Interpreting the Carbon-13 NMR spectrum of E-but-2-ene and Z-but-2-ene (C13 NMR spectra of the cis/trans stereoisomers of but-2-ene, 2-butene)

[Author ©  Dr WP Brown PhD: Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry, IB chemistry students US K12 grade 11 grade 12 organic chemistry courses involving molecular spectroscopy analysing C-13 NMR spectra of E-but-2-ene and Z-but-2-ene [updated Mar 20th 2026 *]

 email doc brown   Re-edit of 13C NMR spectrum of the geometric E/Z isomers of CH₃CH=CHCH₃

(other names: E-2-butene & Z-2-butene, trans-but-2-ene & cis-but-2-ene, trans-2-butene & cis-2-butene)

TMS is the acronym for tetramethylsilane, formula Si(CH₃)₄, whose ¹³C atoms are arbitrarily given a chemical shift of 0.0 ppm. This is the 'standard' in ¹³C NMR spectroscopy and all other ¹³C resonances, called chemical shifts, are measured with respect to the TMS, and depend on the individual (electronic) chemical environment of the ¹³C atoms in an organic molecule - E-but-2-ene and Z-but-2-ene here.

 , structural formula of but-2-ene , but doesn't show the two different spatial arrangements possible due to a high energy barrier to rotation about the double bond, known as the E/Z stereoisomers (cis/trans)

The molecular structure and naming of alkenes

Interpreting the C-13 NMR spectrum of E-but-2-ene and Z-but-2-ene

As you can see from the diagram above there are 2 different chemical shift lines in the C-13 NMR spectrum of E-but-2-ene and Z-but-2-ene indicating 2 different chemical environments of the 4 carbon atoms of E-but-2-ene and Z-but-2-ene (trans-but-2-ene and cis-but-2-ene).

CH₃-CH=CH-CH₃

(Note the 2 different colours indicating the 2 different chemical environments of the 4 carbon atoms in E-but-2-ene and Z-but-2-ene).

¹³C chemical shifts δ (a) and (b) on the C-13 NMR spectrum diagram for E-but-2-ene and Z-but-2-ene. The different stereoisomers of the same structural formula produce two slightly different chemical environments giving rise to two slight different C-13 NMR chemical shifts.

  E/trans isomer and  Z/cis isomer of but-2-ene

(a)  CH₃-CH=CH-CH₃ : ¹³C NMR chemical shift δ of 17(E/trans isomers) and 12 (Z/cis isomers) ppm for the methyl carbon atoms.

In each isomer, both methyl carbon atoms are equivalent to each other because of the symmetry of both of the but-2-ene molecular isomers.

(b)  CH₃-CH=CH-CH₃ : ¹³C NMR chemical shift δ of 126 (E/trans) and 124 (Z/cis) ppm for the CH carbon atoms.

Again, in both isomers, the CH carbon atoms are equivalent to each other because of the symmetry of both of the but-2-ene molecular stereoisomers.

For both isomers the pairs of 13C NMR resonances, albeit wide apart, are quite similar and note the much larger ¹³C NMR chemical shift for the CH proton adjacent to the C=C double bond.

The carbon-13 NMR spectra provides direct evidence of 2 different carbon atom environments for the 4 carbon atoms in the E-but-2-ene and Z-but-2-ene molecule, deduced from the presence of 2 different ¹³C NMR chemical shifts (ppm).

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