Advanced Organic Chemistry: 1H NMR spectrum of but-1-ene

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

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 but-1-ene

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H-1 proton NMR spectroscopy - spectra index

1H proton nmr spectrum of but-1-ene low/high resolution diagrams C4H8 CH3CH2CH=CH3 analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for 1-butene 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 - but-1-ene here.

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

But-1-ene, alkenes structure and naming (c) doc b, alkenes structure and naming (c) doc b , alkene

Interpreting the H-1 NMR spectrum of but-1-ene

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

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

The 8 hydrogen atoms (protons) of but-1-ene seem to occupy 4 different chemical environments in terms of the low resolution NMR spectra, but the very spectrum shows the H2C= protons are slightly different in terms of their H-1 NMR chemical shifts - chemical environments (diagram above for but-1-ene).

H2C=CH-CH2-CH3

Note the proton ratio 2:1:2:3 ratio of the 4 colours of the 8 protons of but-1-ene in the 4 chemically different proton environments  - for the moment ignoring the small difference in the two H2C= proton environments of but-1-ene.

Chemical shifts (a) to (e) on the H-1 NMR spectrum diagram for but-1-ene.

The integrated signal proton ratio 2:1:2:3 observed in the low resolution H-1 NMR spectrum, corresponds with the structural formula of but-1-ene, but at very high resolution the ratio becomes 1:1:1:2:3. At pre-university level the low resolution ratio will probably suffice.

The high resolution 1H NMR spectrum of but-1-ene

The high resolution spectra of but-1-ene shows ? groups of proton resonances and in the ? ratio expected from the structural formula of but-1-ene.

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

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

Resonances (a) and (b) 1H Chemical shifts 4.90 and 4.95: H2C=CH-CH2-CH3

At first you might expect to see the H2C= proton resonance split into a 1:1 doublet by the adjacent CH proton (n+1 = 2).

However, because the H2C= protons have slightly different chemical environments (0.05 ppm difference in chemical shifts), the result is a more complex resonance line pattern because of the lack of equivalence of these two protons (several triplets?).

I presume the difference in the two H2C= proton chemical shifts is due to slightly different shielding effects from the asymmetry of the groups on the other side of the double bond, about which there is no free rotation.

Resonance (c) 1H Chemical shift 5.86 ppm: H2C=CH-CH2-CH3

Again, at first you might expect to see the CH proton resonance split into a 1:4:6:4:1 quintet by the adjacent H2C= and -CH2- protons (n+1 = 5).

However, again, because the H2C= protons have slightly different chemical environments (0.05 ppm difference in chemical shifts), the result is a more complex resonance line pattern because of the lack of equivalence of these two protons (several quartets?).

Resonance (d) 1H Chemical shift 2.04 ppm: H2C=CH-CH2-CH3

This CH2 proton resonance is split by the adjacent CH and CH3 protons into a 1:4:6:4:1 quintet (n+1 = 5).

Evidence for the presence of a CH-C-CH3 grouping in the molecule of but-1-ene.

Resonance (e) 1H Chemical shift 0.95 ppm: H2C=CH-CH2-CH3

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

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

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 but-1-ene

The chemistry of ALKENES revision notes INDEX

The infrared spectrum of but-1-ene

The mass spectrum of but-1-ene

The C-13 NMR spectrum of but-1-ene

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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