The Carbon-13 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 C-13 NMR spectra of 2-methylbut-2-ene (2-methyl-2-butene)

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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 shifts, called chemical shifts, depend on the individual (electronic) chemical environment of the ¹³C atoms in an organic molecule - 2-methylbut-2-ene (2-methyl-2-butene) here.

Interpreting the C-13 NMR spectrum of 2-methylbut-2-ene (2-methyl-2-butene)

As you can see from the diagram above there are 5 different chemical shift lines in the C-13 NMR spectrum of 2-methylbut-2-ene (2-methyl-2-butene) indicating 5 different chemical environments of the carbon atoms.

CH₃C(CH₃)=CHCH₃

(Note the 5 colours indicating the 5 different chemical environments of the carbon atoms in 2-methylbut-2-ene (2-methyl-2-butene).

The carbon-13 NMR spectra a provides direct evidence of 5 different carbon atom environments in the 2-methylbut-2-ene (2-methyl-2-butene) molecule from 5 different chemical shifts (ppm).

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 carbon atoms of the two methyl groups attached to the 'purple' carbon of the C=C bond, because there is a difference in their ¹³C chemical shifts (17.3 and 25.7 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.