Advanced Organic Chemistry: H-1 NMR spectrum E-pent-2-ene and Z-pent-2-ene

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The H-1 hydrogen-1 (proton) NMR spectrum of E-pent-2-ene and Z-pent-2-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 E-pent-2-ene and Z-pent-2-ene (comparing the 1H proton NMR spectra of cis/trans isomers of 2-pentene)

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H-1 proton NMR spectroscopy - spectra index

C5H10 low and high resolution H-1 proton nmr spectrum of E-pent-2-ene 2-pentene analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 1-H nmr for trans-pent-2-ene doc brown's advanced organic chemistry revision notes

The H-1 proton NMR spectrum of the stereoisomer E-pent-2-ene (trans-2-pentene)

 

C5H10 low and high resolution H-1 proton nmr spectrum of 2-pentene Z-pent-2-ene analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 1-H nmr for cis-pent-2-ene doc brown's advanced organic chemistry revision notes

The H-1 proton NMR spectrum of the stereoisomer Z-pent-2-ene (cis-2-pentene)

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 - E-pent-2-ene and Z-pent-2-ene here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of E-pent-2-ene and Z-pent-2-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 E-pent-2-ene and Z-pent-2-ene molecule.

Pent-2-ene C5H10, alkenes structure and naming (c) doc b has two E/Z isomers E-pent-2-ene and Z-pent-2-ene

Z/cis- alkenes structure and naming (c) doc b , E/trans- alkenes structure and naming (c) doc b

Interpreting the H-1 NMR spectrum of E-pent-2-ene and Z-pent-2-ene

For relatively simple molecules, the low resolution H-1 NMR spectrum of E-pent-2-ene and Z-pent-2-ene is a good starting point (low resolution diagram above), but two of the resonances (CH=CH protons) are close together and would show up as a ratio number of (2) compared to CH2 (2) and 2 x CH3 (2 x 3).

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

CH3CH=CHCH2CH3

Note the proton ratio 3:1:1:2:3 of the five colours of the protons in the five chemically different environments

Although there are 10 hydrogen atoms in the molecule, there are only ? possible different chemical environments for the hydrogen atoms in E-pent-2-ene and Z-pent-2-ene molecule.

The integrated signal proton ratio ? observed, corresponds with the structural formula of E-pent-2-ene and Z-pent-2-ene.

The high resolution H-1 NMR spectrum of E-pent-2-ene and Z-pent-2-ene

At very high resolution, the H-1 NMR spectra of E-pent-2-ene and Z-pent-2-ene show 5 groups of proton resonances and in the 3:1:1:2:3 ratio expected from the formula of pent-2-ene.

The ppm quoted on the diagram represent the peak of resonance intensity for a particular proton group in the molecule of E-pent-2-ene and Z-pent-2-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 E-pent-2-ene and Z-pent-2-ene below.

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

The H-1 NMR spectra for the E/Z stereoisomers of pent-2-ene are quite similar, with only small differences in the 1H chemical shifts for each group of protons (a) to (e).

(a) 1H Chemical shift for the CH3 protons CH3CH=CHCH2CH3

1.64 ppm (E) and 1.60 ppm (Z):

Both resonances split into a 1:1 doublet by the 'purple' CH proton (n+1 = 2).

Evidence for the presence of a CH group in the molecule of E-pent-2-ene and Z-pent-2-ene

(b) 1H Chemical shift for the CH proton CH3CH=CHCH2CH3

5.40 ppm (E) and 5.40 ppm (Z):

Both resonances split into a 1:4:6:4:1 quintet by the 'blue' CH3 and 'green' CH protons (n+1 = 5)

Evidence for the presence of a CH3-CH-CH group in the molecule of E-pent-2-ene and Z-pent-2-ene (but can get a quintet from CH2-CH-CH2 grouping too).

(c) 1H Chemical shift for the CH proton CH3CH=CHCH2CH3

5.47 ppm (E) and 5.38 ppm (Z):

Both resonances split into a 1:3:3:1 quartet by the 'purple' CH proton and the 'brown' CH2 protons (n+1 = 4).

(b) and (c) resonances are very close together and need a very high resolution spectra of E/Z pent-2-ene to sort them out.

(d) 1H Chemical shift for the CH2 protons CH3CH=CHCH2CH3

1.97 ppm (E) and 2.04 ppm (Z):

Both resonances split into a 1:4:6:4:1 quintet by the 'red' CH3 and 'green' CH protons (n+1 = 5)

Evidence for the presence of a CHCH2CH3 group in the molecule of E-pent-2-ene and Z-pent-2-ene (but can get a quintet from CH2-CH-CH2 grouping too).

(e) 1H Chemical shift for the CH3 protons CH3CH=CHCH2CH3

0.96 ppm (E) and 0.96 ppm (Z):

Both resonances split into a 1:2:1 triplet by the 'brown' CH2 protons (n+2 = 3)

Evidence for the presence of a CH2 group in the molecule of E-pent-2-ene and Z-pent-2-ene

Number of directly adjacent 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 E-pent-2-ene and Z-pent-2-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)

The infrared spectra of the E/Z isomers of pent-2-ene (cis/trans isomers of 2-pentene)

The mass spectra of the E/Z isomers of pent-2-ene (cis/trans isomers of 2-pentene)

The C-13 NMR spectra of the E/Z isomers of pent-2-ene (cis/trans isomers of 2-pentene)

ALL SPECTROSCOPY INDEXES

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

All Advanced Organic Chemistry Notes

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