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

email doc brown

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.

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..