🌈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 $2100-2300 \text{ cm}^{-1}$ range, a relatively "quiet" region of most spectra.

Alkynes (C≡C stretch)

$2100-2260 \text{ cm}^{-1}$ —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 $\equiv \text{C-H}$ stretch around $3300 \text{ cm}^{-1}$

Nitriles (C≡N stretch)

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

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

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

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

Esters (C=O stretch)

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

Aldehydes (C=O stretch)

$1720-1740 \text{ cm}^{-1}$ —slightly lower than esters, overlapping range requires additional evidence
Distinctive twin C-H stretches—look for two peaks around $2720$ and $2820 \text{ cm}^{-1}$ (Fermi resonance doublet)
Terminal carbonyl position—aldehydes are more reactive than ketones, relevant for mechanism questions

Ketones (C=O stretch)

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

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

$1700-1725 \text{ cm}^{-1}$ for $\text{C}=\text{O}$ —similar to ketones, but the O-H is the giveaway
Very broad O-H stretch from $2500-3300 \text{ cm}^{-1}$ —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)

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

Compare: Aldehydes vs. Ketones—both show $\text{C}=\text{O}$ near $1720 \text{ cm}^{-1}$ , but only aldehydes display the twin C-H peaks around $2720/2820 \text{ cm}^{-1}$ . 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 ( $2500-3300 \text{ cm}^{-1}$ ) while amides show sharper N-H peaks ( $3300-3500 \text{ cm}^{-1}$ ). 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 ( $3000-3700 \text{ cm}^{-1}$ ) because hydrogen's low mass increases vibrational frequency. Hydrogen bonding broadens these peaks dramatically.

Alcohols (O-H stretch)

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

Amines (N-H stretch)

$3300-3500 \text{ cm}^{-1}$ —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 $3200-3600 \text{ cm}^{-1}$ 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)

$2850-2960 \text{ cm}^{-1}$ — $sp^3$ C-H bonds absorb below $3000 \text{ cm}^{-1}$
Strong, multiple peaks—asymmetric and symmetric stretches of $\text{CH}_3$ and $\text{CH}_2$ groups overlap
Benchmark for saturation—if all C-H peaks are below $3000 \text{ cm}^{-1}$ , no $sp^2$ or $sp$ carbons are present

Alkenes (C=C stretch)

$1620-1680 \text{ cm}^{-1}$ —moderate intensity; symmetrical alkenes may show weak or absent peaks
$sp^2$ C-H stretch just above $3000 \text{ cm}^{-1}$ —typically $3020-3100 \text{ cm}^{-1}$ , 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 $3030 \text{ cm}^{-1}$ — $sp^2$ hybridized, appears just above $3000 \text{ cm}^{-1}$
Ring vibrations at $1400-1600 \text{ cm}^{-1}$ —multiple sharp peaks from $\text{C}=\text{C}$ stretching in the ring
Overtone pattern at $1650-2000 \text{ cm}^{-1}$ —weak peaks whose pattern indicates substitution pattern

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

Single-Bond Stretches: The Fingerprint Region

Absorptions below $1500 \text{ cm}^{-1}$ 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)

$1050-1150 \text{ cm}^{-1}$ —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: $1500-1600 \text{ cm}^{-1}$ and $1300-1400 \text{ cm}^{-1}$ —asymmetric and symmetric $\text{NO}_2$ 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)

$600-800 \text{ cm}^{-1}$ —varies by halogen: C-F highest ( $1000-1400 \text{ cm}^{-1}$ ), C-I lowest ( $500-600 \text{ cm}^{-1}$ )
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 $3200-3600 \text{ cm}^{-1}$ that ethers lack. If you see C-O absorption without O-H, you're looking at an ether.

Quick Reference Table

Concept	Best Examples
Triple bond region ( $2100-2300 \text{ cm}^{-1}$ )	Alkynes, Nitriles
Carbonyl stretch ( $1650-1750 \text{ cm}^{-1}$ )	Esters, Aldehydes, Ketones, Carboxylic acids, Amides
Broad O-H (hydrogen bonding)	Alcohols, Carboxylic acids
N-H stretch ( $3300-3500 \text{ cm}^{-1}$ )	Amines, Amides
$sp^3$ C-H (below $3000 \text{ cm}^{-1}$ )	Alkanes
$sp^2$ C-H (above $3000 \text{ cm}^{-1}$ )	Alkenes, Aromatic compounds
Fingerprint region diagnostics	Ethers, Nitro compounds, Halides

Self-Check Questions

You observe a strong, sharp peak at $1715 \text{ cm}^{-1}$ and two smaller peaks near $2720$ and $2820 \text{ cm}^{-1}$ . Which functional group is present, and what feature distinguishes it from a ketone?
Compare the O-H stretch in alcohols versus carboxylic acids. Why does the carboxylic acid absorption extend to much lower wavenumbers ( $2500 \text{ cm}^{-1}$ )?
A compound shows absorption at $2240 \text{ cm}^{-1}$ but nothing near $3300 \text{ cm}^{-1}$ . Is this more likely an alkyne or a nitrile? Explain your reasoning.
How would you distinguish between a primary amine and an alcohol using IR spectroscopy if both absorb in the $3300-3500 \text{ cm}^{-1}$ region?
An FRQ presents a spectrum with strong carbonyl absorption at $1680 \text{ cm}^{-1}$ and N-H peaks at $3350 \text{ cm}^{-1}$ . Identify the functional group and explain why the carbonyl appears at a lower frequency than typical aldehydes or ketones.