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🌈Spectroscopy

Infrared Spectroscopy Functional Groups

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Why This Matters

Infrared spectroscopy is your molecular detective tool—it reveals the functional groups hiding in an unknown compound by measuring how molecules absorb IR radiation and vibrate. You're being tested on your ability to match absorption peaks to functional groups, understand why different bonds absorb at different frequencies, and use this information to identify or confirm molecular structures. This connects directly to broader themes of molecular structure, bonding strength, and intermolecular forces.

The key principle here is that bond strength and reduced mass determine absorption frequency. Stronger bonds and lighter atoms vibrate faster, absorbing at higher wavenumbers. When you see an exam question showing an IR spectrum, don't just hunt for memorized numbers—ask yourself: What type of bond would absorb here? Is this a single, double, or triple bond? Are hydrogen atoms involved? Master these patterns, and you'll decode any spectrum they throw at you.

Triple Bonds: The High-Frequency Region

Triple bonds are the strongest carbon-carbon and carbon-nitrogen bonds, and their high bond order means they vibrate at high frequencies—typically in the 2100−2300 cm−12100-2300 \text{ cm}^{-1} range, a relatively "quiet" region of most spectra.

Alkynes (C≡C stretch)

  • 2100−2260 cm−12100-2260 \text{ cm}^{-1}—this absorption appears in a sparse spectral region, making it easy to spot
  • Weak to moderate intensity—symmetrical alkynes (internal) show weaker or absent peaks due to low dipole change
  • Terminal vs. internal distinction—terminal alkynes also show a sharp ≡C-H\equiv \text{C-H} stretch around 3300 cm−13300 \text{ cm}^{-1}

Nitriles (C≡N stretch)

  • 2210−2260 cm−12210-2260 \text{ cm}^{-1}—slightly higher than alkynes due to the electronegativity of nitrogen
  • Sharp, medium-intensity peak—the polar C≡N\text{C} \equiv \text{N} bond creates a stronger dipole change than C≡C\text{C} \equiv \text{C}
  • No accompanying hydrogen stretch—distinguishes nitriles from terminal alkynes in the same region

Compare: Alkynes vs. Nitriles—both absorb around 2100−2260 cm−12100-2260 \text{ cm}^{-1}, but nitriles appear at the higher end and show stronger peaks. If you see a sharp peak near 2250 cm−12250 \text{ cm}^{-1} with no 3300 cm−13300 \text{ cm}^{-1} signal, think nitrile first.

Carbonyl Compounds: The Diagnostic C=O\text{C}=\text{O} Stretch

The carbonyl group produces one of the most recognizable IR signals—a strong, sharp absorption in the 1650−1750 cm−11650-1750 \text{ cm}^{-1} region. The exact position tells you which carbonyl-containing functional group you're dealing with.

Esters (C=O stretch)

  • 1735−1750 cm−11735-1750 \text{ cm}^{-1}—the highest carbonyl frequency due to resonance with the adjacent oxygen
  • Strong, sharp peak—the polar C=O\text{C}=\text{O} bond creates significant dipole change
  • Look for C-O stretch near 1000−1300 cm−11000-1300 \text{ cm}^{-1}—confirms the ester linkage

Aldehydes (C=O stretch)

  • 1720−1740 cm−11720-1740 \text{ cm}^{-1}—slightly lower than esters, overlapping range requires additional evidence
  • Distinctive twin C-H stretches—look for two peaks around 27202720 and 2820 cm−12820 \text{ cm}^{-1} (Fermi resonance doublet)
  • Terminal carbonyl position—aldehydes are more reactive than ketones, relevant for mechanism questions

Ketones (C=O stretch)

  • 1705−1725 cm−11705-1725 \text{ cm}^{-1}—lower than aldehydes due to greater inductive donation from two alkyl groups
  • No aldehyde C-H peaks—absence of the 2720/2820 cm−12720/2820 \text{ cm}^{-1} doublet distinguishes ketones from aldehydes
  • Conjugation lowers frequency—α,β\alpha,\beta-unsaturated ketones absorb closer to 1680 cm−11680 \text{ cm}^{-1}

Carboxylic Acids (C=O and O-H stretch)

  • 1700−1725 cm−11700-1725 \text{ cm}^{-1} for C=O\text{C}=\text{O}—similar to ketones, but the O-H is the giveaway
  • Very broad O-H stretch from 2500−3300 cm−12500-3300 \text{ cm}^{-1}—hydrogen bonding creates this characteristic "mountain" shape
  • Dimeric hydrogen bonding—carboxylic acids form strong dimers, explaining the unusually broad absorption

