C7H16 infrared spectrum of 2,4-dimethylpentane prominent wavenumbers cm-1 detecting functional groups present finger print for identification of 2,4-dimethylpentane image diagram doc brown's advanced organic chemistry revision notes

Advanced Organic Chemistry: Infrared spectrum of 2,4-dimethylpentane (CH₃)₂CHCH₂CH(CH₃)₂

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Interpreting the infrared spectrum of 2,4-dimethylpentane

[Author © Dr Phil Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11, grade 12 & AP honors chemistry courses: Molecular spectroscopy - analysing infrared spectrum of 2,4-dimethylpentane [spectra updated Mar 19th 2026 *]

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Introductory note on the infrared spectrum of 2,4-dimethylpentane

Students and teachers please note my explanation of the infrared spectrum of 2,4-dimethylpentane is designed for advanced, but pre-university, chemistry courses.

Based in the infrared spectrum diagram for 2,4-dimethylpentane, only some of the most prominent peaks for particular bond vibrations are discussed, particularly if 2,4-dimethylpentane has a functional group with a particular characteristic wavenumber peak.

The infrared spectrum of 2,4-dimethylpentane is unique and the whole, or selected wavenumbers, can be used to fingerprint its identity, sometimes analysing a mixture containing 2,4-dimethylpentane or following its change of concentration in a reaction.

C7H16 infrared spectrum of 2,4-dimethylpentane wavenumbers cm-1 functional group detection fingerprint pattern identification of 2,4-dimethylpentane doc brown's advanced organic chemistry revision notes

Spectra obtained from a liquid film of 2,4-dimethylpentane. The right-hand part of the of the infrared spectrum of 2,4-dimethylpentane, wavenumbers ~1500 to 400 cm^-1 is considered the fingerprint region for the identification of 2,4-dimethylpentane and most organic compounds. It is due to a unique set of complex overlapping vibrations of the atoms of the molecule of 2,4-dimethylpentane.

2,4-dimethylpentane C₇H₁₆

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

Interpretation of the infrared spectrum of 2,4-dimethylpentane

The most prominent infrared absorption lines of 2,4-dimethylpentane

Strong C-H stretching vibration absorptions at wavenumbers 2940 to 2880 cm^-1 for the CH₂ and CH₃ groups in 2,4-dimethypentane.

Several strong C-H deformation vibration absorptions at wavenumbers 1480 to 1365 cm^-1 for the CH₂ and CH₃ groups in 2,4-dimethypentane.

There is a weak -CH- stretching vibration absorption at ~1340 cm^-1.

Several strong C-C skeletal vibration absorptions associated with a C-(CH₃)₂ group occur at wavenumbers 840 to 790 and 1175 to 1140 cm^-1.

There are no specific characteristic lines for alkanes like 2,4-dimethypentane because C-C and C-H vibrations are common to so many organic molecules and alkanes like 2,4-dimethypentane 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 2,4-dimethylpentane molecular structure.

Key points about the infrared spectrum of 2,4-dimethylpentane

Overview: 2,4-dimethylpentane and Its IR Spectrum

2,4-dimethylpentane is a branched alkane with the molecular formula C₇H₁₆.

As a saturated hydrocarbon, it contains only C–C and C–H single bonds, with no functional groups like alcohols, carbonyls, or alkenes.

This makes its IR spectrum relatively simple and characteristic of alkane fingerprints.

Key IR Absorptions for 2,4-dimethylpentane

Wavenumber (cm⁻¹)	Bond Type	Vibration Mode	Comments
~2950–2850	C–H (sp³) stretch	Asymmetric & symmetric	Strong, broad region typical of alkanes
~1465	CH₂ bending (scissoring)	Medium	Common in branched alkanes
~1375	CH₃ symmetric bending	Medium	Indicates methyl groups
~720–740	CH₃ rocking (long chain)	Weak–medium	May appear as a sharp peak in longer chains

Note: No significant peaks above 3000 cm⁻¹ (no O–H, N–H, or C–H sp²(from C=C), and no strong absorptions in the 1700 cm⁻¹region (no C=O)

Common Misconceptions

Misconception	Clarification
"All organic compounds show strong peaks in the 1700 cm⁻¹ region."	Only compounds with C=O (carbonyl) groups do. Alkanes like 2,4-dimethylpentane do not.
"No peaks = no information."	Even a lack of peaks (e.g. no OH or C=O) is diagnostic.
"IR is only useful for identifying functional groups."	While true for many cases, absence of functional groups (as in alkanes) is also informative.
"Branched alkanes have the same IR as straight-chain alkanes."	They share features, but CH₃ bending (~1375 cm⁻¹) is often more prominent in branched alkanes.

