15.7 Spectroscopy of Aromatic Compounds

Spectroscopic Analysis of Aromatic Compounds

Aromatic compounds produce distinctive signals across IR, UV, and NMR spectroscopy. Recognizing these spectral fingerprints lets you confirm aromaticity and determine substitution patterns on a benzene ring.

Spectral Patterns of Aromatic Compounds

Infrared (IR) Spectroscopy

IR spectroscopy reveals three key absorption regions for aromatic compounds:

• Aromatic C-H stretching: weak bands at 3100–3000 cm$^{-1}$. These appear just above 3000 cm$^{-1}$, which is how you distinguish them from sp$^3$ C-H stretches (which fall just below 3000 cm$^{-1}$).
• Aromatic C=C stretching: weak bands at 1600–1475 cm$^{-1}$. You'll typically see two or three bands in this region. They're often weak, so look carefully.
• Out-of-plane C-H bending: strong bands at 900–690 cm$^{-1}$. These are the most useful IR bands for aromatics because their exact positions tell you the substitution pattern (more on this below).

Ultraviolet (UV) Spectroscopy

The delocalized π-electron system in aromatic rings acts as a chromophore, producing characteristic UV absorptions from π → π* transitions. Benzene itself absorbs at 184 nm ($\varepsilon \approx 60{,}000$) and 204 nm ($\varepsilon \approx 8{,}000$).

When substituents are added to the ring, two common effects appear:

• Bathochromic shift (red shift): absorption moves to longer wavelengths
• Hyperchromic effect: absorption intensity increases

Electron-donating groups (like $-\text{NH}_2$ or $-\text{OH}$) and extended conjugation both push absorption to longer wavelengths, which can help you identify what's attached to the ring.

Nuclear Magnetic Resonance (NMR) Spectroscopy

• $^1$H NMR: Aromatic protons resonate in the 6.5–8.5 ppm range. This downfield position is a direct result of the ring-current effect (explained in the next section).
• $^{13}$C NMR: Aromatic carbons appear in the 120–150 ppm range for unsubstituted ring carbons, while carbons bearing electronegative substituents (like $-\text{OH}$ or $-\text{NO}_2$) can shift as far downfield as ~170 ppm.

Symmetry in the substitution pattern simplifies the spectrum. A para-disubstituted benzene, for example, often shows just two doublets in the aromatic region because there are only two sets of equivalent protons.

Ring-Current Effects on NMR Shifts

The ring-current effect is one of the most reliable ways to confirm aromaticity by NMR.

Here's how it works: when an aromatic ring is placed in an external magnetic field, the circulating π-electrons generate their own induced magnetic field. This induced field has two zones:

• Outside the ring (in the plane): the induced field reinforces the external field, so protons here experience a stronger total field. They are deshielded and shift downfield (6.5–8.5 ppm).
• Above and below the ring: the induced field opposes the external field, so anything positioned here is shielded and shifts upfield.

You can see the deshielding effect even on nearby groups. Compare the methyl group in toluene ($\text{CH}_3$ at ~2.3 ppm) to the methyl group in ethane ($\text{CH}_3$ at ~0.9 ppm). That ~1.4 ppm downfield shift comes from the ring current of the adjacent benzene ring, even though the methyl protons aren't on the ring itself.

Aromatic Substitution Patterns in Infrared Spectroscopy

The out-of-plane C-H bending bands (900–690 cm$^{-1}$) are diagnostic for determining how many substituents are on the ring and where they are. Memorize these patterns:

Additional Spectroscopic Tools for Aromatic Compounds

Coupling Constants in $^1$H NMR

The $^3J$ coupling constant between aromatic protons varies with their relative positions, which helps you assign substitution patterns:

• Ortho coupling ($^3J$): 6–10 Hz
• Meta coupling ($^4J$): 1–3 Hz
• Para coupling ($^5J$): 0–1 Hz (often unresolved)

If you see a pair of doublets in the aromatic region with $J \approx 8$ Hz, that's strong evidence for ortho-coupled protons.

UV-Vis and Extended Conjugation

When an aromatic ring is conjugated with additional π-systems (like a carbonyl group or a vinyl group), the chromophore extends. This shifts UV absorption to longer wavelengths and often increases intensity. Comparing the UV spectrum of benzene ($\lambda_{\text{max}}$ = 204 nm) to styrene ($\lambda_{\text{max}}$ = 248 nm) shows this clearly.

Mass Spectrometry

Aromatic compounds often show a strong molecular ion peak ($M^+$) because the aromatic ring stabilizes the radical cation. A common fragmentation for alkylbenzenes is loss of a substituent to give the tropylium cation ($m/z = 91$, $\text{C}_7\text{H}_7^+$), which is unusually stable due to its aromatic 6-π-electron system.