CH₃CH(OH)CH₂CH₃ (2-butanol, sec-butanol, sec-butyl alcohol)

[Author ©  Dr Phil Brown PhD: Doc Brown's advanced level pre-university/college 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 of butan-2-ol (2-butanol) [spectrum page updated Mar 3rd 2026 *]

Introductory note on the infrared spectrum of butan-2-ol

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

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

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

Butan-2-ol    C₄H₁₀O             

Key revision points about the infrared spectrum of butan-2-ol (2-butanol)

The IR spectrum of butan-2-ol shows a broad O–H stretch around 3200–3600 cm⁻¹, strong C–H stretches near 2850–2960 cm⁻¹, and a C–O stretch around 1050–1150 cm⁻¹.

These are the diagnostic peaks for alcohols.

Students often confuse the broad O–H band with N–H stretches or miss the C–O stretch entirely.

Exam tips: always link the broad O–H band to alcohols, check for absence of C=O (~1700 cm⁻¹), and use multiple peaks to confirm identity.

Key IR Features of Butan-2-ol (2-butanol)

• Broad O–H stretch (hydrogen bonding): 3200–3600 cm⁻¹
• C–H stretches (alkyl groups): 2850–2960 cm⁻¹
• C–H bending (alkyl): ~1450 cm⁻¹
• C–O stretch (alcohol functional group): 1050–1150 cm⁻¹
• Fingerprint region: Complex absorptions below 1500 cm⁻¹, less diagnostic but useful for comparison.

Table of Prominent Wavenumbers for the infrared spectrum of butan-2-ol (2-butanol)

Sources: NIST IR spectrum of 2-butanol, Studocu IR assignments, LibreTexts alcohol spectroscopy guide.

Common Student Misconceptions

• Confusing O–H with N–H: Both appear in similar regions, but O–H is broader and stronger.
• Expecting a C=O peak: Students sometimes misidentify alcohols as carbonyl compounds; remember alcohols lack the sharp ~1700 cm⁻¹ peak.
• Ignoring the C–O stretch: Many exam answers miss this diagnostic band, which is crucial for confirming alcohols.
• Over-reliance on one peak: Correct identification requires considering multiple absorptions, not just O–H.

Exam Revision Tips

• Always check for O–H broad band: If present, think alcohol or carboxylic acid (distinguish by C=O at ~1700 cm⁻¹).
• Absence of C=O: Confirms it’s not a ketone/aldehyde/carboxylic acid.
• Look for C–O stretch: Reinforces alcohol identification.
• Compare with alkane spectra: Alcohols show extra O–H and C–O peaks beyond alkane C–H stretches.
• Practice with spectra overlays: Many boards (AQA, Edexcel, OCR, IB, AP) expect students to distinguish alcohols from carbonyls and amines.
• Exam technique: When asked to identify functional groups, state both the wavenumber and the bond (e.g., “Broad O–H stretch at ~3400 cm⁻¹ indicates alcohol”).

Final Guidance

For A level and AP exams, focus on:

• Broad O–H band (alcohol hallmark).
• Absence of C=O (rules out carbonyls).
• Presence of C–O stretch (confirms alcohol).
• Use multiple peaks for a robust answer.

The mass spectrum of butan-2-ol (sec-butyl alcohol)

The H-1 NMR spectrum of butan-2-ol (sec-butyl alcohol)

The C-13 NMR spectrum of butan-2-ol (sec-butyl alcohol)

The chemistry of ALCOHOLS revision notes INDEX

Infrared spectroscopy index

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