be said for all the other resonances (b) to (d)!

1H NMR resonance (b) 1H Chemical shift 4.16 ppm: CH3-CHI-CH2-CH3

At first sight, this CH proton resonance should be split into a 1:5:10:10:5:1 sextet by the adjacent CH2 and CH3 protons (n+5 = 6).

BUT, according the data I obtained, the CH2 protons are NOT quite equivalent, and you can see on the spectrum diagram it looks more like a quintet?

Therefore are we dealing with two closely overlapping 1:4:6:4:1 quintets from two splittings of the CH proton resonance by CH3 protons and a 'CH' proton from the CH2 group? (n+1 = 5).

I'm not entirely sure what is the correct interpretation for resonance (b).

1H NMR resonance (c) 1H Chemical shift 1.69 and 1.81 ppm: CH3-CHI-CH2-CH3

At first sight, this resonance should be split into a 1:4:6:4:1 quintet by the adjacent CH and CH3 protons (n+4 = 5).

BUT, as already mentioned above, according the data I obtained from the internet, the CH2 protons are NOT quite equivalent, and you can see on the spectrum diagram there are two multi-split resonances very close together.

So, each of them could be due to each CH2 proton resonance being split by the adjacent CH and CH3 protons (n+1 = 5), so are we dealing with two overlapping quintets?

I'm not entirely sure what is the correct interpretation for resonance (c).

A very advanced footnote concerning resonances (b) to (d):

2-iodobutane is an asymmetric molecule and exhibits R/S isomerism (optical isomerism, mirror image enantiomers). This asymmetry actually affects the CH2 protons field, and they actually have slightly different chemical shifts of 1.69 and 1.81, slightly different 1H chemical environments. Since this is a pre-university website I have outlined various arguments as a compromise!

1H NMR resonance (d) 1H Chemical shift 1.00 ppm: CH3-CHI-CH2-CH3

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

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

However, this interpretation ignores the fact the CH2 protons are not equivalent, so is this peak an overlap of two doublets?

I'm not entirely sure what is the correct interpretation for resonance (d).

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


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
Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 4 halogenoalkane isomers of C4H9I

NOTE: The images are linked to their original detailed spectral analysis pages AND can be doubled in size with touch screens to increase the definition to the original 1-iodobutane, 2-iodobutane, 1-iodo-2-methylpropane and 2-iodo-2-methylpropane image sizes.  These four molecules are structural isomers of molecular formula C4H9I and exemplify the infrared, mass, 1H NMR and 13C NMR spectra of lower aliphatic halogenoalkanes (haloalkanes, alkyl halides, iodoalkanes, alkyl iodides).

INFRARED SPECTRA (above): Apart from the significant differences in the fingerprint region at wavenumbers 1500 to 400 cm-1, there are no other great striking differences, but each could be identified from its infrared spectrum.

MASS SPECTRA (above): All four give the parent molecular ion of m/z 184, but it is only a relatively tiny peak for 2-iodo-2-methylpropane. All four give the base ion peak of m/z 57. All four give prominent peaks for m/z ions 29 and 41 and all give a tiny peak from an ionised iodine atom at m/z 127. They look quite similar to me and lack a clear fingerprint fragmentation pattern.

1H NMR SPECTRA (above): The 1H NMR spectra of all three molecules give different proton ratios i.e.1-iodobutane four peaks 3:2:2:2, 2-iodobutane four peaks 3:3:2:1, 1-iodo-2-methylpropane three peaks 6:2:1 and 2-iodo-2-methylpropane one peak '1' (effectively no ratio involved), so all four molecular structures can be distinguished from each other by their 1H NMR spectra proton ratios, numbers of peaks and (n+1) rule splitting patterns.

13C NMR SPECTRA (above): The 13C NMR spectra of the four molecules show various numbers of carbon-13 chemical environments i.e 1-iodobutane and 2-iodobutane show four 13C NMR resonances, 1-iodo-2-methylpropane three 13C NMR resonances and 2-iodo-2-methylpropane only two 13C resonances. Therefore 1-iodo-2-methylpropane and 2-iodo-2-methylpropane can be distinguished from the other three by their number of resonances in their 13C NMR spectra, but 1-iodobutane and 2-iodobutane cannot be distinguished from each other from their number of 13C NMR resonance lines - other data would be required.

Key words & phrases: isomer of molecular formula C4H9I CH3CHICH2CH3 CH3CH2CHICH3 Interpreting the proton H-1 NMR spectra of 2-iodobutane, low resolution & high resolution proton nmr spectra of 2-iodobutane, H-1 nmr spectrum of 2-iodobutane, understanding the hydrogen-1 nmr spectrum of 2-iodobutane, explaining the line splitting patterns from spin-spin coupling  in the high resolution H-1 nmr spectra of 2-iodobutane, revising the H-1 nmr spectrum of 2-iodobutane, proton nmr of 2-iodobutane, ppm chemical shifts of the H-1 nmr spectrum of 2-iodobutane, explaining and analyzing spin spin line splitting in the H-1 nmr spectrum, how to construct the diagram of the 1H nmr spectrum of 2-iodobutane, how to work out the number of chemically different protons in the structure of the 2-iodobutane organic molecule, how to analyse the chemical shifts in the hydrogen-1 H-1 proton NMR spectrum of 2-iodobutane using the n+1 rule to explain the spin - spin coupling ine splitting in the proton nmr spectrum of 2-iodobutane deducing the nature of the protons from the chemical shifts ppm in the H-1 nmr spectrum of 2-iodobutane examining the 1H nmr spectrum of 2-iodobutane analysing the 1H nmr spectrum of 2-iodobutane how do you sketch and interpret the H-1 NMR spectrum of 2-iodobutane interpreting interpretation of the 1H proton spin-spin coupling causing line splitting in the NMR spectrum of 2-iodobutane  assignment of chemical shifts in the proton 1H NMR spectrum of 2-iodobutane formula explaining spin-spin coupling for line splitting for 2-iodobutane haloalkane halogenoalkane alkyl iodide sec-butyl iodide functional group

Molecular structure diagram of the proton NMR diagram for the 1H NMR spectrum of 2-iodobutane. The proton ratio in the 1H NMR spectrum of 2-iodobutane. Deducing the number of different chemical environments of the protons in the 2-iodobutane molecule from the 1H chemical shifts in the hydrogen-1 NMR spectrum of 2-iodobutane. Analysing the high resolution 1H NMR spectrum of 2-iodobutane. Analysing the low resolution 1H NMR spectrum of 2-iodobutane. You may need to know the relative molecular mass of 2-iodobutane to deduce the molecular formula from the proton ratio of the 1H NMR spectrum of 2-iodobutane. Revision notes on the proton NMR spectrum of 2-iodobutane. Matching and deducing the structure of the 2-iodobutane molecule from its hydrogen-1 NMR spectrum. Proton NMR spectroscopy of halogenoalkanes iodoalkanes, 1H NMR spectra of 2-iodobutane, an isomer of molecular formula C4H9I


Links associated with 2-iodobutane

The chemistry of HALOGENOALKANES (haloalkanes) revision notes INDEX

The infrared spectrum of 2-iodobutane (sec-butyl iodide)

The mass spectrum of 2-iodobutane (sec-butyl iodide)

The C-13 NMR spectrum of 2-iodobutane (sec-butyl iodide)

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

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