Spectral Characteristics of the Benzene Ring15-4

The UV-visible spectrum of benzene reveals its electronic structure.The energy gap between bonding and antibonding orbitals is greater in benzene than for acyclic trienes.

The UV spectra of benzene shows absorbances at smaller wavelengths (higher energy) than does the spectra of 1,3,5-hexatriene:

The electronic spectra of aromatic compounds varies with the introduction of substituents (useful in designing dye molecules).

Simple substituted benzenes absorb between 250 and 290 nm.

4-Aminobenzoic acid (PABA) has a max of 289 nm and a high extinction coefficient of 18,600. It is used in sunscreen lotions to filter out harmful UV light in this wavelength region.

The infrared spectrum reveals substitution patterns in benzene derivatives.

The IR spectra of benzene and its derivatives have characteristic bands in three regions:

• 3030 cm-1 phenyl-hydrogen stretching

• 1500-2000 cm-1 aromatic ring C-C stretching

• 650-1000 cm-1 C-H out-of-plane bending

The specific substitution pattern determines the precise location of the C-H out-of-plane bending absorptions. For the dimethylbenzenes:

• 1,2-dimethylbenzene (o-) 738 cm-1

• 1,4-dimethylbenzene (p-) 793 cm-1

• 1,3-dimethylbenzene (m-) 690 and 765 cm-1

The mass spectrum of benzene indicates stability.The mass spectrum of benzene shows little fragmentation due to its unusual stability. The (M+1)+. peak shows the correct peak height (6.8%) for the relative abundance of 13C in a six-carbon molecule.

The NMR spectra of benzene derivatives show the effects of an electronic ring current.

The cyclic delocalization of the electrons in the aromatic ring gives rise to unusual deshielding:

Aromatic ring hydrogens: 6.5-8.5 ppm

Alkenyl hydrogens: 4.6-5.7 ppm

Benzene hydrogens: 7.27 ppm (single peak)

This aromatic deshielding is due to ring currents produced by the electrons moving in the external magnet field, H0.

The magnetic field from the ring current opposes H0 inside the loop but opposes it outside the loop where the hydrogens are located, resulting in deshielding.

The effect is strongest closest to the ring and diminishes rapidly with distance.

Benzylic nuclei are deshielded only about 0.4-0.8 ppm more than their allylic counterparts.

Hydrogens farther away from system have chemical shifts similar to those in the alkanes.

Substituted benzenes may have more complicated NMR patterns.

The presence of a substituent renders the ortho, meta and para hydrogens non-equivalent and subject to mutual coupling.

(Benzene: 7.27 ppm)

4-(N,N-dimethylamino)benzaldehyde shows a large chemical shift difference between the two sets of ring hydrogens and a near first-order pattern of two doublets. The 9 Hz coupling constant is typical of splitting between ortho protons.

All three types of coupling can be seen in the first order spectrum of 1-methoxy-2,4-dinitrobenzene (2,4-dinitroanisole).

Ortho hydrogen (to methoxy)

Doublet, =7.23 ppm, 9 Hz coupling

Hydrogen flanked by nitro groups

Doublet, =8.76 ppm, 3 Hz coupling

Remaining ring hydrogen

Doublet of doublets, =8.45 ppm,

Para coupling between C3 and C6 is too small to be resolved.

The 13C NMR spectra of benzene derivatives is not greatly affected by ring current shifts, since the induced ring current flows directly above and below the ring carbons.

Benzene carbons exhibit chemical shifts similar to those in alkanes (120-135 ppm when unsubstituted). Benzene exhibits a single line at =128.7 ppm.