Spectroscopy of aromatic compounds

The IR spectrum of toluene in figure 15.13 shows these characteristic absorptions. 

Aromatic rings are detectable by ultraviolet spectroscopy because they contain a conjugated rr electron system. In general, aromatic compounds show a series of bands, with a fairly intense absorption near 205 nm and a less intense absorption in the 255 to 275 nm range. The presence of these bands in the ultraviolet spectrum of a molecule is a sure indication of an aromatic ring. 

Much of the difference in chemical shift between aromatic protons (6.5-8.0 8) and vinylic protons (4.5-6.5 5) is clue to a property of aromatic 

Induced magnetic field because of ring current 

The presence of a ring-current is characteristic of all Hiickel aromatic molecules and is a good test of aromaticity. For example, benzene, a six-7r-clectron aromatic molecule, absorbs at 7.37 5, but cyclooctatetraene, an eight--iT-electxon nonaromatic molecule, absorbs at 5.78 8. 

1700-2000 cm Combination bands A group of very weak signals ra 1 40-  

An IR spectrum of ethylbenzene indicating the five regions of absorption, characteristic of aromatic compounds. 

In Section 16.5, we first discussed the anisotropic eflFects of an aromatic ring. Specifically, the motion of the it electrons generates a local magnetic field that effectively deshields the protons connected directly to the ring. 

A NMR spectrum of ethylbenzene showing that the deshielding effects of the aromatic ring are strongly dependent on the proximity of the proton to the ring. 

As first mentioned in Section 16.12, the carbon atoms of aromatic rings typically produce signals in the range of 100—150 ppm in a NMR spectrum. The number of signals is very helpful in determining the specific substitution pattern for substituted aromatic rings. Several common substitution patterns are shown below. 

Infrared Spectroscopy (Review) Aromatic compounds are readily identified by their infrared spectra because they show a characteristic C=C stretch around 1600 cm . This is a lower C=C stretching frequency than for isolated alkenes (1640 to 1680 cm ) or conjugated dienes (1620 to 1640 cm ) because the aromatic bond order is only about I 5. The aromatic bond is therefore less stiff than a normal double bond, and it vibrates at a lower frequency. 

Like alkenes, aromatic compounds show unsaturated =C—H stretching just above 3000 cm (usually around 3030 cm ). The combination of the aromatic C = C stretch around 16(X)cm and the =C—H stretch just above 3000 cm leaves little doubt of the presence of an aromatic ring. The sample spectra labeled Compounds 4, 5, and 7 in Chapter 12 (pages 534—536) show compounds containing aromatic rings. 

Aromatic carbon atoms absorb around 6120 to 6150 in the C NMR spectrum. Alkene carbon atoms can also absorb in this spectral region, but the combination of NMR with h NMR or IR spectroscopy usually leaves no doubt about the presence of an aromatic ring. 

The mass spectrum of n-butylbenzene has its base peak at ffj/z 91, corresponding to cleavage of a benzylic bond. The fragments are a benzyl cation and a propyl radical. The benzyl cation rearranges to the tropylium ion, detected at m/z 91. 

All three major bands in the benzene spectrum correspond to transi- 

Azulene, a beautiful blue hydrocarbon, is an isomer of naphthalene. Is azulene aromatic Draw a second resonance form of azulene in addition to that shown. 

How many electrons does each of the four nitrogen atoms in purine contribute to the aromatic 17 system  

Aromatic rings show a characteristic C-H stretching absorption at 3030 cm and a series of peaks in the 1450 to 1600 cm range of the infrared spectrum. The aromatic C H hand at 3030 cm generally has low intensity and occurs just to the left of a typical saturated C-H hand. 

Hydrogens on carbon next to aromatic rings also show distinctive absorptions in the NMR spectrum. Benzylic protons normally absorb downfieid from other alkane protons in the region from 2.3 to 3.0 8. 

Spectroscopy provides many clues to the identity of a compound. Aromatic compounds, because of the delocalization of the electrons, have unique features in their spectra. In fact, spectral evidence can indicate what atoms or functional groups are attached to the aromatic ring or whether the ring itself contains an atom other than carbon. 

The C-H bends are chciracteristic of the substitution ciround the ciromatic ring. Aromatic compounds have a characteristic C-H peak near 3,030 cm. In addition, the infrared spectra can contain the following features  

Up to four ring stretches exist in the 1,450-1,600 cm region and two stronger peaks cire in the 1,500-1,600 cm region. 

In the infrcired spectra of monosubstituted benzenes, usually two very strong peaks appear one between 690 and 710 cm and one between 730 and 770 cm .  

Ortho-disubstituted benzenes show a strong peak between 680 and 725 cm and a very strong peak between 750 and 810 cm .  

In addition, the infrared spectra can contain the following features  

In Section 14.7C we found that absorption takes place far downfield because a ring current generated in the benzene ring creates a magnetic field, called the induced field, which reinforces the applied magnetic field at the position of the protons of the ring. This reinforcement causes the protons of benzene to be highly deshielded. 

We also learned in Section 14.7C that internal hydrogens of large-ring aromatic compounds such as [18]annulene, because of their position, are highly shielded by this induced field. They therefore absorb at unusually low frequency, often at negative delta values. 

The DEPT spectra (not given to save space) show that the signal at S 45 arises from a CH2 group and the one at S 13 arises from a CH3 group. This allows us to assign these two signals immediately to the two carbons of the equivalent ethyl groups. 

This leaves the signals at S 112 and 5 133 and the two sets of carbon atoms of the benzene ring labeled c and d to be accounted for. Both signals are indicated as CH groups in the DEPT spectra. But which signal belongs to which set of carbon atoms Here we find another interesting application of resonance theory. 

If we write resonance structures A-D involving the unshared electron pair of the amino group, we see that contfibutions made by B and D increase the electron density at the set of carbon atoms labeled d  

---

In Part 11 we concentrate on aromatic systems, starting with the basics of structure and properties of benzene and then moving on to related ciromatic compounds. We even throw in a section of spectroscopy of aromatic compounds. Chapters 7 and 8 finish up this pcirt by going into detail about substitution reactions of aromatic compounds. You find out all you ever wanted to know (and maybe more) about electrophilic and nucleophilic substitutions, along with a little about elimination reactions. 

Getting acquainted with heterocyclic aromatic compounds Shedding light on the spectroscopy of aromatic compounds... 

Physical Properties of Benzene and Its Derivatives 742 16-15 Spectroscopy of Aromatic Compounds 743 EssentialTerms 746 Study Problems 748... 

Fleger Y, Mastai Y, Rosenbluh M, Dressier DH (2009) Surface enhanced Raman spectroscopy of aromatic compounds on silver nanoclusters. Surf Sci 603 788-793... 

Spectroscopy of Aromatic Compounds 652 THE CHEMISTRY OF... Sunscreens (Catching the Sun s Rays and What Happens to Them) 656... 

---