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Advanced Organic Chemistry: Infrared spectrum of 3-methylhexane

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Interpreting the infrared spectrum of 3-methylhexane

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 infrared spectra of 3-methylhexane

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Infrared spectroscopy - spectra index

See also comparing the 1H NMR and 13C NMR spectra of the nine alkane structural isomers of C7H16

C7H16 infrared spectrum of 3-methylhexane wavenumbers cm-1 functional group detection fingerprint pattern identification of 3-methylhexane doc brown's advanced organic chemistry revision notes 

Spectra obtained from a liquid film of 3-methylhexane. The right-hand part of the of the infrared spectrum of 3-methylhexane, wavenumbers ~1500 to 400 cm-1 is considered the fingerprint region for the identification of 3-methylhexane and most organic compounds. It is due to a unique set of complex overlapping vibrations of the atoms of the molecule of 3-methylhexane.

 3-methylhexane   C7H16   alkanes structure and naming (c) doc b

 alkanes structure and naming (c) doc b alkanes structure and naming (c) doc b

Interpretation of the infrared spectrum of 3-methylhexane

The most prominent infrared absorption lines of 3-methylhexane

From 2975 to 2845 cm-1 there are multiple peaks amounting to strong absorption due to C-H stretching vibrations in the -CH-, -CH2- or -CH3 groups in 3-methylhexane.

The are other C-H absorptions due to C-H deformation vibrations of the CH2 groups at 1480 to 1440 cm-1.

These overlap with C-H vibrations of the CH3 groups at wavenumbers 1385 to 1370 cm-1 and 1470 to 1435 cm-1.

There are weak C-H vibrations of the -CH- group at ~1340 cm-1, close to the C-H vibrations of the -CH3 groups.

There are strong C-C skeletal vibration absorptions at 1175 to 1140 cm-1 from the C(CH3)2 grouping, and similarly, but, weaker, an absorption at 840 to 790 cm-1.

There are no specific characteristic lines for alkanes like 3-methylhexane because C-C and C-H vibrations are common to so many organic molecules and alkanes like 3-methylhexane have no functional group that gives a characteristic vibration.

The absence of other specific functional group bands will show that a particular functional group is absent from the 3-methylhexane molecular structure.

Comparing the 1H NMR and 13C NMR spectra of the nine alkane structural isomers of C7H16

You can distinguish all 9 isomers from a data combination of their number of 1H NMR chemical shifts,

and their resulting integrated 1H proton ratios, plus, their number of 13C chemical shifts.

Name of the alkane structural isomer of molecular formula C7H16 Abbreviated structural formulae of the nine isomers of molecular formula C7H16 (interpretation complications with 3-methylhexane and 2,3-dimethylpentane because they exhibit R/S isomerism due to a chiral carbon) Skeletal formula of the nine alkane isomers of  molecular formula C7H16 Number of 1H NMR chemical shifts (δ) and proton ratio (links to spectrum) Number of 13C chemical shifts (δ) (links to spectrum)
heptane structural formula skeletal formula alkanes molecular structure naming (c) doc b heptane skeletal formula alkanes molecular structure naming (c) doc b 4 δ: proton ratio: 3:2:2:1 (6:4:4:2 in the molecule) 4 δ shifts
2-methylhexane structural formula skeletal formula alkanes molecular structure naming (c) doc b 2-methylhexane skeletal formula alkanes molecular structure naming (c) doc b 6 δ: proton ratio : 6:3:2:2:2:1 6 δ shifts
3-methylhexane structural formula skeletal formula alkanes molecular structure naming (c) doc b 3-methylhexane skeletal formula alkanes molecular structure naming (c) doc b 7 δ: proton ratio: 3:3:3:2:2:2:1 (simplification) !!! 7 δ shifts
3-ethylpentane structural formula skeletal formula alkanes molecular structure naming (c) doc b 3-ethylpentane skeletal formula alkanes molecular structure naming (c) doc b 3 δ: proton ratio: 9:6:1 3 δ shifts
2,2-dimethylpentane structural formula skeletal formula alkanes molecular structure naming (c) doc b 2,2-dimethylpentane skeletal formula alkanes molecular structure naming (c) doc b 4 δ: proton ratio: 9:3:2:2 5 δ shifts
2,3-dimethylpentane structural formula skeletal formula alkanes molecular structure naming (c) doc b 2,3-dimethylpentane skeletal formula alkanes molecular structure naming (c) doc b 6 δ: proton ratio: 6:3:3:2:1:1 (simplification) !!! 6 δ shifts (simplification) !!!
2,4-dimethylpentane structural formula skeletal formula alkanes molecular structure naming (c) doc b 2,4-dimethylpentane skeletal formula alkanes molecular structure naming (c) doc b 3 δ: proton ratio: 12:2:2 3 δ shifts
3,3-dimethylpentane structural formula skeletal formula alkanes molecular structure naming (c) doc b 3,3-dimethylpentane skeletal formula alkanes molecular structure naming (c) doc b 3 δ: proton ratio: 3:3:2 (6:4:4 in the molecule) 4 δ shifts
2,2,3-trimethylbutane structural formula skeletal formula alkanes molecular structure naming (c) doc b 2,2,3-trimethylbutane skeletal formula alkanes molecular structure naming (c) doc b 3 δ: proton ratio: 9:6:1 4 δ shifts