Exam Revision Tips (AQA, Edexcel, OCR, WJEC, CCEA, CIE, IB, AP)

Interpretation Strategy

Start at 3000 cm⁻¹: Is there a broad OH or sharp C–H sp²? If not, likely an alkane.
Check 1700 cm⁻¹: No peak? Likely no carbonyl.
Look at 1375 & 1465 cm⁻¹: Strong CH₃/CH₂ bends suggest alkane backbone.
Use absence as evidence: No peaks in key regions = no functional groups.

Exam-Style Revision Tips

Focus on identifying presence or absence of functional groups using IR data.
May ask for comparison of IR spectra between compounds.
Expect data interpretation and justification of structure using IR + other spectra.
Emphasizes pattern recognition and elimination of incorrect structures.

Practice Prompt

Given an IR spectrum with peaks at 2950, 1465, and 1375 cm⁻¹, and no peaks above 3000 or near 1700 cm⁻¹, suggest the type of compound.

Correct answer: A branched alkane such as 2,4-dimethylpentane.

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			4 δ: proton ratio: 3:2:2:1 (6:4:4:2 in the molecule)	4 δ shifts
2-methylhexane			6 δ: proton ratio : 6:3:2:2:2:1	6 δ shifts
3-methylhexane			7 δ: proton ratio: 3:3:3:2:2:2:1 (simplification) !	7 δ shifts
3-ethylpentane			3 δ: proton ratio: 9:6:1	3 δ shifts
2,2-dimethylpentane			4 δ: proton ratio: 9:3:2:2	5 δ shifts
2,3-dimethylpentane			6 δ: proton ratio: 6:3:3:2:1:1 (simplification) !	6 δ shifts (simplification) !!!
2,4-dimethylpentane			3 δ: proton ratio: 12:2:2	3 δ shifts
3,3-dimethylpentane			3 δ: proton ratio: 3:3:2 (6:4:4 in the molecule)	4 δ shifts
2,2,3-trimethylbutane			3 δ: proton ratio: 9:6:1	4 δ shifts

Key words & phrases: C7H16 image and diagram explaining the infrared spectrum of 2,4-dimethylpentane, complete infrared absorption spectrum of 2,4-dimethylpentane, comparative spectra of 2,4-dimethylpentane, prominent peaks/troughs for identifying functional groups in the infrared spectrum of 2,4-dimethylpentane, important wavenumber values in cm-1 for peaks/troughs in the infrared spectrum of 2,4-dimethylpentane, revision of infrared spectroscopy of 2,4-dimethylpentane, fingerprint region analysis of 2,4-dimethylpentane, how to identify 2,4-dimethylpentane from its infrared spectrum, identifying organic compounds like 2,4-dimethylpentane from their infrared spectrum, how to analyse the absorption bands in the infrared spectrum of 2,4-dimethylpentane detection of functional groups in the 2,4-dimethylpentane molecule example of the infrared spectrum of a molecule like 2,4-dimethylpentane with a functional group interpreting interpretation of the infrared spectrum of 2,4-dimethylpentane shows presence of no functional groups (CH₃)₂CHCH₂CH(CH₃)₂ Diagram of absorption of wavenumber peaks in the infrared spectrum of 2,4-dimethylpentane. Characteristic peak wavenumbers in the infrared spectrum of 2,4-dimethylpentane. Finger print identification pattern using the infrared spectrum of 2,4-dimethylpentane. Revision notes on the infrared spectrum of 2,4-dimethylpentane. Matching and deducing the structure of the 2,4-dimethylpentane molecule from its infrared spectrum. Infrared spectroscopy of aliphatic alkanes, infrared spectra of 2,4-dimethylpentane, a structural isomer of molecular formula C7H16 How do you interpret the infrared absorption spectrum of 2,4-dimethylpentane How to interpret the infrared spectrum of 2,4-dimethylpentane Explanatory diagram of the infrared spectrum of the 2,4-dimethylpentane molecule in terms of its molecular structure. Listing data of the prominent main wavenumber peaks troughs in the infrared spectrum of 2,4-dimethylpentane. How to explain the infrared spectrum of 2,4-dimethylpentane. Use of the infrared spectrum of 2,4-dimethylpentane, identification of 2,4-dimethylpentane from its infrared spectrum - fingerprint wavenumber pattern to identify the 2,4-dimethylpentane molecule. The uses of the infrared spectrum of the 2,4-dimethylpentane molecule. The distinctive features of the infrared spectrum of the 2,4-dimethylpentane molecule explained interpretation diagram explaining the peaks-trough of the transmittance of the infrared spectrum of 2,4-dimethylpentane what does the infrared spectrum tell you about the structure and properties of the 2,4-dimethylpentane molecule? How is infrared spectrum of 2,4-dimethylpentane used to identify 2,4-dimethylpentane?

Links associated with 2,4-dimethylpentane

The mass spectrum of 2,4-dimethylpentane

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

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

The chemistry of ALKANES revision notes INDEX

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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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