IR Spectroscopy MCAT: Key Peaks and Wavenumbers

Infrared Spectroscopy for the MCAT: Functional Group Peaks Table

By StudyStore · Published 2026-11-09 · 2 min read · 330 words

IR spectroscopy shows which bonds are present. Bonds absorb at characteristic wavenumbers (cm-1). On the MCAT you mainly need a few strong peaks and their shapes.

Peaks to memorize

Bond	Wavenumber (cm-1)	Shape and notes
O-H (alcohol)	about 3200 to 3600	Broad, strong
O-H (carboxylic acid)	about 2500 to 3300	Very broad, overlaps C-H
N-H	about 3300 to 3500	Sharper than O-H; primary amines show two peaks, secondary one
C-H (sp3)	just under 3000	Alkane C-H
C-H (sp2)	just over 3000	Alkene and aromatic C-H
C triple bond C or C triple bond N	about 2100 to 2260	Sharp, in a quiet region
C=O	about 1650 to 1750	Strong, sharp, the easiest peak to spot
C=C	about 1600 to 1680	Weaker than C=O

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A fast routine

Look near 1700 for a strong C=O.
Look above 3200 for an O-H or N-H.
Combine them: C=O plus a very broad O-H is a carboxylic acid; C=O with no O-H or N-H is a ketone, aldehyde or ester; a broad O-H with no C=O is an alcohol.
Check for an aldehyde C-H near 2700 to 2850 (two small peaks).

Common traps

Mixing alcohol and acid O-H. The acid O-H is wider and extends lower.
Missing conjugation. A conjugated C=O absorbs at a slightly lower wavenumber.
Reading only the fingerprint region. Below about 1500 it is too complex for the MCAT.
Expecting an amide to look like a ketone. Amide C=O sits lower, around 1650.

One-minute recap

C=O: strong, sharp, near 1700.
O-H: broad above 3200; acid is very broad.
Triple bonds: near 2100 to 2260.

Check yourself: 5-question mini-quiz

Try each question first, then click it to reveal the answer.

1. Where does a carbonyl C=O absorb?

Near 1700 cm-1, strong and sharp.

2. How does a carboxylic acid O-H differ from an alcohol O-H?

It is much broader and extends lower, about 2500 to 3300.

3. Where do alkyne and nitrile triple bonds absorb?

About 2100 to 2260 cm-1.

4. What distinguishes a primary amine N-H in IR?

Two peaks near 3300 to 3500.

5. Why is the fingerprint region not used on the MCAT?

It is too complex to assign.

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Written by StudyStore from standard scientific facts. Study aid only. Always check values against the reference your exam provides.