Chapter 14 Aromatic Compounds

About The Authors These PowerPoint Lecture Slides were created and prepared by Professor William Tam and his wife, Dr. Phillis Chang. Professor William Tam received his B.Sc. at the University of Hong Kong in 1990 and his Ph.D. at the University of Toronto (Canada) in 1995. He was an NSERC postdoctoral fellow at the Imperial College (UK) and at Harvard University (USA). He joined the Department of Chemistry at the University of Guelph (Ontario, Canada) in 1998 and is currently a Full Professor and Associate Chair in the department. Professor Tam has received several awards in research and teaching, and according to Essential Science Indicators, he is currently ranked as the Top 1% most cited Chemists worldwide. He has published four books and over 80 scientific papers in top international journals such as J. Am. Chem. Soc., Angew. Chem., Org. Lett., and J. Org. Chem. Dr. Phillis Chang received her B.Sc. at New York University (USA) in 1994, her M.Sc. and Ph.D. in 1997 and 2001 at the University of Guelph (Canada). She lives in Guelph with her husband, William, and their son, Matthew.

The Discovery of Benzene In 1825, Faraday isolated benzene from a compressed illuminating gas that had been made by pyrolyzing whale oil

In 1834, a German chemist, Eilhardt Mitscherlich, synthesized benzene by heating benzoic acid with calcium oxide

In 19th century, organic compounds were classified as being either aliphatic or aromatic The chemical behavior of a compound was “fatlike” Aromatic The compound had a low hydrogen-to-carbon ratio and it was “fragrant”

Nomenclature of Benzene Derivatives Naming monosubstituted benzenes In many simple compounds, benzene is the parent name and the substituent is simply indicated by a prefix

For other simple and common compounds, the substituent and the benzene ring taken together may form a commonly accepted parent name

Naming disubstituted benzenes When two substituents are present, their relative positions are indicated by the prefixes ortho-, meta-, and para- (abbreviated o-, m-, and p-) or by the use of numbers

Other examples

The dimethylbenzenes are often called xylenes

Naming benzene rings with more than two groups If more than two groups are present on the benzene ring, their positions must be indicated by the use of numbers The benzene ring is numbered so as to give the lowest possible numbers to the substituents

When more than two substituents are present and the substituents are different, they are listed in alphabetical order

When a substituent is one that, together with the benzene ring gives a new base name, that substituent is assumed to be in position 1 and the new parent name is used

When the C6H5 group is named as a substituent, it is called a phenyl group A hydrocarbon composed of one saturated chain and one benzene ring is usually named as a derivative of the larger structural unit. However, if the chain is unsaturated, the compound may be named as a derivative of that chain, regardless of ring size

Examples

Benzyl is an alternative name for the phenylmethyl group Benzyl is an alternative name for the phenylmethyl group. It is sometimes abbreviated Bn

Reactions of Benzene

Benzene undergoes substitution but not addition

The Kekulé Structure for Benzene

X These 1,2-dibromobenzenes do not exist as isomers There is no such equilibrium between benzene ring bond isomers

The Thermodynamic Stability of Benzene Since p bonds are formed from side-way overlap of p orbitals, p electron clouds are above & below the plane of the double bond p-electrons above and below ring

Modern Theories of the Structure of Benzene 6A. The Resonance Explanation of the Structure of Benzene All bond lengths the same (1.39 Å) (compare with C–C single bond 1.54 Å, C=C double bond 1.34 Å) Extra stabilization due to resonance  aromatic

3-D structure p-electrons above and below ring Planar structure All carbons sp2 hybridized

6B. The Molecular Orbital Explanation of the Structure of Benzene

Hückel’s Rule: The 4n + 2 p Electron Rule Hückel’s rule is concerned with compounds containing one planar ring in which each atom has a p orbital as in benzene Planar monocyclic rings containing 4n + 2 p electrons, where n = 0, 1, 2, 3, and so on (i.e., rings containing 2, 6, 10, 14 . . . etc. p electrons), have closed shells of delocalized electrons like benzene and have substantial resonance energies

