Premed · Premed · Organic Chemistry 1
Lecture 19: Infrared (IR) Spectroscopy
Organic Chemistry I
Learning Objectives
By the end of this lecture, students will be able to:
- Explain the physical basis of infrared spectroscopy (molecular vibrations)
- Describe the key features of an IR spectrum (wavenumber, transmittance)
- Identify the diagnostic absorption regions of an IR spectrum
- Recognize characteristic IR absorptions for common functional groups
- Use IR spectroscopy to determine the presence or absence of functional groups in an unknown compound
- Interpret IR spectra in combination with molecular formula data
- Distinguish between similar functional groups using IR data
Lecture Content
I. Principles of Infrared Spectroscopy
Infrared radiation causes covalent bonds in molecules to vibrate at characteristic frequencies. IR light spans wavelengths of approximately 2.5 to 25 micrometers, more commonly expressed in wavenumbers (reciprocal centimeters) ranging from 4000 to 400 cm-1, where higher wavenumber corresponds to higher energy.
For a bond to absorb infrared radiation, its vibration must produce a change in the dipole moment of the molecule. Symmetric, nonpolar bonds such as those in O2 or N2 do not absorb or absorb only weakly, while polar bonds such as O-H, N-H, and C=O absorb strongly. Molecular vibrations come in two main types: stretching (changes in bond length, either symmetric or asymmetric) and bending (changes in bond angle, including scissoring, rocking, wagging, and twisting). Stretching absorptions are generally stronger and appear at higher wavenumbers than bending absorptions.
The Hooke's law analogy provides a useful framework: vibration frequency depends on bond strength and atomic masses. Stronger bonds vibrate at higher frequencies (higher wavenumber), and lighter atoms vibrate at higher frequencies. Thus triple bonds absorb at higher wavenumber than double bonds, which absorb higher than single bonds, and C-H, N-H, and O-H stretches appear at the highest wavenumbers due to the light mass of hydrogen.
II. The IR Spectrum
An IR spectrum plots percent transmittance on the y-axis against wavenumber on the x-axis, with the x-axis running from high wavenumber (left, approximately 4000 cm-1) to low wavenumber (right, approximately 400 cm-1). Absorptions appear as downward-pointing peaks, representing dips in transmittance where the sample has absorbed radiation.
The spectrum is divided into two major regions. The functional group region (4000-1500 cm-1) contains most diagnostically useful absorptions, including O-H, N-H, and C-H stretches (3600-2700 cm-1), triple bond stretches for C-triple-bond-C and C-triple-bond-N (2300-2100 cm-1), and double bond stretches for C=O, C=C, and C=N (1800-1500 cm-1). The fingerprint region (1500-400 cm-1) contains a complex pattern of many overlapping absorptions from C-O, C-N, and C-C stretches along with various bending modes. While individual peaks in the fingerprint region are difficult to assign, the overall pattern is unique to each compound (like a fingerprint) and useful for matching an unknown to a reference spectrum.
III. O-H and N-H Stretching Region (3600-2500 cm-1)
The alcohol O-H stretch appears as a broad absorption between 3200 and 3550 cm-1 and is one of the most recognizable features in IR spectroscopy. The breadth results from hydrogen bonding: intermolecular H-bonding creates a range of O-H environments that absorb at slightly different frequencies. In dilute solution, where H-bonding is minimal, a free O-H appears as a sharp peak near 3600 cm-1. In neat samples or concentrated solutions, the H-bonded O-H produces the characteristic broad, strong absorption centered around 3300-3400 cm-1.
Carboxylic acid O-H stretches are even broader, extending from approximately 3300 down to 2500 cm-1 in a characteristic "hairy" appearance that overlaps with the C-H stretching region. This extreme breadth reflects the strong hydrogen bonding present in carboxylic acid dimers.
Amine N-H stretches appear between 3300 and 3500 cm-1. Primary amines (R-NH2) show two peaks corresponding to symmetric and asymmetric N-H stretching, secondary amines (R2NH) show a single peak, and tertiary amines (R3N) show no N-H absorption at all. N-H peaks are generally sharper than O-H peaks but still broader than C-H absorptions.
<image>Panel A: A labeled IR spectrum of 1-butanol showing the broad O-H stretch at 3200-3550 cm-1 (highlighted and labeled), C-H stretches at 2850-2960 cm-1, and the C-O stretch at ~1050 cm-1. Panel B: A labeled IR spectrum of butanoic acid showing the extremely broad O-H stretch extending from 2500-3300 cm-1 (overlapping with C-H), and the strong C=O stretch at ~1710 cm-1. Panel C: Comparison of the O-H region for an alcohol (broad around 3300), a carboxylic acid (very broad extending to 2500), and an amine (two medium peaks around 3400 for primary). All three spectra are stacked vertically for comparison.</image>
IV. C-H Stretching Region (3300-2700 cm-1)
The C-H stretching absorptions provide information about the hybridization of carbon. The sp3 C-H stretch appears between 2850 and 2960 cm-1 as a medium to strong absorption present in virtually all organic molecules. The sp2 C-H stretch (from alkene =C-H and aromatic C-H bonds) appears slightly higher, between 3010 and 3100 cm-1, and can help identify the presence of unsaturation. The sp C-H stretch of a terminal alkyne appears as a strong, sharp peak near 3300 cm-1, distinctly above the sp2 range. Its absence indicates an internal alkyne.
