Organic Chemistry: H-1 NMR spectrum of 2-methylbut-2-ene (2-methyl-2-butene)

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H-1 hydrogen-1 NMR spectrum of 2-methylbut-2-ene (2-methyl-2-butene)

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-2-ene (2-methyl-2-butene)

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H-1 proton NMR spectroscopy - spectra index

C5H10 low and high resolution H-1 proton nmr spectrum of 2-methylbut-2-ene (2-methyl-2-butene) analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 1-H nmr for 2-methylbut-2-ene (2-methyl-2-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-2-ene (2-methyl-2-butene) here.

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

2-methylbut-2-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-2-ene (2-methyl-2-butene)

For relatively simple molecules, the low resolution H-1 NMR spectrum of 2-methylbut-2-ene (2-methyl-2-butene) is usually a good starting point (low resolution diagram above), but not in this case

All the methyl proton groups have a similar H-1 chemical shift.

The hydrogen atoms (protons) of 2-methylbut-2-ene (2-methyl-2-butene) occupy 4 (not 3) different chemical environments in a very high resolution NMR spectra showing 5 peaks of different H-1 NMR chemical shifts.

CH3C(CH3)=CHCH3

Note the proton ratio 3:3:1:3 of the four colours of the protons in the four chemically different environments

Although there are 10 hydrogen atoms in the molecule, there are 4 possible different chemical environments for the hydrogen atoms in 2-methylbut-2-ene (2-methyl-2-butene) molecule.

The integrated signal proton ratio ?:? observed at very high resolution, corresponds with the structural formula of 2-methylbut-2-ene, as 'coloured' in the molecule above.

The high resolution H-1 NMR spectrum of 2-methylbut-2-ene (2-methyl-2-butene)

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

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

(a) 1H Chemical shift 1.68 ppm  CH3C(CH3)=CHCH3

The 'blue' methyl proton resonance should be a singlet, with no protons on the neighbouring carbon atom.

(b) 1H Chemical shift 1.56 ppm  CH3C(CH3)=CHCH3

The 'brown' methyl proton resonance will be split into a 1:1 doublet by the 'green' CH proton.

Evidence for the presence of a CH group in the molecule of 2-methylbut-2-ene (2-methyl-2-butene)

(c) 1H Chemical shift 1.60 ppm  CH3C(CH3)=CHCH3

The 'purple' methyl proton resonance should be a singlet, with no protons on the neighbouring carbon atom.

(d) 1H Chemical shift 5.19 for the CH proton CH3C(CH3)=CHCH3

The 'green' CH proton resonance is split into a 1:3:3:1 quartet by the 'brown' methyl protons.

Evidence for the presence of a CH3 group in the molecule of 2-methylbut-2-ene (2-methyl-2-butene)

Note that 2-methylbut-2-ene does NOT exhibit E/Z isomerism (cis/tans) because two of the groups on one of the carbon atoms of the double bond are identical.

See STEREOISOMERISM general definition, E/Z (cis/trans) isomerism

CH3C(CH3)=CHCH3

However, despite this, there are small differences in the field experienced by the protons of the two methyl groups attached to the 'left' carbon of the C=C bond, because there is a difference in their 1H chemical shifts (1.68 and 1.60 ppm).

This must be due to the asymmetry of the groups (H and methyl) attached to the 'right-hand' carbon atom of the C=C double bond.
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-methylbut-2-ene (2-methyl-2-butene)

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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