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

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Interpreting H-1 proton NMR spectrum: 2-methylpropene (2-methylprop-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 2-methylpropene

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H-1 proton NMR spectroscopy - spectra index

1H proton nmr spectrum of 2-methylpropene low/high resolution diagrams C4H8 (CH3)2C=CH2 analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for 2-methylprop-1-ene 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-methylpropene here.

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

2-methylpropene, alkenes structure and naming (c) doc b, alkenes structure and naming (c) doc b, alkenes structure and naming (c) doc b , skeletal formula of methylpropene 2-methylpropene advanced organic chemistry

Interpreting the H-1 NMR spectrum of 2-methylpropene

In terms of spin-spin coupling from the possible proton magnetic orientations, for 2-methylpropene I have only considered the interactions of non-equivalent protons on adjacent carbon atoms e.g. -CH2-CH3, -CH2-CH2- protons etc, BUT this does not apply to the molecular structure of 2-methylpropene.

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

The 8 hydrogen atoms (protons) of 2-methylpropene occupy 2 different chemical environments so that the low resolution NMR spectra should show 2 principal resonance peaks of different H-1 NMR chemical shifts (diagram above for 2-methylpropene).

(CH3)2C=CH2

Note the proton ratio 6:2  of the 2 colours of the 8 protons of 2-methylpropene in the 2 chemically different proton environments

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

Although there are 8 hydrogen atoms in the molecule, the proton NMR spectrum shows there are only 2 possible different chemical environments for the hydrogen atoms in 2-methylpropene molecule.

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

The high resolution 1H NMR spectrum of 2-methylpropene

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

However, there are no adjacent protons on two adjacent carbon atom, so applying the n+1 rule to 2-methylpropene isn't necessary! (but important in working the other way round to deduce the structure of 2-methylpropene or other molecule - everything is a clue!)

1H NMR resonance (a) 1H Chemical shift δ 1.75 ppm: (CH3)2C=CH2

The methyl group protons are detected as a single singlet resonance.

All these 6 protons are equivalent to each other, and equivalent protons do not split each other's resonances.

Evidence for the presence of a CHx grouping, that is not next to another CHx grouping, in the molecule of 2-methylpropene (x = 1 to 3).

1H NMR resonance (b) 1H Chemical shift δ 4.70 ppm: (CH3)2C=CH2

The =CH2 protons are detected as a single singlet resonance.

Both protons are equivalent to each other, and equivalent protons do not split each other's resonances.

Evidence for the presence of a second CHx grouping (different 1H chemical shift), that is not next to another CHx grouping, in the molecule of 2-methylpropene (x = 1 to 3).

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 2-methylpropene

The chemistry of ALKENES revision notes INDEX

The infrared spectrum of 2-methylpropene (2-methylprop-1-ene)

The mass spectrum of 2-methylpropene (2-methylprop-1-ene)

The C-13 NMR spectrum of 2-methylpropene (2-methylprop-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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