Aldehyde C-H stretches appear as two weak absorptions near 2720 and 2820 cm-1, a pattern known as the Fermi resonance doublet. These peaks are diagnostic for aldehydes when observed alongside a carbonyl absorption.
V. Triple Bond Region (2300-2000 cm-1)
The C-triple-bond-C stretch of alkynes appears between 2100 and 2260 cm-1. Terminal alkynes absorb near 2100-2150 cm-1 with medium intensity, while internal alkynes absorb near 2200-2260 cm-1 with weak intensity or may show no absorption at all if the alkyne is symmetric, because a symmetric stretching vibration produces no change in dipole moment.
The C-triple-bond-N stretch of nitriles appears between 2200 and 2260 cm-1 as a sharp, medium-intensity peak. This absorption is highly diagnostic because very few other functional groups absorb in this region, making it easy to identify and distinguishable from C-triple-bond-C by its slightly different position and greater intensity.
VI. Double Bond Region (1800-1500 cm-1)
The C=O stretching absorption is the most intense and most diagnostically valuable peak in all of IR spectroscopy. It appears as a strong, sharp absorption between 1650 and 1800 cm-1, and its exact position varies with the type of carbonyl compound. Ketones absorb near 1715 cm-1, aldehydes near 1725 cm-1, carboxylic acids near 1710 cm-1, esters near 1735-1750 cm-1, amides near 1630-1690 cm-1 (lower due to resonance with nitrogen), acid chlorides near 1800 cm-1 (the highest), and anhydrides show two peaks near 1800 and 1750 cm-1. Conjugation lowers the C=O frequency because resonance weakens the bond, while ring strain raises it.
The C=C stretch appears between 1600 and 1680 cm-1 as a medium to weak absorption, reflecting the smaller change in dipole moment compared to C=O. Alkene C=C stretches fall near 1640-1680 cm-1, and symmetric alkenes may not absorb at all. Aromatic C=C ring stretching modes produce two characteristic absorptions near 1600 and 1475 cm-1.
<image>A reference chart showing the positions of C=O stretching absorptions for different functional groups. A horizontal wavenumber axis from 1600-1850 cm-1 is drawn, with colored bars indicating the typical ranges for: acid chloride (1790-1815), anhydride (two bars at 1800-1850 and 1740-1790), ester (1735-1750), aldehyde (1720-1740), ketone (1705-1725), carboxylic acid (1700-1725), and amide (1630-1690). Each bar is labeled with the functional group name and a representative structural fragment. An arrow at the bottom indicates that conjugation shifts all values lower by ~20-40 cm-1.</image>
VII. Fingerprint Region and C-O Stretches (1500-400 cm-1)
The C-O stretch produces a strong absorption between 1000 and 1300 cm-1, with the exact position varying by functional group: alcohols near 1000-1100 cm-1, ethers near 1000-1300 cm-1, and esters showing a strong absorption in this same region in addition to their C=O peak near 1740 cm-1. The C-N stretch overlaps with the C-O region at approximately 1000-1200 cm-1. Carbon-halogen stretches appear at lower wavenumbers: C-Cl near 600-800 cm-1 and C-Br near 500-600 cm-1, both typically in the fingerprint region.
The fingerprint region below 1500 cm-1 contains many overlapping peaks from various bending and stretching modes. While individual peak assignment is usually impractical, the overall pattern serves as a unique identifier for matching an unknown compound to a reference spectrum in a spectral library.
VIII. Strategy for Interpreting IR Spectra
A systematic approach to interpreting IR spectra proceeds through a series of diagnostic questions. First, look for a broad O-H or N-H stretch between 3200 and 3600 cm-1: if present, the compound may be an alcohol, carboxylic acid, amine, or amide, and if the absorption is extremely broad extending down to 2500 cm-1, a carboxylic acid is likely. Second, examine the C-H region to determine whether sp, sp2, or sp3 C-H bonds are present: a sharp peak at 3300 cm-1 suggests a terminal alkyne, peaks above 3000 cm-1 indicate sp2 carbon (alkene or aromatic), and peaks only below 3000 cm-1 suggest exclusively sp3 carbon. Third, check for triple bond absorptions between 2000 and 2300 cm-1, which would indicate an alkyne or nitrile. Fourth, look for a C=O stretch between 1650 and 1800 cm-1 and use its exact position to identify the type of carbonyl. Fifth, check for C=C stretches or aromatic ring modes between 1600 and 1680 cm-1. Finally, examine the fingerprint region for supporting evidence of C-O, C-N, or C-X bonds.
IR spectroscopy is most effective when combined with molecular formula data, as the degree of unsaturation narrows the structural possibilities. It is important to remember that IR confirms or rules out functional groups but does not by itself reveal the complete molecular structure.
<image>A decision-tree flowchart for interpreting an IR spectrum. Start: "Is there a broad absorption at 3200-3600 cm-1?" → If yes, "Is there also a C=O at 1700-1740?" → If yes, "Carboxylic acid." → If no C=O, "Alcohol or amine (check for N-H doublet)." Back to start → If no broad OH/NH, "Is there a strong C=O at 1650-1800?" → If yes, branch to ketone, aldehyde, ester, or amide based on exact position and other features. → If no C=O, "Check for C≡C/C≡N at 2100-2260" → If present, "Alkyne or nitrile." → If absent, "Likely an alkane, ether, or halide (check fingerprint)." Each terminal node includes the key diagnostic peaks expected.</image>
