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Advanced Organic Chemistry: Carbon-13 NMR spectrum of ethylbenzene

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The Carbon-13 NMR spectrum of ethylbenzene

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 ethylbenzene

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C-13 NMR spectroscopy - spectra index

C8H10 C-13 nmr spectrum of ethylbenzene analysis of chemical shifts ppm interpretation of C-13 chemical shifts ppm of ethylbenzene 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 - ethylbenzene here.

Ethylbenzene, C8H10 , C6H5CH2CH3 , (c) doc b , (c) doc b

Interpreting the C-13 NMR spectrum of ethylbenzene

As you can see from the diagram above there are 6 different chemical shift lines in the C-13 NMR spectrum of ethylbenzene indicating 6 different chemical environments of the carbon atoms.

Chemical shifts (a) to (f) on the C-13 NMR spectrum diagram for ethylbenzene.

13C resonance (a), chemical shift 125.7 ppm, corresponds to benzene ring carbon atom 4, furthest from the side-chain alkyl group.

13C resonance (b), chemical shift 128.4, ppm corresponds to benzene ring carbon atoms 3 and 5 (both in the same chemical environment).

13C resonance (c), chemical shift 127.9 ppm, corresponds to benzene ring carbon atoms 2 and 6 (both in the same chemical environment).

Resonances (a) to (c) are in very similar (electronic) chemical environments and therefore have very similar chemical shifts.

13C resonance (d), chemical shift 144.2 ppm, corresponds to benzene ring carbon atom 1, and, being connected to the alkyl group, shows a significantly greater chemical shift than ring carbon atoms 2 to 6.

The other five ring hydrogen atoms are not substituted, hence the similarity of their chemical environments and chemical shifts of their carbon atoms.

13C resonance (e), chemical shift 29.0 ppm, corresponds to the CH2 carbon atom of the alkyl side-chain.

13C resonance (f), chemical shift 15.6 ppm, corresponds to the CH3 carbon atom of the alkyl side-chain.

Note the 'non-aromatic' chemical environment of these two alkyl carbon atoms results in significant different, and much lower chemical shifts compared to the aromatic carbon atoms of the benzene ring.

The carbon-13 NMR spectra a provides direct evidence of 6 different carbon atom environments in the ethylbenzene molecule from 6 different chemical shifts (ppm), but you need very high resolution to sort out the benzene ring carbon atoms in C-13 NMR spectrum of ethylbenzene..


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