A LITTLE LIGHTBULB MOMENT
Spectroscopy, radicals, pericyclics and synthesis strategy
Broad O-H bands look like a wet towel
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A smeared hill around 3200–3600 means hydrogen bonding.
The idea behind the cue
Hydrogen-bonded O-H stretches are characteristically broad in IR because many slightly different O-H bond strengths are present at once. Their breadth helps distinguish them from sharper N-H or C-H signals. Exact shape varies with concentration, temperature, and hydrogen bonding.
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Short & simple
A smeared hill around 3200–3600 means hydrogen bonding.
Acronym
HBC — Hungry Badgers Cartwheel.
Story
Someone drops a towel on the spectrum and it spreads into a huge soft hill. That fuzzy shape comes from many slightly different O-H environments scrambling together, unlike the neat carbonyl spike.
Connect the pieces
- H (Hungry) → Hydrogenbonding: Hydrogen-bonded O-H stretches are broad because many slightly different bond strengths are present at once.
- B (Badgers) → Broadening: Breadth helps distinguish O-H from sharper N-H or C-H signals.
- C (Cartwheel) → Conditions: Concentration, temperature, and hydrogen bonding can noticeably change the shape.
- Hungry · Hydrogenbonding: Hydrogen-bonded O-H stretches are broad because many slightly different bond strengths are present at once.
- Badgers · Broadening: Breadth helps distinguish O-H from sharper N-H or C-H signals.
- Cartwheel · Conditions: Concentration, temperature, and hydrogen bonding can noticeably change the shape.
Know the limits
Broad O-H features are not equally broad in every sample; hydrogen-bonding conditions can change them noticeably.
Further reading · OpenStax Organic Chemistry ↗Check your understanding
What is the most typical IR appearance of an alcohol O-H stretch?
A. A single peak only in the fingerprint region
B. A pair of equal peaks near 3000 cm⁻¹
C. A broad absorption spanning roughly 3200–3600 cm⁻¹
D. A sharp absorption near 2250 cm⁻¹
Reveal explanation
A broad absorption spanning roughly 3200–3600 cm⁻¹
Alcohol O-H stretches are broad because of hydrogen bonding. The other choices correspond more to nitriles, fingerprint-region complexity, or C-H stretches.