Unit 15: Aromatic Hydrocarbons

1. Introduction and Characteristics of Aromatic Compounds

Aromatic compounds represent a distinct class of organic molecules characterized by specific structural and electronic properties that confer unusual stability. The most fundamental aromatic compound is benzene, which serves as the prototype for understanding this class.

1.1. Defining Aromaticity

Aromatic compounds typically contain a benzene ring or similar stable, cyclic ring systems. These systems are not merely cyclic; they possess a delocalized system of pi electrons that significantly enhances their stability compared to their acyclic or non-aromatic cyclic counterparts.

1.2. Characteristic Properties

• Unusual Stability: Aromatic compounds exhibit an exceptional degree of stability. This stability is often referred to as "aromatic stabilization energy" and is a direct consequence of the delocalization of pi electrons.
• Reactivity: Unlike alkenes, which readily undergo addition reactions across their double bonds, aromatic compounds primarily undergo substitution reactions. This preference for substitution over addition helps maintain the stable aromatic system. For example, benzene resists addition reactions that would disrupt its delocalized pi system, preferring instead to replace a hydrogen atom with another group.

1.3. Huckel's Rule of Aromaticity

To be considered aromatic, a compound must satisfy a set of criteria known as Huckel's Rule. This rule provides a theoretical basis for predicting aromaticity:

1. Planar: The entire ring must be planar, meaning all atoms in the ring lie in the same plane. This allows for effective overlap of p-orbitals.
2. Cyclic: The molecule must have a cyclic structure.
3. Fully Conjugated: The ring must be fully conjugated, meaning there is a continuous overlap of p-orbitals around the entire ring. This typically implies alternating single and double bonds, or the presence of atoms with lone pairs or empty p-orbitals that can participate in conjugation.
4. 4n+2 pi electrons: The cyclic, planar, fully conjugated system must contain a specific number of pi electrons, given by the formula 4n+2, where n is a non-negative integer (n = 0, 1, 2, 3, ...).

Examples:

• For n = 0, 4(0)+2 = 2 pi electrons (e.g., cyclopropenyl cation).
• For n = 1, 4(1)+2 = 6 pi electrons (e.g., benzene, pyridine).
• For n = 2, 4(2)+2 = 10 pi electrons (e.g., naphthalene).
• For n = 3, 4(3)+2 = 14 pi electrons (e.g., anthracene, phenanthrene).

2. Kekule Structure of Benzene

The molecular formula of benzene is C6H6. For many years, its structure posed a significant challenge to chemists. In 1865, Friedrich August Kekule proposed a revolutionary structure for benzene.

2.1. Kekule's Proposal

Kekule suggested that benzene consists of a hexagonal ring of six carbon atoms, with each carbon atom bonded to one hydrogen atom. Within this ring, he proposed alternating single and double bonds. This structure implied that benzene should behave like an alkene, readily undergoing addition reactions due to the presence of double bonds.

However, experimental evidence contradicted this. Benzene did not readily decolorize bromine water (a test for unsaturation) and preferred substitution reactions, indicating a higher stability than typical alkenes.

2.2. Bond Lengths in Benzene

Further experimental studies, particularly X-ray diffraction, revealed another discrepancy with the simple Kekule structure:

• A typical carbon-carbon single bond (C-C) has a bond length of approximately 1.54 Å.
• A typical carbon-carbon double bond (C=C) has a bond length of approximately 1.34 Å.
• However, in benzene, all six carbon-carbon bonds are found to have an identical bond length of 1.40 Å. This value is intermediate between that of a single and a double bond, suggesting that the bonds are neither purely single nor purely double.

2.3. Stability of Benzene

The unusual stability of benzene can be quantified by comparing its heat of hydrogenation to that of a hypothetical cyclohexatriene (a cyclic compound with three isolated double bonds). Cyclohexene, with one double bond, has a heat of hydrogenation of approximately -120 kJ/mol. Therefore, a hypothetical cyclohexatriene (three double bonds) would be expected to have a heat of hydrogenation of roughly 3 * (-120 kJ/mol) = -360 kJ/mol.

However, the experimentally determined heat of hydrogenation for benzene is only about -208 kJ/mol. The difference (360 - 208 = 152 kJ/mol) represents the resonance stabilization energy of benzene. This significantly lower heat of hydrogenation than expected is strong evidence of benzene's enhanced stability due to electron delocalization.

3. Resonance and Isomerism

To reconcile the discrepancies between the Kekule structure and experimental observations, the concept of resonance was applied to benzene.

3.1. Resonance Hybrid of Benzene

Benzene is not accurately represented by a single Kekule structure. Instead, it is best described as a resonance hybrid of two contributing Kekule structures. These structures differ only in the placement of their pi electrons, with the double and single bonds interchanging positions.

The actual structure of benzene is an average of these two resonance forms. This means there are no true double bonds and no true single bonds; instead, all carbon-carbon bonds are equivalent and possess partial double bond character. This delocalization of pi electrons over the entire ring is often depicted by a circle inside the hexagon, symbolizing the continuous cloud of pi electrons above and below the plane of the ring.

