In 1825, Faraday isolated benzene from compressed illuminating gas made by pyrolyzing whale oil.
In 1834, German chemist Eilhardt Mitscherlich synthesized benzene by heating benzoic acid with calcium oxide:
COOH + CaO \xrightarrow{heat} Benzene + CaCO_3
In the 19th century, organic compounds were classified as aliphatic or aromatic.
Aliphatic: Compounds with "fatlike" chemical behavior.
Aromatic: Compounds with a low hydrogen-to-carbon ratio and a fragrant scent.
2. Nomenclature of Benzene Derivatives
Naming Monosubstituted Benzenes
For many simple compounds, benzene is the parent name, and the substituent is indicated by a prefix (e.g., fluorobenzene, chlorobenzene, bromobenzene, nitrobenzene).
For other simple and common compounds, the substituent and benzene ring together form a commonly accepted parent name.
Naming Disubstituted Benzenes
When two substituents are present, their relative positions are indicated by the prefixes ortho- (o-), meta- (m-), and para- (p-) or by using numbers.
Examples:
2-Nitrobenzoic acid (o-Nitrobenzoic acid)
3-Methylphenol (m-Methylphenol)
4-Chlorotoluene (p-Chlorotoluene) or (1-Chloro-4-methylbenzene)
Dimethylbenzenes are often called xylenes:
1,2-Dimethylbenzene (o-xylene)
1,3-Dimethylbenzene (m-xylene)
1,4-Dimethylbenzene (p-xylene)
Naming Benzene Rings with More Than Two Groups
If more than two groups are present, their positions must be indicated using numbers.
Number the benzene ring to give the lowest possible numbers to the substituents.
Examples:
1,2,3-Trichlorobenzene
1,2,4-Tribromobenzene (not 1,3,4-Tribromobenzene)
When more than two substituents are present and different, list them in alphabetical order.
Example: 4-Bromo-1-chloro-2-fluorobenzene
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.
Examples:
3,5-Dichlorophenol
5-Bromo-2-methylbenzoic acid
When the C6H5 group is named as a substituent, it's 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:
Butylbenzene
Isopropylbenzene
trans-1-Phenyl-1-butene
(R)-3-Phenyloctane
Benzyl is an alternative name for the phenylmethyl group (abbreviated Bn). Example: Benzyl chloride (phenylmethyl chloride or BnCl).
3. Reactions of Benzene
Benzene undergoes substitution, but not addition due to its stability.
Example:
C6H6 + Br2 \xrightarrow{FeBr3} C6H5Br + HBr
4. The Kekulé Structure for Benzene
The Kekulé formula for benzene is a six-membered ring with alternating single and double bonds.
The 1,2-dibromobenzenes do not exist as isomers, indicating that there is no such equilibrium between benzene ring bond isomers.
5. The Thermodynamic Stability of Benzene
π bonds are formed from the sideways overlap of p orbitals; π electron clouds are above and below the plane of the double bond.
Benzene is more stable than expected due to resonance.
Resonance (stabilization) energy = 152 kJ/mol
6. Modern Theories of the Structure of Benzene
All C-C bond lengths are the same (1.39 Å), which is between a C-C single bond (1.54 Å) and a C=C double bond (1.34 Å).
Extra stabilization is due to resonance.
Planar structure with all carbons sp^2 hybridized.
7. Hückel’s Rule: The (4n + 2) 𝜋 Electron Rule
Hückel’s rule concerns compounds containing one planar ring in which each atom has a 𝜋 orbital as in benzene.
Planar monocyclic rings containing (4n + 2) 𝜋 electrons, where n = 0, 1, 2, 3, and so on (i.e., rings containing 2, 6, 10, 14, etc. 𝜋 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… delocalized electrons should 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, and that the molecule has a tub-like shape, which is not aromatic.
8. Other Aromatic Compounds
Benzenoid polycyclic aromatic hydrocarbons consist of molecules having two or more benzene rings fused together.
Nonbenzenoid Aromatic Compounds (e.g., Azulene)
9. Heterocyclic Aromatic Compounds
Cyclic compounds that include an element other than carbon are called heterocyclic compounds.
Examples of useful heterocyclic aromatic compounds:
Serotonin (neurotransmitter)
Penicillin (antibiotic)
Nitrofurantoin (urinary antibacterial)
Viagra
10. Aromatic Compounds in Biochemistry
Two amino acids necessary for protein synthesis contain the benzene ring: Phenylalanine and Tyrosine.