Hückel’s rule states that planar monocyclic rings with 2, 6, 10, 14 Hückel’s rule states that planar monocyclic rings with 2, 6, 10, 14 . . . delocalized electrons should be aromatic

7A. How To Diagram the Relative Energies of p Molecular Orbitals in Monocyclic Systems Based on Hückel’s Rule

The p molecular orbitals that cyclooctatetraene would have if it were planar. Notice that, unlike benzene, this molecule is predicted to have two nonbonding orbitals, and because it has eight p electrons, it would have an unpaired electron in each of the two nonbonding orbitals. Such a system would not be expected to be aromatic.

The bonds of cyclooctatetraene are known to be alternately long and short; X-ray studies indicate that they are 1.48 and 1.34 Å, respectively

7B. The Annulenes Hückel’s rule predicts that annulenes will be aromatic if their molecules have 4n + 2 p electrons and have a planar carbon skeleton

All these (4n + 2)p, planar annulenes are aromatic

Non-planar (4n + 2)p annulenes are antiaromatic

(4n)p non-planar annulenes are antiaromatic

7C. NMR Spectroscopy: Evidence for Electron Delocalization in Aromatic Compounds The 1H NMR spectrum of benzene consists of a single unsplit signal at d 7.27 The signal occurs at relatively high frequency, which is compelling evidence for the assertion that the p electrons of benzene are delocalized

The circulation of p electrons in benzene creates an induced magnetic field that, at the position of the protons, reinforces the applied magnetic field. This reinforcement causes the protons to be strongly deshielded and to have a relatively high frequency (d ~ 7) absorption

(d 9.3) (d -3.0)

7D. Aromatic Ions pka = 36 pka = 16

6 p electrons  aromatic sp3 sp2

7E. Aromatic, Antiaromatic, and Nonaromatic Compounds An aromatic compound has its p electrons delocalized over the entire ring and it is stabilized by the p-electron delocalization

Based on sound calculations or experiments One way to evaluate whether a cyclic compound is stabilized by delocalization of p electrons through its ring is to compare it with an open-chain compound having the same number of p electrons Based on sound calculations or experiments If the ring has lower p-electron energy, then the ring is aromatic If the ring and the chain have the same p-electron energy, then the ring is nonaromatic If the ring has greater p-electron energy than the open chain, then the ring is antiaromatic

Cyclobutadiene Benzene

Other Aromatic Compounds 8A. Benzenoid Aromatic Compounds Benzenoid polycyclic aromatic hydrocarbons consist of molecules having two or more benzene rings fused together

8B. Nonbenzenoid Aromatic Compounds

8C. Fullerenes

Heterocyclic Aromatic Compounds Cyclic compounds that include an element other than carbon are called heterocyclic compounds

Examples of useful heterocyclic aromatic compounds

Aromaticity

Aromaticity Evidence: 1H NMR shift

Basicity of nitrogen-containing heterocycles

Basicity of nitrogen-containing heterocycles

Non-basic nitrogen basic nitrogen

Aromatic Compounds in Biochemistry Two amino acids necessary for protein synthesis contain the benzene ring

Derivatives of purine and pyrimidine are essential parts of DNA and RNA

Nicotinamide adenine dinucleotide, one of the most important coenzymes in biological oxidations and reductions, includes both a pyridine derivative (nicotinamide) and a purine derivative (adenine) in its structure

Spectroscopy of Aromatic Compounds 11A. 1H NMR Spectra The ring hydrogens of benzene derivatives absorb downfield in the region between d 6.0 and d 9.5 ppm 11B. 13C NMR Spectra The carbon atoms of benzene rings generally absorb in the d 100–170 ppm region of 13C NMR spectra

11C. Infrared Spectra of Substituted Benzenes

11D. Ultraviolet–Visible Spectra of Aromatic Compounds

11E. Mass Spectra of Aromatic Compounds

 END OF CHAPTER 14 