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Advanced Organic Chemistry: Carbon-13 NMR spectrum of 3-methylpentane

The Carbon-13 NMR spectrum of 3-methylpentane

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 3-methylpentane

See also comparing infrared, mass, 1H NMR & 13C NMR spectra of the structural alkane isomers of C6H14

C-13 nmr spectrum of 3-methylpentane analysis of chemical shifts ppm interpretation of C-13 chemical shifts ppm of 3-methylpentane 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 - 3-methylpentane here.

3-methylpentane C6H14, alkanes structure and naming (c) doc b , alkanes structure and naming (c) doc b , alkanes structure and naming (c) doc b

For more see The molecular structure, classification and naming of alkanes

Interpreting the C-13 NMR spectrum of 3-methylpentane

As you can see from the diagram above there are 4 different chemical shift lines in the C-13 NMR spectrum of 3-methylpentane indicating 4 different chemical environments of the carbon atoms.

CH3CH2CH(CH3)CH2CH3  or  (CH3CH2)2CHCH3  

(Note the 4 colours indicating the 4 different chemical environments of the six carbon atoms in 3-methylpentane).

Note the symmetry of the molecule - six carbon atoms, but only four different chemical environments for the in the 3-methylhexane molecule.

The end methyl group carbon atoms are equivalent to each other, as are the pair of CH2 group carbon atoms - same chemical environments.

The carbon-13 NMR spectra a provides direct evidence of 4 different carbon atom environments in the 3-methylpentane molecule from 4 different chemical shifts (ppm).

Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the five structural alkane isomers of C6H14

NOTE: The images are linked to their original detailed spectral analysis pages AND can be doubled in size with touch screens to increase the definition to the original hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane image sizes.  These five molecules are structural isomers of saturated alkanes of molecular formula C6H14 and exemplify the infrared, mass, 1H NMR and 13C NMR spectra of lower aliphatic alkanes (non-cyclic alkanes).

Infrared spectra below.

INFRARED SPECTRA:

Apart from the significant differences in the fingerprint region at wavenumbers 1500 to 400 cm-1, there are no other great striking differences, but each could be identified from its infrared spectrum.

All the absorption bands are typical of molecules containing saturated alkyl structure and there are no characteristic infrared absorptions due to a specific functional group.

Infrared spectra above, mass spectra below.

MASS SPECTRA: Base ion peaks plus m/z comments.

Hexane: m/z 57, 42 and 56 prominent

2-methylpentane: m/z 43, 42 and 71 prominent

3-methylpentane: m/z 57, 41 and 56 prominent

2,2-dimethylbutane: m/z 43, 41, 57 and 71 prominent

2,3-dimethylbutane: m/z 43, 41, 42 and 71 prominent

Mass spectra above, 1H NMR spectra below.

1H NMR SPECTRA: They can all be distinguished by their different integrated proton ratios - need very high resolution.

Hexane: 3 1H δ shifts, H ratio 3:2:2 (6:4:4 in formula)

2-methylpentane: 5 1H δ shifts, H ratio 6:3:2:2:1

3-methylpentane: 4 1H δ shifts, H ratio 6:4:3:1

2,2-dimethylbutane: 3 1H δ shifts, H ratio 9:3:2

2,3-dimethylbutane: 2 1H δ shifts, H ratio 6:1 (12:2 in formula)

1H NMR spectra above, 13C NMR spectra below.

13C NMR SPECTRA: From the number of shifts, you can't distinguish (iii) and (iv) but you can distinguish them from (i), (ii) and (v). (i) Hexane: 3 13C δ shifts

(ii) 2-methylpentane: 5 13C δ shifts

(iii) 3-methylpentane: 4 13C δ shifts

(iv) 2,2-dimethylbutane: 4 13C δ shifts

(v) 2,3-dimethylbutane: 2 13C δ shifts

13C NMR spectra above.

Key words & phrases: Interpreting the C-13 NMR spectra of 3-methylpentane, C-13 nmr spectrum of 3-methylpentane, understanding the carbon-13 nmr spectrum of 3-methylpentane, explaining the line pattern in the high resolution C-13 nmr spectra of 3-methylpentane, revising the C-13 nmr spectrum of 3-methylpentane, ppm chemical shifts of the C-13 nmr spectrum of 3-methylpentane, how to construct the diagram of the C-13 nmr spectrum of 3-methylpentane, how to analyse the chemical shifts in the carbon-13 NMR spectrum of 3-methylpentane deducing the chemical environment of all the carbon atoms in 3-methylpentane examining the c13 nmr spectrum of  3-methylpentane analysing the 13-c nmr spectrum of 3-methylpentane how do you sketch and interpret the C-13 NMR spectrum of 3-methylpentane interpreting interpretation of the C-13 NMR spectrum of 3-methylpentane

Molecular structure diagram of the carbon-13 NMR diagram for the 13C NMR spectrum of 3-methylpentane. Deducing the number of different chemical environments of the carbon atoms in the 3-methylpentane molecule from the 13C chemical shifts in the carbon-13 NMR spectrum of 3-methylpentane. Revision notes on the carbon-13 NMR spectrum of 3-methylpentane. Matching and deducing the structure of the 3-methylpentane molecule from its 13C NMR spectrum. Carbon-13 NMR spectroscopy of  aliphatic alkanes, 13C NMR spectra of 3-methylpentane, a structural isomer of molecular formula C6H14


Links associated with 3-methylpentane

The chemistry of ALKANES revision notes INDEX

The infrared spectrum of 3-methylpentane

The mass spectrum of 3-methylpentane

The H-1 NMR spectrum of 3-methylpentane

C-13 NMR spectroscopy index

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