Key words & phrases: C7H16 image and diagram explaining the infrared spectrum of 3-methylhexane, complete infrared absorption spectrum of 3-methylhexane, comparative spectra of 3-methylhexane, prominent peaks/troughs for identifying functional groups in the infrared spectrum of 3-methylhexane, important wavenumber values in cm-1 for peaks/troughs in the infrared spectrum of 3-methylhexane, revision of infrared spectroscopy of 3-methylhexane, fingerprint region analysis of 3-methylhexane, how to identify 3-methylhexane from its infrared spectrum, identifying organic compounds like 3-methylhexane from their infrared spectrum, how to analyse the absorption bands in the infrared spectrum of 3-methylhexane detection of no functional groups in the 3-methylhexane molecule example of the infrared spectrum of a molecule like 3-methylhexane with a functional group  interpreting interpretation of the infrared spectrum of 3-methylhexane shows presence of no functional group  CH3CH2CH(CH3)CH2CH2CH3 Diagram of absorption of wavenumber peaks in the infrared spectrum of 3-methylhexane. Characteristic peak wavenumbers in the infrared spectrum of 3-methylhexane. Finger print identification pattern using the infrared spectrum of 3-methylhexane. Revision notes on the infrared spectrum of 3-methylhexane. Matching and deducing the structure of the 3-methylhexane molecule from  its infrared spectrum. Infrared spectroscopy of aliphatic alkanes, infrared spectra of 3-methylhexane, a structural isomer of molecular formula C7H16


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Infrared spectra of the isomers of C7H16

The infrared spectrum of heptane

The infrared spectrum of 2-methylhexane

The infrared spectrum of 3-methylhexane

The infrared spectrum of 3-ethylpentane

The infrared spectrum of 2,2-dimethylpentane

The infrared spectrum of 2,3-dimethylpentane

The infrared spectrum of 2,4-dimethylpentane

The infrared spectrum of 3,3-dimethylpentane

The infrared spectrum of 2,2,3-trimethylbutane

Mass spectra of the isomers of C7H16

The mass spectrum of heptane

The mass spectrum of 2-methylhexane

The mass spectrum of 3-methylhexane

The mass spectrum of 3-ethylpentane

The mass spectrum of 2,2-dimethylpentane

The mass spectrum of 2,3-dimethylpentane

The mass spectrum of 2,4-dimethylpentane

The mass spectrum of 3,3-dimethylpentane

The mass spectrum of 2,2,3-trimethylbutane

H-1 proton NMR spectra of ALKANES

1H NMR spectra of the isomers of C7H16

The H-1 NMR spectrum of heptane

The H-1 NMR spectrum of 2-methylhexane

The H-1 NMR spectrum of 3-methylhexane

The H-1 NMR spectrum of 3-ethylpentane

The H-1 NMR spectrum of 2,2-dimethylpentane

The H-1 NMR spectrum of 2,3-dimethylpentane

The H-1 NMR spectrum of 2,4-dimethylpentane

The H-1 NMR spectrum of 3,3-dimethylpentane

The H-1 NMR spectrum of 2,2,3-trimethylbutane

C-13 carbon-13 NMR spectra of ALKANES

13C NMR spectra of the isomers of C7H16

The C-13 NMR spectrum of heptane

The C-13 NMR spectrum of 2-methylhexane

The C-13 NMR spectrum of 3-methylhexane

The C-13 NMR spectrum of 3-ethylpentane

The C-13 NMR spectrum of 2,2-dimethylpentane

The C-13 NMR spectrum of 2,3-dimethylpentane

The C-13 NMR spectrum of 2,4-dimethylpentane

The C-13 NMR spectrum of 3,3-dimethylpentane

The C-13 NMR spectrum of 2,2,3-trimethylbutane

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