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Advanced Organic Chemistry: H-1 hydrogen-1 proton NMR spectrum of Pent-1-ene

H-1 hydrogen-1 (proton) NMR spectrum of Pent-1-ene (1-pentene)

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

low and high resolution H-1 proton nmr spectrum of pent-1-ene 1-pentene analysis interpretation of chemical shifts ppm spin spin line splitting diagram 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 - pent-1-ene here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of pent-1-ene represent the peaks of the intensity of the chemical shifts of pent-1-ene (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 pent-1-ene molecule.

Pent-1-ene, alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b (1-pentene)

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

For relatively simple molecules, the low resolution H-1 NMR spectrum of ? is a good starting point.

The hydrogen atoms (protons) of pent-1-ene occupy 5 different chemical environments so that the H-1 proton low resolution NMR spectra should show 5 peaks, which it does (diagram above).

CH3CH2CH2CH=CH2  (note the five different colours of the protons in pent-1-ene)

Since all 5 hydrogen atoms in the molecule are in different chemical environments, pent-1-ene will show 5 different chemical shifts in its H-1 NMR spectrum.

The ratio of the area under the NMR 'peaks' should be 3 : 2 : 2 : 1 : 2 from the molecular structured of pent-1-ene (colour coded above) i.e. the proton ratios in the molecule.

As you can see, the high resolution spectrum of pent-1-ene is complex

The end CH3 group is split into a triplet (1 : 2 : 1 lines at 0.91 ppm) by the adjacent CH2 group.

The end =CH2 group is split into a doublet (1 : 1 lines at 4.97 ppm) by the adjacent CH= group.

Both sets of CH2 protons and the HC= proton are split on both sides by adjacent non-equivalent protons into multiple resonance lines.

CH3CH2CH2CH=CH2

The 1.43 ppm chemical shift:

From the n+1 rule, the 'left-hand' 1st CH2 (H2) protons are split by the CH3 (H3) protons and by the 'right-hand' 2nd CH2 protons (H2), (5 protons in total), into a 1:5:10:10:5:1 sextet of resonance lines.

This is pattern of resonances is a good indication of a propyl group (CH3CH2CH2).

The 2.02 ppm chemical shift:

The 'middle' CH2 protons (H2) are split by the left CH2 protons (H2) and the CH proton on the right (H), (3 protons in total), in to a 1:3:3:1 quartet of resonance lines.

The 5.81 ppm chemical shift:

The CH proton (H) is split by the left CH2 protons (H2) and the end CH2 protons (H), (4 protons in total), into a 1:4:6:4:1 quintet of resonance lines.

Non of these is clear on the H-1 NMR spectra of pent-1-ene shown above.


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

Key words & phrases: Interpreting the proton H-1 NMR spectra of pent-1-ene 1-pentene, low resolution & high resolution proton nmr spectra of pent-1-ene 1-pentene, H-1 nmr spectrum of pent-1-ene 1-pentene, understanding the hydrogen-1 nmr spectrum of pent-1-ene 1-pentene, explaining the line splitting patterns in the high resolution H-1 nmr spectra of pent-1-ene 1-pentene, revising the H-1 nmr spectrum of pent-1-ene 1-pentene, proton nmr of pent-1-ene 1-pentene, ppm chemical shifts of the H-1 nmr spectrum of pent-1-ene 1-pentene, explaining and analyzing spin spin line splitting in the H-1 nmr spectrum, how to construct the diagram of the H-1 nmr spectrum of pent-1-ene 1-pentene, how to work out the number of chemically different protons in the structure of the pent-1-ene 1-pentene organic molecule


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