Amides (C=O and N-H stretch)

  • 1650−1700 cm−11650-1700 \text{ cm}^{-1} for C=O\text{C}=\text{O}—the lowest carbonyl frequency due to resonance with nitrogen
  • N-H stretch at 3300−3500 cm−13300-3500 \text{ cm}^{-1}—primary amides show two peaks, secondary amides show one
  • "Amide I and II bands"—the carbonyl stretch plus N-H bending near 1550 cm−11550 \text{ cm}^{-1} are key protein markers

Compare: Aldehydes vs. Ketones—both show C=O\text{C}=\text{O} near 1720 cm−11720 \text{ cm}^{-1}, but only aldehydes display the twin C-H peaks around 2720/2820 cm−12720/2820 \text{ cm}^{-1}. This is a classic FRQ distinction—always check for that doublet.

Compare: Carboxylic Acids vs. Amides—both have carbonyl groups, but carboxylic acids show an extremely broad O-H (2500−3300 cm−12500-3300 \text{ cm}^{-1}) while amides show sharper N-H peaks (3300−3500 cm−13300-3500 \text{ cm}^{-1}). The carbonyl position also differs: acids absorb higher than amides.

O-H and N-H Stretches: The Hydrogen-Bonding Region

Bonds to hydrogen vibrate at high frequencies (3000−3700 cm−13000-3700 \text{ cm}^{-1}) because hydrogen's low mass increases vibrational frequency. Hydrogen bonding broadens these peaks dramatically.

Alcohols (O-H stretch)

  • Broad absorption at 3200−3600 cm−13200-3600 \text{ cm}^{-1}—hydrogen bonding between molecules creates the characteristic width
  • Sharp peak near 3600 cm−13600 \text{ cm}^{-1} in dilute solution—"free" O-H without hydrogen bonding appears sharp
  • C-O stretch at 1000−1260 cm−11000-1260 \text{ cm}^{-1}—confirms alcohol; position varies with primary/secondary/tertiary

Amines (N-H stretch)

  • 3300−3500 cm−13300-3500 \text{ cm}^{-1}—slightly lower than alcohols due to nitrogen's lower electronegativity
  • Primary amines show two peaks—symmetric and asymmetric N-H stretches; secondary amines show one
  • Medium intensity, moderately broad—less hydrogen bonding than alcohols means narrower peaks

Compare: Alcohols vs. Amines—both absorb in the 3200−3600 cm−13200-3600 \text{ cm}^{-1} region, but alcohol O-H peaks are broader and often more intense. Primary amines show a characteristic doublet while alcohols show a single broad peak. If an FRQ asks you to distinguish these, peak shape is your answer.

Carbon-Carbon and Carbon-Hydrogen Frameworks

These absorptions establish the hydrocarbon skeleton of your molecule. While less diagnostic than functional groups, they confirm saturation, unsaturation, and aromaticity.

Alkanes (C-H stretch)

  • 2850−2960 cm−12850-2960 \text{ cm}^{-1}—sp3sp^3 C-H bonds absorb below 3000 cm−13000 \text{ cm}^{-1}
  • Strong, multiple peaks—asymmetric and symmetric stretches of CH3\text{CH}_3 and CH2\text{CH}_2 groups overlap
  • Benchmark for saturation—if all C-H peaks are below 3000 cm−13000 \text{ cm}^{-1}, no sp2sp^2 or spsp carbons are present

Alkenes (C=C stretch)

  • 1620−1680 cm−11620-1680 \text{ cm}^{-1}—moderate intensity; symmetrical alkenes may show weak or absent peaks
  • sp2sp^2 C-H stretch just above 3000 cm−13000 \text{ cm}^{-1}—typically 3020−3100 cm−13020-3100 \text{ cm}^{-1}, distinguishing from alkane C-H
  • Indicates unsaturation—presence suggests the molecule can undergo addition reactions

Aromatic Compounds (C-H stretch and ring vibrations)

  • C-H stretch around 3030 cm−13030 \text{ cm}^{-1}—sp2sp^2 hybridized, appears just above 3000 cm−13000 \text{ cm}^{-1}
  • Ring vibrations at 1400−1600 cm−11400-1600 \text{ cm}^{-1}—multiple sharp peaks from C=C\text{C}=\text{C} stretching in the ring
  • Overtone pattern at 1650−2000 cm−11650-2000 \text{ cm}^{-1}—weak peaks whose pattern indicates substitution pattern

