Advanced Organic Chemistry: H-1 NMR spectrum of 2-methylbut-1-ene

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

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H-1 proton NMR spectroscopy - spectra index

low and high resolution H-1 proton nmr spectrum of 2-methylbut-1-ene analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 1-H nmr for 2-methylbut-1-ene 2-methyl-1-butene 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 shifts, called chemical shifts, depend on the individual (electronic) chemical environment of the hydrogen atoms in an organic molecule - 2-methylbut-1-ene here.

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

2-methylbut-1-ene C5H10, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b

Interpreting the H-1 NMR spectrum of 2-methylbut-1-ene

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

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

H2C=C(CH3)CH2CH3

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

Although there are 10 hydrogen atoms in the molecule, there only 4 possible different chemical environments for the hydrogen atoms in 2-methylbut-1-ene molecule.

The integrated signal proton ratio ? observed, corresponds with the structural formula of 2-methylbut-1-ene.

The high resolution H-1 NMR spectrum of 2-methylbut-1-ene

All low and high resolution spectra of 2-methylbut-1-ene show ? groups of proton resonances and in the ratio expected from the formula of 2-methylbut-1-ene.

The ppm quoted on the diagram represent the peak of resonance intensity for a particular proton group in the molecule of 2-methylbut-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 2-methylbut-1-ene below.

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

(a) 1H Chemical shift 4.67 ppm CH2 protons H2C=C(CH3)CH2CH3

The 'blue' CH2 proton resonance is not affected by any directly adjacent protons.

(b) 1H Chemical shift 1.73 ppm CH3 protons H2C=C(CH3)CH2CH3

The 'purple' CH3 proton resonance is not affected by any directly adjacent protons.

(c) 1H Chemical shift 2.02 ppm CH2 protons H2C=C(CH3)CH2CH3

The 'green' CH2 proton resonance is split into a 1:3:3:1 quartet by the 'brown' CH3 protons (n+1 = 4).

Evidence for the presence of a CH3 group in the molecule of 2-methylbut-1-ene

(d) 1H Chemical shift 1.03 ppm CH3 protons H2C=C(CH3)CH2CH3

The 'brown' CH3 proton resonance is split into a 1:2:1 triplet by the 'green' CH2 protons (n+1 = 3).

Evidence for the presence of a CH2 group in the molecule of 2-methylbut-1-ene

Number of protons 1H causing splitting Splitting pattern produced from the n+1 rule 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-methylbut-1-ene

The chemistry of ALKENES revision notes INDEX

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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