The resonance concept explains:

• The equal bond lengths of 1.40 Å.
• The unusual stability of benzene, as delocalization of electrons lowers the overall energy of the molecule.
• The preference for substitution reactions, as addition would destroy the stable delocalized pi system.

3.2. Isomerism in Disubstituted Benzene

When two substituents are present on the benzene ring, their relative positions give rise to distinct isomers. These isomers are designated using specific prefixes:

• Ortho (o-): The two substituents are on adjacent carbon atoms, specifically at positions 1,2.

  Example: 1,2-dichlorobenzene

• Meta (m-): The two substituents are separated by one carbon atom, at positions 1,3.

  Example: 1,3-dichlorobenzene

• Para (p-): The two substituents are on opposite sides of the ring, at positions 1,4.

  Example: 1,4-dichlorobenzene

4. Preparation of Benzene

Benzene can be prepared through several laboratory and industrial methods:

4.1. Decarboxylation of Sodium Benzoate

Heating sodium benzoate with soda lime (a mixture of sodium hydroxide and calcium oxide) results in the removal of a carboxyl group as carbon dioxide, producing benzene.

C6H5COONa + NaOH --(CaO, heat)--> C6H6 + Na2CO3

Here, C6H5COONa is sodium benzoate, NaOH is sodium hydroxide, C6H6 is benzene, and Na2CO3 is sodium carbonate. Calcium oxide (CaO) is used to keep the soda lime dry and prevent the glass from cracking.

4.2. From Phenol

Phenol can be reduced to benzene by heating it with zinc dust. The zinc removes the oxygen atom from the hydroxyl group.

C6H5OH + Zn --(heat)--> C6H6 + ZnO

Here, C6H5OH is phenol, Zn is zinc, C6H6 is benzene, and ZnO is zinc oxide.

4.3. From Ethyne (Cyclic Polymerization)

When ethyne (acetylene) gas is passed through a red hot iron tube, three molecules of ethyne undergo cyclic polymerization to form one molecule of benzene. This is a classic example of a cyclotrimerization reaction.

3C2H2 --(Red hot iron tube)--> C6H6

Here, C2H2 is ethyne, and C6H6 is benzene.

5. Physical Properties of Benzene

Benzene exhibits several characteristic physical properties:

• Appearance: It is a colorless liquid at room temperature.
• Odor: It possesses a distinctive, pleasant odor. However, benzene is carcinogenic and its vapors should not be inhaled.
• Solubility: Benzene is insoluble in water due to its nonpolar nature. It is, however, readily soluble in organic solvents such as ether, alcohol, and chloroform, and is itself an excellent solvent for many organic compounds.
• Boiling Point: It has a relatively low boiling point of 80 °C (353 K).
• Flammability: Benzene is highly flammable and burns with a sooty flame, indicative of its high carbon content.
• Density: It is less dense than water (density approximately 0.876 g/mL at 20 °C).

6. Chemical Properties of Benzene

The chemical properties of benzene are primarily dictated by its aromatic nature, leading to a preference for substitution reactions over addition, though addition reactions can occur under specific, harsh conditions.

6.1. Addition Reactions

These reactions involve the breaking of the aromatic system to form saturated or partially saturated products. They require more vigorous conditions than for alkenes.

6.1.1. Hydrogenation

Benzene can be completely hydrogenated to cyclohexane in the presence of a catalyst like nickel, platinum, or palladium at elevated temperatures and pressures.

C6H6 + 3H2 --(Ni catalyst, heat, pressure)--> C6H12

Here, C6H6 is benzene, H2 is hydrogen, and C6H12 is cyclohexane.

6.1.2. Halogenation (under UV light)

In the presence of UV light, benzene reacts with halogens (like chlorine) via a free radical addition mechanism, leading to the addition of halogen atoms across the double bonds, disrupting the aromaticity.

C6H6 + 3Cl2 --(UV light)--> C6H6Cl6

Here, C6H6 is benzene, Cl2 is chlorine, and C6H6Cl6 is Benzene Hexachloride (BHC), also known as Lindane, which is an insecticide.

6.2. Electrophilic Substitution Reactions

These are the most characteristic reactions of benzene. In these reactions, an electrophile (an electron-deficient species) attacks the electron-rich benzene ring, and a hydrogen atom on the ring is replaced by the electrophile, preserving the aromatic system.

6.2.1. Nitration

Benzene reacts with a nitrating mixture (concentrated nitric acid and concentrated sulfuric acid) to form nitrobenzene. The electrophile is the nitronium ion (NO2+), generated from the reaction of the two acids.

C6H6 + HNO3 --(conc. H2SO4, 50-60°C)--> C6H5NO2 + H2O

Here, C6H6 is benzene, HNO3 is nitric acid, H2SO4 is sulfuric acid, C6H5NO2 is nitrobenzene, and H2O is water.