Compare: Alkanes vs. Alkenes vs. Aromatics—all show C-H stretches, but position tells you hybridization. Below 3000 cm−13000 \text{ cm}^{-1} = sp3sp^3 (alkane); above 3000 cm−13000 \text{ cm}^{-1} = sp2sp^2 (alkene or aromatic). Aromatics add distinctive ring vibrations at 1400−1600 cm−11400-1600 \text{ cm}^{-1}.

Single-Bond Stretches: The Fingerprint Region

Absorptions below 1500 cm−11500 \text{ cm}^{-1} fall in the "fingerprint region"—complex patterns unique to each molecule. C-O, C-N, and C-X stretches appear here.

Ethers (C-O-C stretch)

  • 1050−1150 cm−11050-1150 \text{ cm}^{-1}—strong absorption from asymmetric C-O-C stretching
  • No O-H or carbonyl peaks—distinguishes ethers from alcohols and esters
  • Difficult to identify in isolation—often requires additional spectroscopic data for confirmation

Nitro Compounds (N-O stretch)

  • Two strong absorptions: 1500−1600 cm−11500-1600 \text{ cm}^{-1} and 1300−1400 cm−11300-1400 \text{ cm}^{-1}—asymmetric and symmetric NO2\text{NO}_2 stretches
  • Very characteristic pattern—the two-band system is diagnostic for the nitro group
  • Electron-withdrawing effect—relevant for understanding reactivity in aromatic substitution

Halides (C-X stretch)

  • 600−800 cm−1600-800 \text{ cm}^{-1}—varies by halogen: C-F highest (1000−1400 cm−11000-1400 \text{ cm}^{-1}), C-I lowest (500−600 cm−1500-600 \text{ cm}^{-1})
  • Heavier halogens absorb at lower frequencies—increased reduced mass lowers vibrational frequency
  • Often outside typical scanning range—C-Cl, C-Br, and C-I may be missed on standard instruments

Compare: Ethers vs. Alcohols—both contain C-O bonds, but alcohols show a broad O-H stretch at 3200−3600 cm−13200-3600 \text{ cm}^{-1} that ethers lack. If you see C-O absorption without O-H, you're looking at an ether.

Quick Reference Table

ConceptBest Examples
Triple bond region (2100−2300 cm−12100-2300 \text{ cm}^{-1})Alkynes, Nitriles
Carbonyl stretch (1650−1750 cm−11650-1750 \text{ cm}^{-1})Esters, Aldehydes, Ketones, Carboxylic acids, Amides
Broad O-H (hydrogen bonding)Alcohols, Carboxylic acids
N-H stretch (3300−3500 cm−13300-3500 \text{ cm}^{-1})Amines, Amides
sp3sp^3 C-H (below 3000 cm−13000 \text{ cm}^{-1})Alkanes
sp2sp^2 C-H (above 3000 cm−13000 \text{ cm}^{-1})Alkenes, Aromatic compounds
Fingerprint region diagnosticsEthers, Nitro compounds, Halides

Self-Check Questions

  1. You observe a strong, sharp peak at 1715 cm−11715 \text{ cm}^{-1} and two smaller peaks near 27202720 and 2820 cm−12820 \text{ cm}^{-1}. Which functional group is present, and what feature distinguishes it from a ketone?

  2. Compare the O-H stretch in alcohols versus carboxylic acids. Why does the carboxylic acid absorption extend to much lower wavenumbers (2500 cm−12500 \text{ cm}^{-1})?

  3. A compound shows absorption at 2240 cm−12240 \text{ cm}^{-1} but nothing near 3300 cm−13300 \text{ cm}^{-1}. Is this more likely an alkyne or a nitrile? Explain your reasoning.

  4. How would you distinguish between a primary amine and an alcohol using IR spectroscopy if both absorb in the 3300−3500 cm−13300-3500 \text{ cm}^{-1} region?

  5. An FRQ presents a spectrum with strong carbonyl absorption at 1680 cm−11680 \text{ cm}^{-1} and N-H peaks at 3350 cm−13350 \text{ cm}^{-1}. Identify the functional group and explain why the carbonyl appears at a lower frequency than typical aldehydes or ketones.