Organic Level Organic Chemistry: Infrared spectrum of 2-methylbut-2-ene (2-methyl-2-butene)
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Interpreting the infrared spectrum of 2-methylbut-2-ene (2-methyl-2-butene)

[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 and AP honors chemistry courses: Molecular spectroscopy - analysing the infrared spectrum of 2-methylbut-2-ene  [spectra page updated Mar 13th 2026 *]

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Introductory note on the infrared spectrum of 2-methylbut-2-ene

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

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

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

C5H10 infrared spectrum of 2-methylbut-2-ene (2-methyl-2-butene) wavenumbers cm-1 functional group detection fingerprint pattern identification of 2-methylbut-2-ene (2-methyl-2-butene) doc brown's advanced organic chemistry revision notes 

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

2-methylbut-2-ene C5H10 (CH3)2C=CHCH3 alkenes structure and naming (c) doc b , alkenes structure and naming (c) doc b

The molecular structure and naming of alkenes

Interpretation of the infrared spectrum of 2-methylbut-2-ene (2-methyl-2-butene)

The most prominent infrared absorption lines of 2-methylbut-2-ene (2-methyl-2-butene)

The most characteristic absorption is ~1660 cm-1 due to the C=C vibration (stretching).

It is quite a weak absorption in the infrared spectrum of 2-methylbut-2-ene, C=C absorption is usually much stronger.

You don't find this C=C absorption band in the infrared spectra of saturated alkanes.

There are also characteristic lines due to various C-H vibration absorptions group at wavenumbers ~3100, ~2900, ~1290-1420 and ~800 to 900 cm-1.

The absence of other specific functional group bands will show that particular functional group is absent from the 2-methylbut-2-ene (2-methyl-2-butene) molecular structure.

Summary of key points for the infrared spectrum of 2-methylbut-2-ene plus extra exam revision comments

A structured breakdown tailored for advanced A-level chemistry revision, with emphasis on spectroscopy interpretation, common pitfalls, and exam strategies.

Key infrared Spectrum Features of 2-Methylbut-2-ene

2-Methylbut-2-ene (C5H10) is a branched alkene with a non-terminal C=C bond and methyl substituents. Its IR spectrum reflects characteristic vibrations of alkenes and alkyl groups.

Functional Groups Present:

  • C=C (alkene) — internal double bond
  • C–H (sp² and sp³) — from methyl and vinylic hydrogens
  • No O–H, C=O, or N–H groups

Prominent IR Absorptions in the infrared spectrum of 2-methylbut-2-ene (see also below)

Bond Type Wavenumber Range (cm⁻¹) Expected Peak Characteristics Notes
C=C stretch (alkene) 1620–1680 Medium intensity, sharp Confirms presence of double bond
C–H stretch (sp²) 3020–3100 Weak to medium, sharp Vinylic C–H
C–H stretch (sp³) 2850–2960 Strong, sharp Methyl and methylene groups
C–H bend (alkene) ~890–990 Medium intensity Indicates non-terminal alkene
Fingerprint region 1500–400 Complex, unique pattern Useful for compound identification

Common Misconceptions about the infrared spectrum of 2-methylbut-2-ene (see also above/below)

Misconception Clarification
All alkenes show the same IR peaks Terminal versus internal alkenes differ in C–H bending regions
Absence of strong peaks means no functional groups Alkene C=C stretch is often medium and easily overlooked
Fingerprint region is irrelevant It’s crucial for distinguishing isomers and confirming identity
C=C always gives a strong peak Not necessarily — intensity depends on symmetry and dipole change

Exam Revision Tips for questions involving the infrared spectrum of 2-methylbut-2-ene (AQA, Edexcel, OCR, WJEC, CCEA, CIE, IB) (see also above)

What to Focus On:

  • Recognising C=C stretch: Look for 1620–1680 cm⁻¹. It’s often medium intensity.
  • Distinguishing terminal versus internal alkenes: Terminal alkenes show C–H bending ~910–990 cm⁻¹; internal alkenes like 2-methylbut-2-ene lack this.
  • Using the fingerprint region: Compare unknown spectra to known references for exact identification.
  • Avoid overinterpreting weak peaks: Not all peaks are diagnostic — focus on those with clear assignments.

Exam-Style Strategy:

  • Step 1: Identify strong, sharp peaks above 1500 cm⁻¹.
  • Step 2: Check for absence of O–H or C=O peaks to rule out alcohols or carbonyls.
  • Step 3: Confirm alkene presence via C=C stretch and C–H bending.
  • Step 4: Use fingerprint region for final confirmation or to distinguish isomers.

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The mass spectrum of 2-methylbut-2-ene

The H-1 NMR spectrum of 2-methybut-2-ene

The C-13 NMR spectrum of 2-methylbut-2-ene

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