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Advanced Organic Chemistry: Carbon-13 NMR spectrum of 2-methylbut-2-ene (2-methyl-2-butene)

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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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C-13 NMR spectroscopy - spectra index

C5H10 C-13 nmr spectrum of 2-methylbut-2-ene (2-methyl-2-butene) analysis of chemical shifts ppm interpretation of C-13 chemical shifts ppm of 2-methylbut-2-ene (2-methyl-2-butene) C13 13-C nmr doc brown's advanced organic chemistry revision notes 

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.

2-methylbut-2-ene C5H10 alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b

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.


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