6.2.2. Sulphonation

Benzene reacts with fuming sulfuric acid (or concentrated sulfuric acid at higher temperatures) to form benzene sulphonic acid. The electrophile is sulfur trioxide (SO3).

C6H6 + H2SO4(fuming) --(heat)--> C6H5SO3H + H2O

Here, C6H6 is benzene, H2SO4 is sulfuric acid, C6H5SO3H is benzene sulphonic acid, and H2O is water.

6.2.3. Halogenation (with Lewis acid catalyst)

Benzene reacts with halogens (e.g., chlorine, bromine) in the presence of a Lewis acid catalyst (e.g., FeCl3, AlCl3, FeBr3) to form haloarenes. The Lewis acid helps generate a strong electrophile (e.g., Cl+).

C6H6 + Cl2 --(FeCl3)--> C6H5Cl + HCl

Here, C6H6 is benzene, Cl2 is chlorine, FeCl3 is ferric chloride, C6H5Cl is chlorobenzene, and HCl is hydrogen chloride.

6.2.4. Friedel-Crafts Alkylation

Benzene reacts with an alkyl halide in the presence of a Lewis acid catalyst (e.g., anhydrous AlCl3) to form an alkylbenzene. The electrophile is an alkyl carbocation.

C6H6 + CH3Cl --(anhyd. AlCl3)--> C6H5CH3 + HCl

Here, C6H6 is benzene, CH3Cl is chloromethane, AlCl3 is aluminum chloride, C6H5CH3 is toluene (methylbenzene), and HCl is hydrogen chloride.

6.2.5. Friedel-Crafts Acylation

Benzene reacts with an acyl halide (or acid anhydride) in the presence of a Lewis acid catalyst (e.g., anhydrous AlCl3) to form an acylbenzene (ketone). The electrophile is an acylium ion.

C6H6 + CH3COCl --(anhyd. AlCl3)--> C6H5COCH3 + HCl

Here, C6H6 is benzene, CH3COCl is acetyl chloride, AlCl3 is aluminum chloride, C6H5COCH3 is acetophenone (methyl phenyl ketone), and HCl is hydrogen chloride.

6.2.6. Combustion of Benzene

Benzene is highly flammable and undergoes combustion. In the presence of sufficient oxygen (free combustion), it burns to produce carbon dioxide and water, releasing a large amount of energy.

2C6H6 + 15O2 --> 12CO2 + 6H2O

In limited oxygen, incomplete combustion occurs, leading to the formation of soot (carbon) and carbon monoxide, which is why it burns with a sooty flame.

6.3. Orientation of Benzene Derivatives

When a monosubstituted benzene undergoes a second electrophilic substitution, the existing substituent directs the incoming electrophile to specific positions (ortho, meta, or para) on the ring. Substituents are categorized based on their directing ability:

6.3.1. Activating Groups (Ortho/Para Directors)

These groups donate electron density to the benzene ring, making it more reactive towards electrophilic substitution. They activate the ortho and para positions more than the meta position, thus directing incoming electrophiles to these positions.

• Examples: Hydroxyl (-OH), Amino (-NH2), Alkyl groups (-CH3, -C2H5), Alkoxy (-OCH3), Halogens (-F, -Cl, -Br, -I - halogens are deactivating but ortho/para directing).

6.3.2. Deactivating Groups (Meta Directors)

These groups withdraw electron density from the benzene ring, making it less reactive towards electrophilic substitution. They deactivate the ortho and para positions more strongly than the meta position, thus directing incoming electrophiles to the meta position.

• Examples: Nitro (-NO2), Carboxyl (-COOH), Carbonyl (-CHO, -COR), Cyano (-CN), Sulfonic acid (-SO3H).

7. Uses of Benzene

Benzene and its derivatives are vital in various industrial applications:

• Solvent: Benzene is an excellent nonpolar solvent for fats, oils, resins, waxes, and many other organic compounds. However, due to its toxicity, its use as a general solvent has been largely replaced by less harmful alternatives like toluene.
• Manufacture of Dyes: It is a crucial starting material for the synthesis of various synthetic dyes, providing the basic aromatic skeleton.
• Drugs and Pharmaceuticals: Benzene derivatives are intermediates in the synthesis of a wide range of pharmaceutical products.
• Explosives: Nitration products of benzene and its derivatives are used in the production of explosives, such as Trinitrotoluene (TNT), derived from toluene.
• Polymers and Plastics: Benzene is a precursor for the production of monomers like styrene (for polystyrene), phenol (for nylon, bakelite), and aniline, which are used in the manufacture of various polymers and plastics. For example, nylon is produced from benzene derivatives.
• Detergents: Alkylbenzenes, particularly linear alkylbenzene sulfonates (LAS), are key components in the manufacture of synthetic detergents.
• Other Chemicals: It is used to produce cyclohexane, maleic anhydride, chlorobenzene, and other important industrial chemicals.