infrared spectroscopy
Summary
Infrared spectroscopy identifies functional groups by the infrared frequencies a molecule absorbs as its bonds stretch and bend. Peaks are read in wavenumber (cm⁻¹); the most testable features are the strong C=O peak near 1700 cm⁻¹ and the broad O–H peak near 3300 cm⁻¹.
Detail
Meaning: IR light has the right energy to make bonds stretch and bend. A bond absorbs when the light matches its vibration and the vibration changes the bond dipole, so polar bonds give strong peaks. Stronger bonds and lighter atoms absorb at higher wavenumber: triple above double above single, with X–H bonds at the high end.
Equation: E = hcṽ = hc/λ, where E is photon energy (J), h is Planck's constant (J·s), c is the speed of light (m/s), ṽ is wavenumber (m⁻¹, reported in cm⁻¹), and λ is wavelength (m).
On the MCAT: A spectrum plots percent transmittance against wavenumber, with peaks pointing down. Match peaks to groups: alcohol O–H broad, 3200–3600 cm⁻¹; carboxylic acid O–H very broad, 2500–3300; N–H 3300–3500; C≡N about 2250; C=O strong and sharp, 1700–1750. Questions often ask which peak appears or vanishes in a reaction, such as the C=O peak disappearing and a broad O–H peak appearing when a ketone is reduced to an alcohol.
Common confusion: IR identifies functional groups, not the full carbon skeleton; NMR does that. Also, higher wavenumber means higher energy and shorter wavelength, not longer wavelength.
Sources
- Organic Chemistry (Wade)
- Organic Chemistry (Klein)
- Organic Chemistry (McMurry)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.