IR spectroscopy

Meaning: A bond absorbs infrared light when the light's frequency matches the bond's vibrational frequency and the vibration changes the dipole moment. The spectrum plots % transmittance (absorptions point downward) against wavenumber in cm⁻¹. Triple bonds absorb at higher wavenumber than double bonds, and double bonds higher than single bonds. Bonds to light atoms such as H also absorb at high wavenumber. Key peaks: O–H (alcohol): broad, about 3200–3600 O–H (carboxylic acid): very broad, about 2500–3300 N–H: about 3300–3500 C≡N or C≡C: about 2100–2260 C=O: strong, about 1700 C=C: about 1600–1680

Equation: ν̃ = (1/2πc)√(k/μ), where ν̃ is the wavenumber (m⁻¹, usually reported in cm⁻¹), c is the speed of light (3.00 × 10⁸ m/s), k is the bond force constant (N/m), and μ is the reduced mass of the two atoms (kg).

On the MCAT: You are usually given a spectrum or a reaction and asked which functional group appeared or disappeared. For example, oxidizing an alcohol to a ketone removes the broad O–H peak and adds a strong C=O peak near 1700 cm⁻¹.

Common confusion: Higher wavenumber means higher energy and shorter wavelength, not longer. IR shows which functional groups are present. It does not give the carbon skeleton (NMR does) or the molecular mass (mass spectrometry does).