MCAT Physical : Infrared (IR) Spectroscopy

Study concepts, example questions & explanations for MCAT Physical

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Example Questions

Example Question #1 : Spectroscopy

An IR spectroscopy reading of a compound shows a large absorption at \(\displaystyle 3500cm^{-1}\). What is the identity of this compound?

Possible Answers:

Alcohol

Amide

Amine

Aldehyde

Correct answer:

Alcohol

Explanation:

Compounds with an alcohol group show absorptions in an IR sprectrum from \(\displaystyle 3300cm^{-1}\) through \(\displaystyle 3500cm^{-1}\). Therefore an absorption of \(\displaystyle 3500cm^{-1}\) would correspond to an alcohol. Amide groups show absorptions from \(\displaystyle 3250cm^{-1}\) through \(\displaystyle 3400cm^{-1}\). Amine groups show absorptions from \(\displaystyle 665cm^{-1}\) through \(\displaystyle 910cm^{-1}\)\(\displaystyle 3250cm^{-1}\)through \(\displaystyle 3400cm^{-1}\)\(\displaystyle 1580cm^{-1}\) through \(\displaystyle 1650cm^{-1}\) and \(\displaystyle 1250cm^{-1}\) through \(\displaystyle 1335cm^{-1}\) (aromatic amines). Carbonyl compounds such as aldehydes show strong absorptions in the IR spectrum at \(\displaystyle 1665cm^{-1}\) through \(\displaystyle 1760cm^{-1}\).

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